Evidence-based Synthesis Program Department of Veterans Affairs 

Health Services Research & Development Service 

December 2012 

Comparative Effectiveness 

of Newer Oral Anticoagulants 

and Standard Anticoagulant 

Regimens for Thromboprophylaxis 

in Patients Undergoing Total Hip 

or Knee Replacement 

Prepared for: 

Department of Veterans Affairs 

Veterans Health Administration 

Quality Enhancement Research Initiative 

Health Services Research & Development Service 

Washington, DC 20420 

Prepared by: 

Evidence-based Synthesis Program (ESP) Center 

Durham Veterans Affairs Healthcare System 

Durham, NC 

John W. Williams Jr., M.D., M.H.Sc., Director 

Investigators: 

Principal Investigator: 

Soheir S. Adam, M.D. 

Co-Investigators: 

Jennifer R. McDuffie, Ph.D. 

Paul F. Lachiewicz, M.D. 

Thomas L. Ortel, M.D., Ph.D. 

John W. Williams Jr., M.D., M.H.Sc. 

Research Associate:
 

Avishek Nagi, M.S.
 

Medical Editor:
 

Liz Wing, M.A.
 
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Comparative Effectiveness of New Oral Anticoagulants

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PREFACE
 

Quality Enhancement Research Initiative’s (QUERI’s) Evidence-based Synthesis Program (ESP) 

was established to provide timely and accurate syntheses of targeted healthcare topics of particular 

importance to Veterans Affairs (VA) managers and policymakers, as they work to improve the 

health and healthcare of Veterans. The ESP disseminates these reports throughout VA. 

QUERI provides funding for four ESP Centers and each Center has an active VA affiliation. The ESP 

Centers generate evidence syntheses on important clinical practice topics, and these reports help: 

• develop clinical policies informed by evidence, 

• guide the implementation of effective services to improve patient 

outcomes and to support VA clinical practice guidelines and performance 

measures, and 

• set the direction for future research to address gaps in clinical knowledge. 

In 2009, the ESP Coordinating Center was created to expand the capacity of QUERI Central 

Office and the four ESP sites by developing and maintaining program processes. In addition, 

the Center established a Steering Committee comprised of QUERI field-based investigators, 

VA Patient Care Services, Office of Quality and Performance, and Veterans Integrated Service 

Networks (VISN) Clinical Management Officers. The Steering Committee provides program 

oversight, guides strategic planning, coordinates dissemination activities, and develops 

collaborations with VA leadership to identify new ESP topics of importance to Veterans and the 

VA healthcare system. 

Comments on this evidence report are welcome and can be sent to Nicole Floyd, ESP 

Coordinating Center Program Manager, at nicole.floyd@va.gov. 

Recommended citation: Adam SS, McDuffie JR, Lachiewicz PF, Ortel TL, Williams JW Jr. 

Comparative Effectiveness of Newer Oral Anticoagulants and Standard Anticoagulant Regimens 

for Thromboprophylaxis in Patients Undergoing Total Hip or Knee Replacement. VA ESP Project 

#09-010; 2012. 

This report is based on research conducted by the Evidence-based Synthesis Program (ESP) 

Center located at the Durham VA Medical Center, Durham, NC, funded by the Department of 

Veterans Affairs, Veterans Health Administration, Office of Research and Development, Health 

Services Research and Development. The findings and conclusions in this document are those of 

the author(s) who are responsible for its contents; the findings and conclusions do not necessarily 

represent the views of the Department of Veterans Affairs or the United States government. 

Therefore, no statement in this article should be construed as an official position of the 

Department of Veterans Affairs. Potential conflicts of interest: Dr. Ortel: Grants–GlaxoSmithKline, 

Eisai, Daichi Sankyo, Pfizer, Instrumentation Laboratory; Consultancy–Boehringer Ingelheim, 

Pfizer, Instrumentation Laboratory. No other investigators have any affiliations or financial 

involvement (e.g., employment, consultancies, honoraria, stock ownership or options, expert 

testimony, grants or patents received or pending, or royalties) that conflict with material presented 

in the report. To limit conflict of interest, Dr. Ortel participated in the design and critical review of 

the report but did not participate in data abstraction or drafting of the report. 
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TABLE OF CONTENTS
 

ExEcutivE Summary
 

Background......................................................................................................................................................1
 

Methods ...........................................................................................................................................................2
 

Data Synthesis .................................................................................................................................................2
 

Rating the Body of Evidence...........................................................................................................................2
 

Peer Review.....................................................................................................................................................3
 

Results .............................................................................................................................................................3
 

Recommendations for Future Research...........................................................................................................9
 

Conclusion.......................................................................................................................................................9
 

Abbreviations Table.......................................................................................................................................10
 

introduction 

Pharmacological Treatment Options for VTE Thromboprophylaxis ............................................................11
 

mEthodS
 

Topic Development........................................................................................................................................13
 

Analytic Framework......................................................................................................................................13
 

Search Strategy..............................................................................................................................................14
 

Study Selection..............................................................................................................................................14
 

Data Abstraction ............................................................................................................................................15
 

Quality Assessment .......................................................................................................................................16
 

Data Synthesis ...............................................................................................................................................16
 

Rating the Body of Evidence.........................................................................................................................17
 

Peer Review...................................................................................................................................................18
 

rESultS
 

Literature Search ...........................................................................................................................................19
 

Study Characteristics.....................................................................................................................................20
 

Participant Characteristics.............................................................................................................................22
 

KQ 1. Comparative effectiveness of newer oral anticoagulants and standard drug classes..........................23
 

KQ 2. Comparative effects of combined pharmacological and mechanical modalities................................27
 

KQ 3. Comparative efficacy of newer oral anticoagulants............................................................................28
 

Summary and diScuSSion
 

Summary of Evidence by Key Question .......................................................................................................30
 

Clinical and Policy Implications ...................................................................................................................34
 

Applicability..................................................................................................................................................36
 

Strengths and Limitations..............................................................................................................................36
 

Recommendations for Future Research.........................................................................................................36
 

Conclusion.....................................................................................................................................................37
 

rEfErEncES ..............................................................................................................................................................38
 
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Comparative Effectiveness of New Oral Anticoagulants

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appEndix a SEarch StratEgiES.............................................................................................................42
 

appEndix B ExcludEd StudiES...............................................................................................................43
 

appEndix c SamplE data aBStraction form..................................................................................46
 

appEndix d critEria uSEd in Quality aSSESSmEnt of SyStEmatic rEviEwS.......................49
 

appEndix E pEEr rEviEw commEntS ..................................................................................................52
 

appEndix f gloSSary ...............................................................................................................................56
 

figurES
 

Figure 1. Analytic framework for the comparative effectiveness of newer oral anticoagulants..............13
 

Figure 2. Literature flow diagram.............................................................................................................19
 

taBlES
 

Table 1. Summary of the strength of evidence for KQ 1 ..........................................................................5
 

Table 2. Summary of the strength of evidence for KQ 3 ..........................................................................8
 

Table 3. Evidence gaps and future research..............................................................................................9
 

Table 4. Summary of inclusion and exclusion criteria............................................................................15
 

Table 5. Characteristics of included systematic reviews.........................................................................21
 

Table 6. Characteristics of patient samples.............................................................................................22
 

Table 7. Summary of the strength of evidence for KQ 1 ........................................................................31
 

Table 8. Summary of the strength of evidence for KQ 3 ........................................................................33
 

Table 9. U.S. guideline recommendations related to specific thromboprophylaxis strategies................35
 

Table 10. Evidence gaps and future research............................................................................................37
 
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EVIDENCE REPORT 

INTRODUCTION 

Venous thromboembolic (VTE) events are important causes of morbidity in elective total hip 

replacement (THR) and total knee replacement (TKR) procedures, which are being performed 

with increasing frequency in an aging population. Because of the substantial risk of VTE, 

current guidelines recommend thromboprophylaxis in patients undergoing THR or TKR.1-3 Low 

molecular weight heparin (LMWH) and adjusted-dose warfarin are the most commonly used 

anticoagulants for thromboprophylaxis in the United States,4 but a number of pharmacological 

treatment options are available including unfractionated heparin, aspirin, and newer oral 

anticoagulants. These drug classes differ in practical applications such as a predictable doseresponse and the need for laboratory monitoring, oral versus injection administration, dosing 

frequency, drug-drug interactions, and the availability of an appropriate reversal mechanism in 

case of over anticoagulation. In addition, mechanical thromboprophylaxis, most frequently used 

in combination with anticoagulants, is commonly used in the United States. 

Risk factors for VTE include venous stasis, endothelial injury, and hypercoagulability. Venous 

stasis can result from the positioning of the limb, localized postoperative swelling, or limited 

mobility in the postoperative period.5,6 Endothelial injury can result from positioning and 

manipulation of the limb.5,7 Markers of thrombin generation, indicating hypercoagulability, 

have been shown to be elevated in total hip arthroplasty.8

 Prior to 1980, rates of symptomatic 

VTE were 15 to 30 percent. However, changes in surgical care, including earlier ambulation, 

and changes to surgical technique that are less invasive and possibly less thrombogenic have 

decreased the rate of symptomatic VTE. A recent analysis that incorporated data from trials and 

observational studies estimated the contemporary 35-day rate of symptomatic VTE without 

thromboprophylaxis at 4.3 percent.1 

Pharmacological thromboprophylaxis for THR or TKR surgery decreases VTE by approximately 

50 percent but with the tradeoff of increased bleeding.9 Surgical procedures may also increase 

bleeding risk; major bleeding is estimated to occur in 1.5 percent of patients undergoing THR or 

TKR, even without thromboprophylaxis.1 The risk of bleeding is a concern because bleeding can 

lead to infections, reoperation, delayed wound healing, and extended hospital stay.10 Considering 

both benefits and risks, guideline panels have issued moderate to strong recommendations for 

thromboprophylaxis in patients without a contraindication.1-3 The choice of which antithrombotic 

thus becomes pivotal for balancing the prevention of thromboembolism with the risk of bleeding. 

PHARMACOLOGICAL TREATMENT OPTIONS FOR VTE 

THROMBOPROPHYLAXIS 

The most commonly used anticoagulants are LMWH, fondaparinux, and warfarin.4,11,12 

Unfractionated heparin and antiplatelet agents are rarely used. The efficacy and safety of LMWH 

for postoperative thromboprophylaxis has been established in more than 30 studies.3,9,13 LMWH 

binds to antithrombin and accelerates the inhibition of thrombin and factor X. LMWH has a long 

half-life, which allows a once-daily dosing schedule and good bioavailability after subcutaneous 
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injection. Disadvantages of LMWH include the need for parenteral administration, high drug 

cost, and a small risk of heparin-induced thrombocytopenia.14 Similarly, fondaparinux has 

good bioavailability when given once daily subcutaneously. Due to the length of the molecule, 

fondaparinux mainly acts by catalyzing the inhibition of factor Xa (FXa) with essentially no 

inhibition of thrombin.15 

For nearly 50 years, warfarin has been successfully used for prophylaxis and treatment of 

VTE. Warfarin is administered orally once daily and is inexpensive. However, it has several 

disadvantages, including the need for regular monitoring with international normalized ratio 

(INR) and numerous interactions with a host of drugs, herbs, and dietary products. Its delayed 

onset of action can leave patients unprotected in the early postoperative period. In fact, registry 

data show that surgeons using warfarin are less likely to meet guideline recommendations than 

with other agents due to failure to meet the target INR.4 

Newer oral anticoagulants have been developed with the goal to overcome the limitations 

of warfarin and the available parenteral agents. These new anticoagulants belong to two 

drug classes, based on their target coagulation protein: FXa inhibitors and direct thrombin 

inhibitors (DTIs). These are given as fixed oral doses and have the advantage of a more 

predictable anticoagulant effect, eliminating the need for monitoring when used for short term 

thromboprophylaxis. Disadvantages of newer oral anticoagulants include drug costs and the 

lack of specific antidotes to reverse their anticoagulant effect in a timely fashion in case of 

bleeding. Rivaroxaban, an oral FXa inhibitor, was approved on July 1, 2011, by the U.S. Food 

and Drug Administration (FDA) for prophylaxis of VTE in adults undergoing orthopedic surgery. 

Other oral FXa inhibitors that are currently under clinical development include apixaban, 

edoxaban, and betrixaban. Apixaban is under FDA review for thromboprophylaxis in orthopedic 

surgery. Dabigatran etexilate is an oral DTI that has been approved in the United States for 

stroke prevention in atrial fibrillation. Renal excretion is the predominant elimination pathway 

for dabigatran, with more than 80 percent of systemically available dabigatran eliminated 

unchanged.16 Dabigatran has a better drug interaction profile compared with warfarin and is 

currently under review for FDA approval in patients undergoing elective orthopedic surgery. 

Given the emerging data on new oral anticoagulants, this report was commissioned by 

the VA to assess the comparative effectiveness of newer oral anticoagulants and standard 

thromboprophylaxis regimens in total hip and knee replacement surgery. 
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METHODS
 

TOPIC DEVELOPMENT 

This review was commissioned by the VA’s Evidence-based Synthesis Program. The topic was 

nominated after a topic refinement process that included a preliminary review of published 

peer-reviewed literature, consultation with internal partners and investigators, and consultation 

with key stakeholders. We further developed and refined the key questions (KQs) based on a 

preliminary review of published peer-reviewed literature in consultation with VA and non-VA 

experts. 

The final KQs were: 

KQ 1. For patients undergoing total hip or total knee replacement, what is the comparative 

effectiveness of newer oral anticoagulants and standard drug classes (low molecular weight 

heparin, injectable factor Xa inhibitors, unfractionated heparin, warfarin, aspirin) on the 

incidence of symptomatic, objectively confirmed venous thromboembolism (VTE), other VTE 

events, total mortality, and bleeding outcomes? 

KQ 2. For patients undergoing total hip or total knee replacement, what are the effects of 

combined pharmacological and mechanical modalities versus pharmacological treatment alone 

on the incidence of symptomatic, objectively confirmed VTE, other VTE events, total mortality, 

and bleeding outcomes? 

KQ 3. For patients undergoing total hip or total knee replacement, what is the comparative 

efficacy of individual newer oral anticoagulants on the incidence of symptomatic, objectively 

confirmed VTE, other VTE events, total mortality, and bleeding outcomes? 

ANALYTIC FRAMEWORK 

We followed a standard protocol for all steps of this review; certain methods map to the PRISMA 

checklist.17 Our approach was guided by the analytic framework shown in Figure 1. 

Figure 1. Analytic framework for the comparative effectiveness of newer oral anticoagulants 

KQs 1–3 

Adverse effects 

of treatment 

Rates of venous 

thromboembolic 

events and bleeding 

•	 Anticoagulants including LMWH, FXa 

inhibitors, unfractionated heparin, 

warfarin, aspirin 

•	 Combined pharmacological and 

mechanical interventions 

Adults having total 

hip or total knee 

replacement surgery 

All-cause mortality 

Abbreviations: FXa=factor Xa; KQs=key questions; LMWH=low molecular weight heparin 
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SEARCH STRATEGY 

During the topic development phase of this study, we identified a number of published highquality systematic reviews that addressed our KQs. We concluded that a synthesis of these 

reviews as they pertained to the KQs and the Veteran population would be the most effective 

approach to summarizing the evidence. This approach is particularly useful when different 

intervention options or outcomes are evaluated in multiple recent reviews and when the 

audience is policymakers. We searched MEDLINE® (via PubMed®), Embase®, and the Cochrane 

Database of Systematic Reviews for systematic review publications comparing the newer oral 

anticoagulants to other types of anticoagulation (aspirin, warfarin, LMWH, unfractionated 

heparin, etc.) from January 1, 2009, through May 30, 2012. Our search strategy used the 

National Library of Medicine’s medical subject headings (MeSH) keyword nomenclature and 

text words for newer oral anticoagulants, the conditions of interest, and validated search terms 

for systematic reviews.18,19 

Our final search terms included new or novel oral anticoagulants; DTIs, including dabigatran, 

FXa inhibitors, including edoxaban, rivaroxaban, apixaban, betrixaban, YM150; the MeSH 

descriptor “orthopedic procedures”; and terms for the specific procedures of interest, total knee 

replacement or total hip replacement surgery. We limited the search to systematic reviews and 

meta-analyses and articles published in the English language involving human subjects 18 years 

of age and older. The full search strategy is provided in Appendix A. We supplemented the 

electronic searches with a manual search of citations from a set of key systematic reviews20-23 

and clinical guidelines.1,24 We developed our search strategy in consultation with an experienced 

search librarian and updated the search during the course of analysis so as not to miss any recent, 

pertinent reviews; the last update was conducted September 2012. A supplementary search of 

the primary literature was conducted in September 2012 to identify relevant trials published 

since May 2012. All citations were imported into an electronic database (DistillerSR; Evidence 

Partners, Inc., Manotick, ON, Canada) for citation screening. 

STUDY SELECTION 

Using prespecified inclusion and exclusion criteria, two reviewers assessed titles and abstracts 

for relevance to the KQs. Full-text systematic reviews identified by either reviewer as potentially 

relevant were retrieved for further review. Each article retrieved was examined by two reviewers 

against the eligibility criteria. Disagreements on inclusion, exclusion, or major reason for 

exclusion were resolved by discussion or by a third reviewer. 

The criteria to screen articles for inclusion or exclusion at both the title-and-abstract and full-text 

screening stages are detailed in Table 4. Studies excluded at the full-text review stage are listed 

with the reasons for exclusion in Appendix B. 
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Table 4. Summary of inclusion and exclusion criteria 

Study 

Characteristic Inclusion Criteria Exclusion Criteria 

Population Adults (≥18 years) of age undergoing elective 

orthopedic surgery for total hip or total knee 

replacement 

Pregnant women 

Intervention KQ 1: Newer oral anticoagulants: Direct thrombin 

inhibitors and factor Xa inhibitors 

KQ 2: Combined pharmacological and mechanical 

modalities 

KQ 3: Newer oral anticoagulants 

Newer anticoagulants requiring 

intravenous or subcutaneous 

administration 

Comparator KQ 1: Warfarin, low molecular weight heparin, 

unfractionated heparin, aspirin 

KQ 2: Pharmacological treatment alone 

KQ 3: Within-class or between-class comparison with 

another newer oral anticoagulant 

Comparators other than those 

specified by the KQ inclusion criteria 

Outcome Primary outcomes: Symptomatic, objectively confirmed 

deep vein thrombosis or pulmonary embolism 

Secondary outcomes: Major bleeding, surgical-site 

bleeding, or mortality 

No relevant outcomes 

Timing Outcomes reported >1 week postoperatively Less than 1 week postoperatively 

Setting Inpatient surgical settings None 

Study design Systematic reviews that evaluated randomized 

controlled trials (RCTs) or secondary data analysis 

from an RCT 

Not an SR of at least fair or good 

quality 

Publications English-language only 

Published from 2009 to present 

Peer-reviewed article 

Non-English language publication 

Published before 2009 

Abbreviations: KQ=key question; RCT=randomized controlled trial 

DATA ABSTRACTION 

Before general use, the abstraction form templates designed specifically for this report were 

pilot-tested on a sample of included articles and revised to ensure that all relevant data elements 

were captured and that there was consistency and reproducibility between abstractors. Select data 

from published reports were then abstracted into the final abstraction form by a trained reviewer 

(Appendix C). All data abstractions were confirmed by a second reviewer. Disagreements were 

resolved by consensus or by obtaining a third reviewer’s opinion when consensus could not be 

reached. We abstracted the following key information for each included study: 

• Systematic review design features 

○ Databases used in searches and dates of searches 

○ Inclusion/exclusion criteria 

○ Number of primary studies that apply to each KQ 

○ Method of analysis 

○ Types of comparisons 

○ Tests for heterogeneity 

○ Assessment of publication bias 
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• Characteristics of the included studies 

○		 Average or range of ages included 

○		 Average or range of sex distribution 

○		 Inclusion of Veteran Health Care Facilities 

○		 Indication for anticoagulation 

○		 Baseline bleeding risk or factors associated with increased risk (e.g., creatinine >1.5, 

history of gastrointestinal bleeding), if given 

○		 Countries included in primary studies 

○		 Study drug and comparator, route of administration, and dosage 

○		 Length of treatment and followup duration 

○		 Funding source 

•		 Results of the systematic review 

○		 Number of studies and subjects and completion rates 

○		 Quality of the primary literature and strength of evidence, if given 

○		 Outcomes (including definition of outcome, if given) 

○		 Results from subgroup or sensitivity analyses 

○		 Author conclusions 

In addition, we examined included articles for subgroup analyses of particular relevance to the 

population served by Veterans Health Administration. Data on the inclusion of Veteran Health 

Care Facilities was not provided at the systematic review level; therefore, we returned to the 

primary literature to abstract this information. 

QUALITY ASSESSMENT 

We also abstracted data necessary for assessing the quality of systematic reviews, adapted 

from the AMSTAR criteria.25-27 These key quality criteria consist of (1) search methods are 

adequate for replication and are comprehensive, (2) selection bias is avoided, (3) data are 

abstracted reliably, (4) characteristics of primary literature are reported and quality is assessed 

appropriately, (5) results are synthesized using appropriate methods, (6) publication bias is 

assessed, (7) conflict of interest is reported, and (8) conclusions are supported by results. We 

supplemented these criteria for studies that used multiple treatment comparisons based on the 

guidance by Mills et al.28 Based on these criteria, systematic reviews were categorized as good, 

fair, or poor quality (Appendix D). Poor-quality reviews were excluded. The criteria were applied 

for each study by the reviewer abstracting the article; this initial assessment was then overread 

by a second reviewer. Disagreements were resolved between the two reviewers or, when needed, 

by arbitration from a third reviewer. 

DATA SYNTHESIS 

We categorized each systematic review by the key research questions they addressed and 

critically analyzed them to compare their characteristics, methods, and findings. We summarized 

the key findings and conclusions from each included review and produced summary tables for 

comparison across reviews. We prioritized the evidence from these reviews by higher quality of 

methodological designs, more complete drug comparisons (e.g., by class and drug rather than 
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by drug only), and more detailed information about population, specific drug intervention (e.g., 

dosage), and definitions of outcomes. In addition to summary measures of relative effects (e.g., 

risk ratios), we report absolute risk differences in the summary strength of evidence tables. For 

FXa inhibitors, we used the risk differences reported by Neumann et al.20 To standardize the 

reporting of risk differences, which are dependent on the baseline risk of events, we adopted the 

approach used by Neumann et al. for other drugs. Risk difference was estimated by using the 

baseline risk from the control group and the risk ratio from the relevant meta-analysis. Baseline 

risk for patients treated with LMWH, the common comparator for newer anticoagulants, was 

estimated for each major outcome as symptomatic deep vein thrombosis (DVT), 9 per 1000 

patients; nonfatal pulmonary embolism (PE), 3 per 1000 patients; mortality, 3 per 1000 patients; 

and major bleeding, 7 per 1000 patients.20 

Our synthesis focused on documenting and identifying patterns in efficacy and safety of the 

different drugs. To determine the consistency of results and conclusions, we then compared 

each additional review that addressed the same key question. If findings or conclusions differed 

importantly across reviews, we analyzed potential reasons for discrepancies such as the primary 

literature included (both the type of studies and the dates of the searches), review inclusion/ 

exclusion criteria, differences in outcome definition, analytic approach, and conflict of interest. 

In the event that our supplemental search of the primary literature identified additional eligible 

studies, we planned a qualitative summary of these studies to determine if the outcomes observed 

were consistent with the results from the systematic reviews. However, our search did not 

identify any additional relevant RCTs. 

RATING THE BODY OF EVIDENCE 

In addition to rating the quality of individual studies, we evaluated the overall quality of the 

evidence for each KQ as described in the Methods Guide for Effectiveness and Comparative 

Effectiveness Reviews.

29 In brief, this approach requires assessment of four domains: risk of 

bias, consistency, directness, and precision. For risk of bias, we considered basic (e.g., RCT) 

and detailed study design (e.g., adequate randomization) using the quality assessments of the 

primary literature reported in the systematic reviews. We used results from meta-analyses when 

evaluating consistency (forest plots, tests for heterogeneity), precision (confidence intervals), 

strength of association (odds ratio), and whether publication bias was detected (e.g., funnel plots 

or Begg’s test). Optimal information size and consideration of whether the confidence interval 

crossed the clinical decision threshold for a therapy were also used when evaluating precision.30 

These domains were considered qualitatively, and a summary rating of high, moderate, low, or 

insufficient strength of evidence was assigned after discussion by two reviewers. This four-level 

rating scale consists of the following definitions: 

•	 High—Further research is very unlikely to change our confidence on the estimate of 

effect. 

•	 Moderate—Further research is likely to have an important impact on our confidence in 

the estimate of effect and may change the estimate. 

•	 Low—Further research is very likely to have an important impact on our confidence in 

the estimate of effect and is likely to change the estimate. 
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•	 Insufficient—Evidence on an outcome is absent or too weak, sparse, or inconsistent to 

estimate an effect. 

When a rating of high, moderate, or low was not possible or was imprudent to make, a grade of 

insufficient was assigned.31 

PEER REVIEW 

A draft version of the report was reviewed by technical experts and clinical leadership. A 

transcript of their comments can be found in Appendix E, which elucidates how each comment 

was considered in the final report. 
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RESULTS
 

LITERATURE SEARCH
 

The flow of articles through the literature search and screening process is illustrated in Figure 2. 

Our search for systematic reviews (SRs) identified 162 unique citations from a combined search 

of MEDLINE via PubMed (n=117), Embase (n=42), and the Cochrane Database of Systematic 

Reviews (n=3). Manual searching of included study bibliographies and review articles added 20 

more citations for a total of 182 unique citations. After applying inclusion and exclusion criteria 

at the title-and-abstract level, 47 full-text articles were retrieved and screened. Of these, 38 were 

excluded at the full-text screening stage, leaving 9 articles (representing 9 unique studies) for 

data abstraction. After further review, we excluded three systematic reviews32-34 because they 

reviewed only one drug of interest and all of the primary studies included in these systematic 

reviews were already represented in another, more comprehensive included review. Thus, the 

final set of articles used in this evidence report comprises six systematic reviews. 

Appendix B provides a complete listing of published articles excluded at the full-text screening 

stage, with reasons for exclusion. We did not search www.clinicaltrials.gov for randomized 

controlled trials (RCTs) currently underway, as we relied on methods in the included systematic 

reviews to ascertain publication bias. We grouped the studies by key question (Figure 2). 

Figure 2. Literature flow diagram
 

Search results: 

182 referencesa 

Excluded: 135 references at the 

title/abstract level 

Retrieved for full-text review: 

47 references 

Excluded: 38 references 

Not a systematic review: 32 

Poor-quality systematic review: 1 

Did not address KQs: 5 

Note: 20 of these 38 were flagged 

for background. 

Potential for abstraction: 

9 references 

Excluded: 3 references that examined a 

single drug and all primary studies were 

represented in other reviewsb 

Abstracted: 

6 systematic reviews (SRs) 

KQ 1: New oral 

anticoagulants 

vs. comparator 

(6 SRs) 

KQ 2: Mechanical 

interventions 

(1 SR) 

KQ 3: Within-class 

comparisons 

(2 SRs) 

a

Search results for systematic reviews from PubMed (117), Embase (42), Cochrane (3), previous database (14), and manual (6) 

were combined.
 b

Cao, 2010; Huang, 2011; and Turun, 2011. 

Note: The reference list of this report includes additional references cited for background and methods plus Web sites relevant to 

the key questions.
 

Abbreviations: KQ=key question; SR=systematic review
 
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STUDY CHARACTERISTICS 

We identified six recent, good-quality SRs9,20-23,35 that were relevant to our KQs (Table 5). 

All of the SRs compared newer oral anticoagulants with other drug classes used for 

thromboprophylaxis in THR or TKR (KQ 1), although one considered only safety outcomes such 

as major bleeding.35 Two of the six SRs compared one newer oral anticoagulant with another 

(KQ 3) though all results were based on indirect comparisons; i.e., through common comparison 

with enoxaparin.21,23 Only one SR9 compared a pharmacological agent plus mechanical modality 

versus mechanical modality alone (KQ 2). Five of the six SRs included trials examining 

thromboprophylaxis for both THR and TKR, while one also included hip fracture surgery.9 

Characteristics of the SRs are summarized in Table 5; detailed quality assessments are presented 

in Appendix D. 

Search dates ranged from May 2009 to December 2011. All literature search strategies included 

MEDLINE, and all but one21 included some aspect of the Cochrane Library. Other databases or 

sources of information were meeting abstracts (5), Embase (3), regulatory Web sites (4), clinical 

trial registries (3), and the Center for Reviews and Dissemination (1). Four studies9,20-22 also 

involved a manual search of the bibliographies of exemplary primary articles. The searches were 

limited only to RCTs in four of the SRs. One included SRs,22 and one included observational 

studies of more than 750 subjects9 in addition to RCTs. Language limits were used in only two of 

the studies. 

All reviews assessed the quality of included trials. Overall trial quality was judged to be good, 

with the most common quality problems being unclear allocation concealment and incomplete 

reporting of outcome data. Publication bias was assessed most commonly with funnel plots, 

which did not indicate any publication bias that would favor newer oral anticoagulants. All 

studies conducted random-effects meta-analyses, but specific strategies varied. Two SRs 

compared drug classes as a whole (for example, FXa inhibitors versus LMWH9,20), while the 

other four SRs compared individual drugs, some analyzing THR and TKR studies separately. All 

of the SRs performed meta-analysis using direct comparisons, and two also provided indirect 

comparisons.21,23 Every SR except one21 evaluated major bleeding using the same definition: 

bleeding that was fatal, involved a critical organ, required reoperation, or where bleeding was 

associated with a fall in hemoglobin level of at least 2 g/dL or required infusion of 2 or more 

units of whole blood or packed cells. Our other prespecified primary outcomes—all-cause 

mortality, symptomatic DVT, and nonfatal PE—were reported in three of the SRs.9,20,22 It was 

difficult to evaluate the other SRs21,23,35 due to the individuality of the definitions given for the 

outcomes reported, many of which were composite outcomes. For example, in the study by 

Loke et al.,21 the authors state the composite primary outcome as “total VTE” and define it as 

“DVT, non-fatal PE and all-cause mortality” (emphasis added). The study also reports bleeding 

as a combination of major bleeding (using the standard definition given above) and “clinically 

relevant non-major bleeding.” 

Three of the SRs were unfunded and reported no conflicts of interest.21,23,35 One SR was unfunded 

but did report a conflict of interest for one author.20 Two SRs were funded by government 

agencies.9,22 
21 

ew Oral Anticoagulants for Thromboprophylaxis Evidence-based Synthesis Program

uded systematic reviews 

mann, 201220 Sobieraj, 20129 Gómez-Outes, 201223 Loke, 201121 Ringerike, 201122 Alves, 201135 

Good Good Good Good Good 

KQs 1, 2 KQs 1, 3 KQs 1, 3 KQ 1 KQ 1 (safety only) 

KR THR, TKR, HFS THR, TKR THR, TKR THR, TKR THR, TKR 

g classes: 

s. LMWH 

H/warfarin in one 

tudies) 

As drug classes: - FXa vs. LMWH - DTI vs. LMWH - DTI vs. UFH 

As individual drugs vs. LMWH: - Apixaban - Dabigatran - Rivaroxaban 

As individual drugs vs. LMWH: - Dabigatran - Rivaroxaban 

As individual drugs vs. LMWH: - Dabigatran - Rivaroxaban 

As individual drugs vs. LMWH: - Apixaban - Rivaroxaban 

NE, Embase, 

T, meeting 

cts 

MEDLINE, CCRCT, Scopus, clinical trial registries, meeting abstracts, regulatory Web sites 

MEDLINE, CCRCT, clinical trial registries, meeting abstracts, regulatory Web sites 

MEDLINE, Embase, clinical trial registries, regulatory Web sites 

MEDLINE, Embase, Cochrane Library and Center for Reviews and Dissemination 

MEDLINE, Cochrane Library; meeting abstracts, regulatory Web sites 

ber 2011 May 2011 April 2011 May 2009 September 2010 June 2011 

None None None English, Scandinavian None 

RCTs, observational if more than 750 subjects RCTs RCTs RCTs, SRs RCTs 

nalysis, direct, 

up by dosage 

Meta-analysis, direct, indirect, network MA 

Meta-analysis, direct, indirect, subgroup by surgery type 

Meta-analysis, direct, indirect, subgroup by protocol 

Meta-analysis, subgroup by type of surgery 

Meta-analysis, direct, subgroup by surgery type 

ty, symptomatic 

onfatal PE, 

bleeding 

Mortality, symptomatic DVT, nonfatal PE, major bleeding 

Symptomatic DVT, nonfatal PE, major bleeding 

None included Mortality, symptomatic 

DVT, nonfatal PE, major bleeding 

Major bleeding 

anial bleeding, 

g leading to 

ation 

Symptomatic VTE, major VTE, PE, surgical site bleeding, readmission 

Total VTE or mortality, symptomatic VTE, clinically relevant bleeding 

Total VTE (mortality + DVT + nonfatal PE), bleeding (major + clinically relevant nonmajor) 

None Safety variables: 

Other types of bleeding, adverse events 

Government None None Government None 

sclosed No No No NR No 

 2=combined pharmacological and mechanical vs. pharmacological monotherapy; KQ 3=within-class comparisons of newer oral anticoagulants, all of which 

tral Registry of Controlled Trials; CINAHL=Cumulative Index of Nursing and Allied Health Literature; COI=conflict of interest; DTI=direct thrombin inhibitor; 

r Xa inhibitor; HFS=hip fracture surgery; KQ=key question; LMWH=low molecular weight heparin; MA=meta-analysis; NR=not reported; PE=pulmonary em

al; SR=systematic review; THR=total hip replacement; TKR=total knee replacement; UFH=unfractionated heparin; VTE=venous thromboembolism 
22 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

PARTICIPANT CHARACTERISTICS 

Information on the populations studied was very limited in all of the included SRs (Table 6). The 

number of primary articles included ranged from 5 to 45; total sample size ranged from just over 

19,000 to almost 240,000, but two articles did not report the total number of subjects. Females 

made up approximately 50 to 75 percent of the population when reported. Mean age ranged 

from 55 to 68 years in the 5 studies reporting age. Weight or body mass index was reported in 

four studies and indicated most subjects were moderately overweight to slightly obese. Risk 

factors for VTE were limited to a prior history of VTE (two studies) or history of cancer (one 

study); only a small proportion of patients had one of these risk factors. No other risk profiles 

or population characteristics were reported, and no study reported whether or not Veterans 

were included in the sample. However, our review of the primary studies found that no studies 

specifically included Veterans or were conducted at VA medical centers. 

Table 6. Characteristics of patient samples 

Study Neumann, 

201220 

Sobieraj, 

20129 

GómezOutes, 201223 Loke, 201121 Ringerike, 

201122 Alves, 201135 

Studies 

(N patients)a 22 (32,159) 45 (36,152) 16 (38,747) 9 (19,218) 5 (NR) 12 (28,483) 

Female 

(% range) 44.6 to 72.5 

RCTs 

36.05–84.1% 

observational 

63–65% 

50 to 74 55 to 70 NRb 51 to 71 

Mean age 

(range) 57.8 to 67.6 

RCTs 

52.4 to 78.3; 

observational 

66.4 to 71 

61 to 68 63.2 to 67.7 NRb 60.6 to 67.6 

Weight 

(kg range) 

26.5 to 32.7c 

RCTs 

64.2 to 89 kg; 

observational 

NR 

75 to 89 76 to 89 NR NR 

Veterans? No No No No No No 

Risk factors: 

History of VTE 

History of 

cancer 

NR

 0 to 14.5%

 0 to 12.4% 

1 to 4% 

(9 studies) NR NR NR 

Risk factorsd NR NR NR NR NR NR 

a

Numbers are for RCTs, with the exception of Ringerike et al., which reviewed three RCTs and two SRs. Sobieraj et al. also 

reviewed three observational studies including over 239,000 participants.
 b

Ringerike et al. did not give demographics on study populations but did give national statistics on who had these surgeries 

performed in Sweden: average age is 69.1 and 69.4 yrs and % female is 68.4 and 67.4% on average for THR and TKR, 

respectively.
 c

Value is in BMI units (kg/m2).
 d

Risk factors sought were prior gastrointestinal bleeding, anemia, chronic kidney disease, and diabetes mellitus. 

Abbreviations: kg=kilogram; NR=not reported; RCT=randomized controlled trial; VTE=venous thromboembolism
 
23 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

KEY QUESTION 1. For patients undergoing total hip or total knee 

replacement, what is the comparative effectiveness of newer oral 

anticoagulants and standard drug classes (low molecular weight 

heparin, injectable FXa inhibitors, unfractionated heparin, warfarin, 

aspirin) on the incidence of symptomatic, objectively confirmed 

venous thromboembolism (VTE), other VTE events, total mortality, 

and bleeding outcomes? 

Key Points 

•		 For all-cause mortality and nonfatal PE, there were no important differences between 

oral FXa inhibitors and LMWH (high strength of evidence). Using a base rate of 9 events 

per 1000 patients with LMWH, FXa inhibitors were associated with lower symptomatic 

DVT (4 fewer events per 1000 patients; 95% CI, 3 to 6). Overall, FXa inhibitors were 

associated with an increased risk of major bleeding, but major bleeding did not differ 

importantly at low to moderate doses (moderate strength of evidence). Based on subgroup 

analyses, there was not a consistent pattern of differences in treatment effects for THR 

and TKR. 

•		 There were fewer studies evaluating oral DTIs than oral FXa inhibitors; all trials 

compared dabigatran with enoxaparin. Although estimates of effect were often imprecise, 

there were no significant differences between oral DTIs and enoxaparin for any major 

outcome. 

•		 Neither oral FXa inhibitors nor DTIs have been compared directly with adjusted-dose 

warfarin, oral antiplatelet drugs, or unfractionated heparin in existing SRs. 

We identified six good-quality SRs9,20-23,35 that evaluated thromboprophylaxis using newer oral 

anticoagulants versus LMWH. For each comparison, we focus our discussion on the review 

having the most recent search date and comprehensive analysis, and which reported our 

prespecified outcomes of interest. Other reviews are described briefly when findings differed 

importantly or additional analyses provided relevant results. 

Effects of Oral FXa Inhibitors Compared With Low Molecular Weight Heparin 

A good-quality SR20 (search date December 2011) included 22 RCTs and a total of 32,159 

patients that compared FXa inhibitors with LMWH for surgical thromboprophylaxis. Eleven of 

the included studies were on hip replacement, 10 were on knee replacement, and 1 was on either 

procedure. FXa inhibitors included apixaban (four studies), rivaroxaban (eight studies), edoxaban 

(four studies), YM150 (two studies), and LY1517717, TAK442, razaxaban, and betrixaban (one 

study each). Of these drugs, only rivaroxaban is currently available in the United States. In the 

majority of trials, the European-approved dose of enoxaparin, 40 mg daily, was the comparator 

instead of the U.S.-approved dose, 30 mg twice daily. The duration of thromboprophylaxis was 

14 days or less in all but 4 trials. Patients were followed for less than 14 days in 9 trials, 30 to 70 

days in 12 trials, and up to 90 days in one trial. In addition to a random-effects meta-analysis of 

drug class comparisons, this sophisticated review performed a multiple-treatment-comparison 

meta-analysis to evaluate effects of FXa dose, and sensitivity analyses to examine the effects 

of missing outcomes. Pooled estimates of effect were presented as summary odds ratios and 
24 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

summary risk differences. In addition, risk differences were estimated by applying the relative 

risk reduction from meta-analysis to the baseline risk estimated from a large cohort study. 

This SR by Neumann et al.20 found high strength of evidence suggesting no important difference 

between oral FXa inhibitors and LMWH for all-cause mortality (OR 0.95; 95% CI, 0.55 to 

1.63; I2=43%) and nonfatal PE (OR 1.07; CI, 0.65 to 1.73; I2=35%) in patients undergoing THR 

or TKR. However, high strength of evidence indicated that the risk of symptomatic DVT is 

decreased by 4 events for every 1000 patients treated using FXa thromboprophylaxis compared 

with LMWH (OR 0.46; CI, 0.30 to 0.70; I2=0%). There was moderate strength of evidence 

because of inconsistency, suggesting that the risk of major bleeding may be increased with 

oral FXa inhibitors compared with LMWH (OR 1.27; CI, 0.98 to 1.65; I2=55%). This finding 

represents an increase of 2 major bleeding events per 1000 patients treated with FXa, for 1 to 

5 weeks compared with LMWH. The pooled effect estimate of bleeding that led to reoperation 

also was increased (OR 1.62; CI, 0.82 to 3.19; I2=1%), but the confidence interval included the 

possibility of a chance association. 

In a subgroup analysis, higher doses of FXa inhibitors, but not intermediate or lower doses, 

were associated with increased risk of bleeding (OR 2.50; 95% CI, 1.38 to 4.53; p=0.02). The 

authors did not report the drug doses used for this subgroup analysis. Total daily doses were 

reported for the primary studies and ranged from 5 to 20 mg for apixaban and 5 to 60 mg for 

rivaroxaban. In an analysis that adjusted for FXa dose, there was no significant difference in 

thrombotic or bleeding outcomes for different FXa inhibitors. Sensitivity analyses that accounted 

for missing outcomes did not differ appreciably from the main analyses. Thus this SR concluded 

that while there is no important difference between low-dose oral FXa inhibitors and LMWH for 

the outcomes of all-cause mortality, nonfatal PE, and major bleeding, there is a small absolute 

reduction in symptomatic DVT events (4 fewer events per 1000 patients treated). However, 

most studies included in this SR reported bleeding as a composite outcome and did not include 

details; this introduces uncertainty about the importance of the reported bleeding events. Other 

limitations of the included trials in this SR were (1) missing outcomes for 3 to 41 percent of 

randomized patients, (2) the short duration of followup in many trials, (3) the nonstandard dosing 

of enoxaparin, and (4) the short duration of prophylaxis in patients undergoing THR. 

The other SRs9,21-23,35 were generally in agreement with the results and conclusions of Neumann 

et al. Where disagreements occurred, they were mainly due to different outcomes (e.g., 

composite outcomes), differences in approach to data analysis (separate analyses for each drug), 

and fewer included studies due to earlier search dates and more restrictive inclusion criteria 

(e.g., only FDA-approved drugs). Also, most SRs reported on outcomes by individual new oral 

anticoagulants, whereas those by Neumann et al. and Sobieraj et al. reported on outcomes by 

drug class. 

We summarize below the notable findings from these other SRs: 

• In a good-quality review23 that separately analyzed the effects of apixaban (4 trials) and 

rivaroxaban (8 trials), both drugs were associated with lower symptomatic DVT than 

enoxaparin (apixaban, RR 0.41; 95% CI, 0.18 to 0.95, and rivaroxaban, RR 0.40; CI, 

0.22 to 0.72). Symptomatic VTE (DVT or PE) was decreased with rivaroxaban (RR 0.48; 

CI, 0.31 to 0.75; I2

=5%) but not apixaban (RR 0.82; CI, 0.41 to 1.64; I2=40%). All-cause 
25 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

mortality was not reported as a separate outcome. Symptomatic PE and major bleeding 

did not differ significantly from LMWH, but confidence intervals for these estimates were 

wide and included the potential for clinically important differences. Notably, to increase 

the consistency of outcome definitions and results, major bleeding rates for the RECORD 

studies of rivaroxaban36-39 were analyzed using data reported to the FDA—a definition that 

included wound bleeding. Subgroup analyses showed no differences in treatment effect by 

type of surgery (THR vs. TKR) for symptomatic VTE or clinically relevant bleeding. 

•		 In a review limited by the exclusion of rivaroxaban,9 the pooled effect from four RCTs 

comparing LMWH with FXa inhibitors did not show a significant difference in major 

bleeding leading to reoperation (OR 0.67; 95% CI, 0.28 to 1.61). 

• In a review limited by the exclusion of apixaban,21 the risk of hemorrhage (major and 

clinically relevant nonmajor bleeding) did not differ significantly for rivaroxaban 

compared with LMWH (RR 1.26; 95% CI, 0.94 to 1.69; I2=28%). Hemorrhage was 

defined as major bleeding leading to death, reoperation, blood transfusion of two or more 

units, a drop in hemoglobin level of more than two g/dL, or bleeding into a critical organ. 

In contrast to the review by Gómez-Outes et al., published rates of bleeding rather than 

rates reported to the FDA (that included wound bleeding) were used for these analyses. 

•		 A report on adverse outcomes by type of surgery compared two oral FXa inhibitors with 

enoxaparin.35 There was a lower risk of major bleeding with apixaban compared with 

LMWH in TKR (RR 0.56; 95% CI, 0.32 to 0.96) but not in THR (RR 1.22; 95% CI, 0.65 

to 2.26). Major bleeding events were not different with rivaroxaban treatment compared 

with LMWH in both types of surgeries. Subgroup analysis showed an increased risk of 

bleeding with the 30-mg twice-daily dosing regimen of LMWH compared with the 40­

mg once-daily dose. 

Effects of Direct Thrombin Inhibitors Compared with Low Molecular Weight Heparin 

Only four SRs9,21-23 included comparisons of dabigatran—the only available DTI—with standard 

thromboprophylaxis using LMWH. A good-quality SR (search date April 2011) included 4 trials 

involving 12,897 patients that compared dabigatran with enoxaparin for thromboprophylaxis of 

THR or TKR.23 The surgical procedure was THR and TKR in two trials each. In three trials, the 

comparator was enoxaparin at 40 mg daily, and in one trial the dose was 30 mg twice daily. The 

duration of thromboprophylaxis was 15 days or less in the two TKR studies and 28 to 35 days in 

the two THR studies. The duration of followup was approximately 3 months. Three studies used 

a three-arm design; the dabigatran 150 mg and dabigatran 220 mg treatment arms were combined 

for meta-analysis. The two-arm trial evaluated dabigatran 220 mg, a dose that is not approved by 

the FDA. All-cause mortality was not reported as a separate outcome. 

In a random-effects meta-analysis, the risk of symptomatic PE (RR 0.69; 95% CI, 0.31 to 

1.54; I2=NR) and symptomatic DVT (RR 0.82; CI, 0.17 to 3.99; I2=NR) did not differ between 

dabigatran and enoxaparin.23 Similarly, there was no statistically significant difference in 

symptomatic VTE (DVT and PE), but treatment effects differed substantially across studies 

(I2

=73%). Clinically relevant bleeding events (major bleeding and clinically relevant nonmajor 

bleeding) were not different with dabigatran treatment (RR 0.94; CI, 0.58 to 1.52; p=0.79). In 

subgroup analyses, there was no statistically significant interaction between type of surgery and 

effects on symptomatic VTE or clinically relevant bleeding. 
26 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

Three other SRs9,21,22 reported additional outcomes, including mortality, major bleeding, and 

bleeding leading to rehospitalization, summarized below: 

•		 The SR by Ringerike et al.22 included an additional study (BISTRO-II)40 of patients 

undergoing either THR or TKR, but anticoagulation was given for 7 days only. The SR 

by Sobieraj et al.9 evaluated an injectable DTI (desirudin) but did not include the most 

recent trial of oral DTI,40 which was also omitted in the SR by Loke et al.21 Despite these 

differences in approach, mortality did not differ significantly for DTIs compared with 

enoxaparin in any of these analyses. Consistent with the findings by Gómez-Outes et al.23 

for clinically relevant bleeding, major bleeding did not differ between drug classes when 

analyzed by surgical procedure22 or in aggregate.9,21 

•		 Sobieraj et al.9 found no significant difference between LMWH and dabigatran for 

bleeding leading to rehospitalization (RR 1.27; 95% CI, 0.43 to 3.75; moderate strength 

of evidence). 

Other Comparisons of Interest 

Only one good-quality SR by Sobieraj et al.9

 (search date May 2011) addressed drug class 

comparisons between older antithrombotics. We summarize results for key drug class 

comparisons and outcomes below. 

Low molecular weight heparin versus vitamin K antagonists. Sobieraj et al.9 reported 

on the comparative effects of LMWH thromboprophylaxis versus adjusted-dose warfarin. 

LMWHs included enoxaparin (30 mg every 12 hours) and logiparin. Other details such as 

duration of treatment and duration of followup were not reported uniformly for the included 

trials. Depending on outcomes, 3 to 7 trials were included in the meta-analyses. There was no 

significant difference in mortality (OR 0.79; 95% CI, 0.42 to 1.50; I2=0%), nonfatal PE (OR 

1.00; CI, 0.20 to 4.95; I2=NR), or symptomatic DVT (OR 0.87; CI, 0.61 to 1.24; I2=28.4%). The 

risk of major bleeding was significantly higher in the LMWH treatment group (OR 1.92; CI, 1.27 

to 2.91; I2=0; high strength of evidence). 

Oral FXa inhibitors versus unfractionated heparin. Sobieraj et al.9 reported on the 

comparative effects of oral FXa inhibitors versus unfractionated heparin. There were no RCTs 

comparing oral or injectable FXa inhibitors with unfractionated heparin. One observational study 

compared an injectable FXa inhibitor (fondaparinux) with unfractionated heparin; drug doses 

were not reported. The injectable FXa inhibitor was associated with lower mortality compared 

with unfractionated heparin. The risk of major bleeding was found to be increased in the 

unfractionated heparin treatment group compared with the injectable FXa inhibitor group (OR 

1.27; 95% CI, 1.06 to 1.52). Effects on symptomatic DVT and nonfatal PE were not reported. 

Low molecular weight heparin versus oral antiplatelet agents. Sobieraj et al.9

 reported on the 

comparative effects of LMWH versus oral antiplatelet agents but identified no studies comparing 

these drug classes. 

Antiplatelet agents versus vitamin K antagonists. Sobieraj et al.9 reported on the comparative 

effects of antiplatelet agents versus vitamin K antagonists, identifying a single RCT. Among 

patients undergoing hip fracture surgery, the risk of mortality was similar in both treatment 

arms (RR 0.98; 95% CI, 0.32 to 3.05). Nonfatal PE was evaluated, but there were no events 
27 

Comparative Effectiveness of New Oral Anticoagulants

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in either treatment arm. The risk of major bleeding was also reported in this trial and did not 

show a statistically significant difference (RR 0.20; CI 0.03 to 1.23). In addition to this RCT, 

two observational studies compared aspirin prophylaxis with vitamin K antagonists in patients 

undergoing THR or TKR. One study showed higher mortality with aspirin prophylaxis (0.3 

percent vs. 0 percent; p=0.013); the other study found no significant difference in mortality. 

There were no reports on symptomatic DVT or symptomatic VTE. 

KEY QUESTION 2. For patients undergoing total hip or total knee 

replacement, what are the effects of combined pharmacological and 

mechanical modalities versus pharmacological treatment alone on 

the incidence of symptomatic, objectively confirmed VTE, other VTE 

events, total mortality, and bleeding outcomes? 

Key Points 

•		 In the included SRs, no studies were identified that compared the combination of newer 

oral anticoagulants and mechanical thromboprophylaxis with pharmacological treatment 

alone. 

•		 Few studies have compared older antithrombotics (LMWH, oral antiplatelet agents, or 

unfractionated heparin) combined with mechanical prophylaxis to pharmacological or 

mechanical prophylaxis alone. 

•		 The strength of evidence is insufficient to determine the comparative effectiveness for 

combined pharmacological and mechanical prophylaxis compared with pharmacological 

prophylaxis alone for all major outcomes prioritized for this report. 

One good-quality SR9 (search date May 2011) included 6 trials and a total of 995 patients 

that compared a combined-modality thromboprophylaxis (pharmacological and mechanical 

agents) with a single modality and found a paucity of data. Four of the included studies were 

on hip replacement, one was on knee replacement, and one included both surgeries. No trial 

evaluated the combination of a newer oral anticoagulant together with mechanical prophylaxis. 

Combination treatments included LMWH, aspirin, or unfractionated heparin together with 

mechanical prophylaxis. Of the six trials, the comparator was pharmacologic prophylaxis alone 

(4 trials), mechanical prophylaxis alone (1 trial), and both pharmacological and mechanical 

comparators (1 trial). Duration of followup ranged from the postoperative period to 90 days. 

Three trials reported effects on mortality, but treatment effects were not pooled. Two of 

these trials had no mortality events; the third trial comparing the combination of aspirin plus 

pneumatic compression to aspirin alone found no effects on mortality (OR 7.72; 95% CI, 0.15 

to 389.59), but the trial was underpowered for clinically significant differences. Two trials 

evaluated the effects on nonfatal PE, but there were no events in either trial. A single older 

trial evaluated the effects of sequential unfractionated heparin for 3 days, then aspirin together 

with a venous foot pump versus sequential pharmacologic prophylaxis alone or a venous foot 

pump alone. The combined modality had a lower risk of symptomatic DVT compared with 

pharmacologic prophylaxis only (RR 0.14; 95% CI, 0.01 to 1.42), but there were few events, 

and the confidence interval included no effect. Only one trial reported the effects of combined 
28 

Comparative Effectiveness of New Oral Anticoagulants

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thromboprophylaxis (aspirin plus venous foot pump) compared with aspirin alone for major 

bleeding. However, there were no major bleeding events in either treatment arm. The authors 

concluded that there was insufficient evidence for all outcomes when comparing pharmacologic 

plus mechanical prophylaxis to pharmacologic prophylaxis alone, with the exception of overall 

DVT (including asymptomatic DVT). For overall DVT, combined treatment was more effective 

than pharmacologic prophylaxis alone. 

KEY QUESTION 3. For patients undergoing total hip or total knee 

replacement, what is the comparative efficacy of individual newer 

oral anticoagulants on the incidence of symptomatic, objectively 

confirmed VTE, other VTE events, total mortality, and bleeding 

outcomes? 

Key Points 

•		 No clinical trials directly compared newer oral anticoagulants with each other for 

thromboprophylaxis of TKR or THR.
 

•		 The included SRs did not estimate the comparative efficacy of newer oral anticoagulants 

for symptomatic DVT, nonfatal PE, all-cause mortality, or surgical site bleeding. 

•		 Based on indirect comparisons, there were few differences between newer oral 

anticoagulants for the outcomes examined. Rivaroxaban was associated with more 

major bleeding than apixaban (RR 1.59; 95% CI, 0.84 to 3.02). In contrast, the risk of 

symptomatic VTE was lower for rivaroxaban than apixaban or dabigatran, but confidence 

intervals included the possibility of a chance association. 

In the absence of direct comparisons between the newer oral anticoagulants, two good-quality 

SRs used indirect comparisons21,23 to analyze these drugs. 

A good-quality comprehensive review23 (search date April 2011) evaluated apixaban (4 trials), 

dabigatran (4 trials), and rivaroxaban (8 trials) against a common comparator (enoxaparin). 

These indirect comparisons utilized pooled risk ratios and yielded an unbiased estimate of effect 

“when there is no interaction between covariates defining subgroups of patients (reflected, for 

instance, in different inclusion criteria in different studies) and the magnitude of the treatment 

effect.”41 Of the 16 trials (total of 38,747 patients), 8 were of total hip replacement and 8 of 

total knee replacement. Outcomes reported in the indirect comparisons included symptomatic 

venous thromboembolism (DVT or PE), clinically relevant bleeding (major bleeding or clinically 

relevant nonmajor bleeding), major bleeding, and a net clinical endpoint—defined as a composite 

of symptomatic VTE, major bleeding, and all-cause death. Overall, the primary trials were rated 

low risk of bias. Individual drug comparisons across these 4 outcomes (12 comparisons) showed 

only one statistically significant difference: rivaroxaban resulted in more clinically relevant 

bleeding compared with apixaban (RR 1.52; 95% CI, 1.19 to 1.95). The risk of major bleeding 

was also increased with rivaroxaban compared with apixaban (RR 1.59; CI, 0.84 to 3.02), but 

the difference was not statistically significant. Rivaroxaban was associated with lowest risk of 

symptomatic VTE compared with dabigatran (RR 0.68; CI, 0.21 to 2.23) and apixaban (RR 0.59; 

CI, 0.26 to 1.33), but neither comparison was statistically significant. Overall, differences in the 
29 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

number of VTE events were offset by the number of major bleeding episodes. Thus, there was no 

difference on the net clinical endpoint among apixaban, dabigatran, and rivaroxaban. The review 

concluded that “higher efficacy of new anticoagulants was generally associated with higher 

bleeding tendency. The new anticoagulants did not differ significantly for efficacy and safety.”23 

The other SR provided few additional relevant findings. Similar to the review described above, 

Loke et al.21 (search date May 2009) used indirect analysis methods but excluded studies of 

apixaban, yielding a less informative analysis. In addition, a dabigatran trial published after 2009 

and three rivaroxaban studies were excluded due to more restrictive eligibility criteria. Despite 

these differences, findings regarding rivaroxaban compared with dabigatran were generally 

similar. The authors concluded that rivaroxaban was superior to dabigatran in preventing VTE 

(RR 0.50; 95% CI, 0.37 to 0.68) although with an increased risk of bleeding (RR 1.14; CI, 0.80 

to 1.64). The decreased risk of VTE with rivaroxaban was consistent across different doses of 

dabigatran (150 mg vs. 220 mg), different dosing regimens of enoxaparin in the control groups 

(30 mg twice daily vs. 40 mg once daily), and the type of surgery (THR vs. TKR). 
30 

Comparative Effectiveness of New Oral Anticoagulants

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SUMMARY AND DISCUSSION 

We identified six good-quality SRs that evaluated thromboprophylaxis using newer oral 

anticoagulants versus LMWH. One SR evaluated additional drug classes, including 

unfractionated heparin, aspirin, and vitamin K antagonists. Although we identified no direct 

comparisons of newer oral anticoagulants, two good-quality SRs indirectly compared one newer 

oral anticoagulant with another through common comparison to enoxaparin.21,23 Only one SR 

compared combined pharmacologic and mechanical thromboprophylaxis to either method 

alone.9 FXa inhibitors have been studied more extensively than DTIs. In the absence of head-tohead comparisons between newer oral anticoagulants, it is difficult to draw strong conclusions 

on inter- or intra-drug class differences. The main findings and strength of evidence from our 

literature synthesis are summarized by key question in the section that follows. 

SUMMARY OF EVIDENCE BY KEY QUESTION 

KQ 1: Newer Oral Anticoagulants Versus Standard Treatments 

FXa inhibitors. Rivaroxaban and apixaban are the most commonly studied FXa inhibitors. 

The risk of symptomatic DVT was reduced with FXa inhibitors thromboprophylaxis compared 

with LMWH, while the risk of nonfatal PE and mortality was not significantly different (all 

high strength of evidence). The estimated absolute risk difference was 4 fewer symptomatic 

DVT events for each 1000 patients receiving thromboprophylaxis with FXa inhibitors over 5 

weeks compared with LMWH. However, these benefits were offset by an increase in major 

bleeding (moderate strength of evidence). The absolute risk difference was 2 more major 

bleeding events per 1000 patients on FXa thromboprophylaxis over a period of 5 weeks. Higher 

doses of FXa inhibitors but not intermediate or low doses were associated with increased major 

bleeding.20 Subgroup analysis by specific drug and type of surgery showed a reduced risk of 

bleeding with apixaban compared with LMWH in TKR but not in THR; risk of major bleeding 

with rivaroxaban did not differ significantly for either surgery.35 No reviews identified trials 

comparing oral FXa inhibitors with warfarin, UFH, or oral antiplatelet agents. 

Direct thrombin inhibitors. Dabigatran is the only FDA-approved oral DTI, and the only 

DTI evaluated in existing SRs. Compared with LMWH, dabigatran was not associated with 

significant differences for any outcome examined. The strength of evidence was low for 

most outcomes due to few events and imprecise estimates of effect; also, effects on mortality 

varied substantially across studies. In addition to the major outcomes, a subgroup analysis in 

one SR found no significant difference between both treatment groups on bleeding requiring 

rehospitalization.9

 No reviews identified trials comparing oral DTIs with warfarin, unfractionated 

heparin, or oral antiplatelet agents. 

Table 7 summarizes the findings and strength of evidence for the effects of newer oral 

anticoagulant drug classes compared with enoxaparin in patients undergoing THR or TKR 

surgery. 
ew Oral Anticoagulants for Thromboprophylaxis Evidence-based Synthesis Program

gth of evidence for KQ 1 

Domains Pertaining to SOE 

Effect Estimate (95% CI) SOE 

mber 

tudies 

bjects) 

Study Design/ 

Quality 

Consistency 

Directness 

Precision 

Publication Bias 

38) RCT/Good Consistent 

Direct 

Precise None detected 

OR=0.95 (0.55 to 1.63) RD=0 (2 fewer to 1 more) deaths/1000 patients 

High 

77) RCT/Good Consistent 

Direct 

Precise None detected 

OR=0.46 (0.30 to 0.70) RD=4 fewer (3 to 6 fewer) events/1000 patients 

High 

98) RCT/Good Consistent 

Direct 

Precise None detected 

OR=1.07 (0.65 to 1.73) RD=0 (1 fewer to 2 more) events/1000 patients 

High 

24) RCT/Good Inconsistent 

Direct 

Precise None detected 

OR=1.27 (0.98 to 1.65) RD=2 more (0 to 4 more) events/1000 patients 

Moderate 

0) RCT/Good Inconsistent 

Direct 

Imprecise None detected 

TKR RR=1.06 (0.36 to 3.12) 

RD=0 (2 fewer to 6 more) events/1000 patients 

THR RR=1.17 (0.04 to 36.52) RD=0 (3 fewer to 107 more) events/1000 

patients 

Low 

4) RCT/Good Consistent 

Direct 

Imprecise None detected 

RR=0.82 (0.17 to 3.99) RD=2 fewer (7 fewer to 27 more) events/1000 

patients 

Low 

4) RCT/Good Consistent 

Direct 

Imprecise None detected 

OR=0.69 (0.31 to 1.54) RD=1 fewer (2 fewer to 2 more) events/1000 

patients 

Low 

4) RCT/Good Consistent 

Direct 

Imprecise None detected 

RR=0.94 (0.58 to 1.52) RD=0 (3 fewer to 3 more) events/1000 patients 

Moderate 

otics 

NA NA NA Not estimable Insufficient 

nd Gómez-Outes, 2012; risk difference calculated; SOE ratings from Sobieraj, 2012.

may be some differences for hip versus knee replacement (different baseline risk and different duration of anticoagulation in existing studies); there is some 

oses increase risk of bleeding.

DVT=deep vein thrombosis; DTI=direct thrombin inhibitor; FXa=factor X inhibitor; LMWH=low molecular weight heparin; NA=not applicable; OR=odds 

andomized controlled trial; RD=risk difference; RR=risk ratio; SOE=strength of evidence; THR=total hip replacement; TKR=total knee replacement 

31
32 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

KQ 2: Combined Pharmacological and Mechanical Prophylaxis 

No reviews identified trials comparing newer oral anticoagulants combined with mechanical 

prophylaxis to pharmacological prophylaxis alone. Even when considering standard treatments, 

very little data are available comparing combined-modality thromboprophylaxis and 

pharmacologic prophylaxis only. One SR found moderate strength of evidence that combinedmodality thromboprophylaxis was associated with a decreased risk of overall DVT (including 

asymptomatic events) compared with pharmacologic prophylaxis only.9 The evidence was 

insufficient for all other outcomes. 

KQ 3: Comparisons of Individual Newer Oral Anticoagulants 

Only indirect comparisons of rivaroxaban, apixaban, and dabigatran were performed through 

common comparison with LMWH.23,35 These comparisons were made for only two of our major 

outcomes—symptomatic VTE (DVT or PE) and major bleeding. There were no significant 

differences in treatment effect for symptomatic VTE or major bleeding. Because these indirect 

comparisons are subject to confounding and the treatment effects were imprecise, we considered 

the strength of evidence low. Other outcomes reported included clinically relevant bleeding 

and net clinical endpoints. Rivaroxaban was found to be associated with an increased risk of 

clinically relevant bleeding, but there was no significant difference in net clinical endpoints 

(symptomatic VTE, major bleeding and death). Table 8 summarizes the findings and strength 

of evidence for between-drug comparisons of newer oral anticoagulants in patients undergoing 

THR or TKR. 
ew Oral Anticoagulants for Thromboprophylaxis Evidence-based Synthesis Program

gth of evidence for KQ 3 

Domains Pertaining to SOE 

Effect Estimate (95% CI) SOE 

er 

dies 

cts) 

Study Design/ 

Quality 

Consistency 

Directness 

Precision 

Publication Bias 

ana 

NA NA NA Outcome not reported Insufficient 

NA NA NA Outcome not reported Insufficient 

NA NA NA Outcome not reported Insufficient 

 RCT/Good NA 

Indirect 

Imprecise None detected 

Rivaroxaban vs. dabigatran RR=0.68 (0.21 to 2.23) 

RD=3 fewer (11 fewer to 4 more) events/1000 patients 

Rivaroxaban vs. apixaban RR=0.59 (0.26 to 1.33) RD=4 fewer (9 fewer to 1 more)/1000 patients Apixaban vs. dabigatran RR=1.16 (0.31 to 4.28) RD=1 more (7 fewer to 8 more)/1000 patients 

Low Low Low 

 RCT/Good NA 

Indirect 

Imprecise None detected 

Rivaroxaban vs. dabigatran RR=1.37 (0.21 to 2.23); 

RD=4 more (2 fewer to 11 more) events/1000 patients 

Rivaroxaban vs. apixaban RR=1.59 (0.84 to 3.02); RD=5 more (2 fewer to 12 more)/1000 patients Apixaban vs. dabigatran RR=1.16 (0.31 to 4.28); RD=0 (8 fewer to 7 more)/1000 patients 

Low Low Low 

osis; NA=not applicable; NR=not reported; PE=pulmonary embolism; RD=risk difference; RR=risk ratio; SOE=strength of evidence; VTE=venous thrombo

33
34 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

CLINICAL AND POLICY IMPLICATIONS
 

Patients undergoing total knee replacement or total hip replacement are at a significant 

risk for VTE. The precise risk for VTE in contemporary orthopedic surgery is difficult to 

estimate because of changes in surgical management, the paucity of recent trials comparing 

thromboprophylaxis to placebo, and the high frequency of thromboprophylaxis in routine 

practice, making observational studies of natural history difficult to conduct. A recent, 

careful analysis estimated the prevalence of symptomatic VTE without thromboprophylaxis 

at 2.8 percent for the initial 14 days and 4.3 percent at 35 days following major orthopedic 

surgery.1 Appropriate use of perioperative thromboprophylaxis significantly reduces the risk of 

postoperative proximal VTE, but the evidence is much more limited for effects on symptomatic 

DVT, PE, and mortality.1,9 

Our evidence synthesis primarily addresses the comparative effectiveness of newer oral 

anticoagulants compared with standard antithrombotic agents for VTE prophylaxis. These newer 

drugs have been compared only to LMWH and show similar effects on most major clinical 

outcomes, although the strength of evidence varies by drug class and specific drug. In evaluating 

whether to add these newer agents to the VA formulary and whether to promote a specific 

thromboprophylaxis strategy, consideration should be given to the evidence of effectiveness 

and the importance and variability of patient values and preferences, costs, and health care 

system resources for successfully implementing competing strategies. In the following section, 

we summarize recommendations from the two major U.S. clinical guideline panels that have 

addressed this issue. 

Guidelines 

Both the American College of Chest Physicians (ACCP) and the American Academy of 

Orthopaedic Surgeons (AAOS) have recently issued guidelines on thromboprophylaxis in 

patients undergoing TKR or THR.1,2,24 The ACCP recommends antithrombotic prophylaxis over 

no prophylaxis for patients undergoing TKR or THR. The AAOS guidelines suggest individual 

assessment of patients for thromboprophylaxis. For patients at average risk, the guidelines do 

not include a recommendation for a specific thromboprophylactic strategy, considering the 

evidence for comparative effectiveness to be inconclusive. In contrast, the ACCP guidelines make 

recommendations for specific strategies and include the following options: LMWH, fondaparinux, 

apixaban, dabigatran, rivaroxaban, low-dose unfractionated heparin, adjusted-dose vitamin K 

antagonists, aspirin, or an intermittent pneumatic compression device. However, in the absence of 

elevated bleeding risk, LMWH is recommended in preference to other agents. Factors identified as 

increasing the risk of bleeding include previous major bleeding, severe renal failure, concomitant 

antiplatelet use, and a history of or difficult-to-control surgical bleeding during the current operative 

procedure, extensive surgical dissection, and revision surgery.1

 For patients with increased bleeding 

risk, ACCP recommends intermittent pneumatic compression device or no prophylaxis. 

In making recommendations, both guideline panels considered benefits and potential harms but 

had different approaches to considering costs. The AAOS did not conduct cost analyses and 

instructed guideline members to consider costs only when the impact was likely to be substantial. 

The ACCP process considered costs when it was plausible that resource use might change 
35 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

	 	 	 	 	 	 	 	

the direction or strength of recommendation and when high-quality economic analyses were 

available; it is not stated whether costs were considered in the recommendations pertaining to 

major orthopedic surgery. Guideline recommendations regarding choice of thromboprophylaxis 

are summarized in Table 9. Finally, we note that these guidelines address other clinical 

management issues, including duration and timing of thromboprophylaxis and use of routine 

DVT screening, which are not summarized here since they are not directly germane to our key 

questions. 

Table 9. U.S. guideline recommendations related to specific thromboprophylaxis strategies

ACCP 2012a AAOS 2011b 

In patients undergoing THR or TKR, we recommend We suggest the use of pharmacologic agents and/or 

use of one of the following for minimum of 10–14 days mechanical compressive devices for the prevention 

rather than no antithrombotic prophylaxis: LMWH, of VTE in patients undergoing elective hip or knee 

fondaparinux, apixaban, dabigatran, rivaroxaban low- arthroplasty, and who are not at elevated risk beyond 

dose unfractionated heparin (LDUH), adjusted-dose that of the surgery itself for venous thromboembolism 

VKA, aspirin (all Grade 1B) or an intermittent pneumatic or bleeding. 

compression device (IPCD) (Grade 1C). Grade: Moderate 

In patients undergoing THR or TKR, irrespective of the Current evidence is unclear about which prophylactic 

concomitant use of an IPCD or length of treatment, strategy is optimal or suboptimal. Therefore, we 

we suggest the use of LMWH in preference to the are unable to recommend for or against specific 

other agents we have recommended as alternatives: prophylactics in these patients. 

fondaparinux, apixaban, dabigatran, rivaroxaban lowdose unfractionated heparin (LDUH) (all Grade 1B), Grade: Inconclusive 

adjusted-dose VKA, aspirin (all Grade 2C). 

In patients undergoing major orthopedic surgery, we In the absence of reliable evidence, patients who have 

suggest using dual prophylaxis with an antithrombotic had a previous VTE should receive pharmacologic 

agent and an IPCD during the hospital stay (Grade 2C). prophylaxis and mechanical compressive devices. 

Grade: Consensus 

In patients undergoing major orthopedic surgery and 

increased risk of bleeding, we suggest using an IPCD 

or no prophylaxis rather than pharmacologic treatment 

(Grade 2C). 

In the absence of reliable evidence, patients with a 

known bleeding disorder and/or active liver disease 

should use mechanical compressive devices for 

preventing VTE. 

Grade: Consensus 

In patients undergoing major orthopedic surgery and In the absence of reliable evidence about how long to 

who decline or are uncooperative with injections or an employ these prophylactic strategies, it is the opinion 

IPCD, we recommend using apixaban or dabigatran of this work group that patients discuss the duration of 

(alternatively, rivaroxaban or adjusted-dose VKA if prophylaxis with their treating physicians. 

apixaban or dabigatran are unavailable) rather than 

other forms of prophylaxis (Grade 1B). Grade: Consensus 

a

ACCP evidence grading of evidence was as follows: grade 1 recommendations are strong and indicate that the benefits do or 

do not outweigh risks, burden, and costs, while grade 2 suggestions imply that individual patient values may lead to different 

choices. Furthermore, level A indicates consistent results from RCTs or observational studies with very strong association and 

secure generalization (high), B indicates inconsistent results from RCTs or RCTs with methodological limitations (moderate), C 

indicates unbiased observational studies (low), and D indicates other observational studies (e.g. case series) (very low). 

b

AAOS grading was as follows: strong when good-quality evidence, moderate when fair-quality evidence, weak when poorquality evidence, inconclusive when insufficient or conflicting evidence, or consensus in the absence of reliable evidence. 

Abbreviations: IPCD=intermittent pneumatic compression device; LDUH=low-dose unfractionated heparin; LMWH=low 

molecular weight heparin; THR=total hip replacement; TKR=total knee replacement; VKA=vitamin K antagonist; VTE=venous 

thromboembolism 
36 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

APPLICABILITY
 

We think these results are likely to apply to Veteran populations, but recommend some caution 

when applying trial data to standard clinical practice. There were strict exclusion criteria for 

patients enrolled in these studies, including severe renal or hepatic impairment or high risk of 

bleeding. Patients enrolled in trials are often more adherent to treatment plans and are monitored 

more closely than patients in routine clinical care. As a result, treatment effects in standard 

clinical practice may differ from those observed in clinical trials. Furthermore, the definition of 

bleeding was not consistent across studies, and it did not always include surgical site bleeding, 

which can lead to infection, dehiscence, and reoperation. Another limitation is the treatments 

compared. Newer anticoagulants were compared exclusively with LMWH, an appropriate 

and widely used comparator, but there were no direct comparisons to other treatment options 

recommended by guideline panels. Finally, none of these studies included patients from the VA 

health care system. Compared to patients with private sector insurance, VA patients on average 

have a greater burden of chronic disease, which would likely increase bleeding risk. If the 

comparative treatment effects vary by the presence of certain comorbid conditions, these results 

may not be reproducible in VA settings. 

STRENGTHS AND LIMITATIONS 

Our study has a number of strengths, including a protocol-driven review, a comprehensive 

search, and careful quality assessment. Another strength is the opportunity for meta-synthesis 

from existing systematic reviews and the opportunity to carefully evaluate reasons for different 

findings or conclusions across published reviews. Limitations include the lack of head-to-head 

comparisons of the newer oral anticoagulants, which precludes strong conclusions on their 

comparative effectiveness. Further, the length of experience with these new anticoagulants is 

too short to allow identification of longer term adverse events that may only emerge with more 

widespread use. 

RECOMMENDATIONS FOR FUTURE RESEARCH 

We used the framework recommended by Robinson et al.42 to identify gaps in evidence and 

classify why these gaps exist (Table 10). 

This approach considers PICOTS (population, intervention, comparator, outcomes, timing, and 

setting) to identify gaps and classifies them as due to: (1) insufficient or imprecise information, 

(2) biased information, (3) inconsistency or unknown consistency, and (4) not the right 

information. VA and other health care systems should consider their clinical and policy needs 

when deciding whether to invest in research to address gaps in evidence. Specific research 

questions can be evaluated quantitatively, using value-of-information analysis, which uses 

Bayesian methods to estimate the potential benefits of gathering further information through 

research.43 
37 

Comparative Effectiveness of New Oral Anticoagulants

for Thromboprophylaxis Evidence-based Synthesis Program

Table 10. Evidence gaps and future research 

Evidence Gap Reason Type of Studies to Consider 

Absence of direct comparisons between 

newer anticoagulant drugs 

Insufficient information Multicenter RCTs 

High-quality network meta-analyses 

Observational comparative 

effectiveness studies 

Absence of direct comparisons between 

newer anticoagulants and agents other 

than LMWH 

Insufficient information Multicenter RCTs 

Observational comparative 

effectiveness studies 

Absence of comparisons between 

combined treatment with newer 

anticoagulants and mechanical 

thromboprophylaxis to pharmacological 

thromboprophylaxis alone 

Insufficient information Multicenter RCTs 

Observational comparative 

effectiveness studies 

Adverse effects with long-term use and 

in usual clinical practice 

Insufficient information Observational studies 

Abbreviations: LMWH=low molecular weight heparin; RCT=randomized controlled trial 

CONCLUSION 

For THR or TKR, the 35-day rate of symptomatic VTE without thromboprophylaxis is estimated 

to be 4.3 percent. Pharmacological thromboprophylaxis decreases VTE by approximately 50 

percent but with the tradeoff of increased bleeding. Newer oral anticoagulants have a more 

convenient route of administration compared with LMWH, and unlike adjusted dose warfarin, 

they do not require regular laboratory monitoring. Compared with LMWH, FXa inhibitors 

are associated with a reduced risk of symptomatic DVT, but mortality and nonfatal PE are not 

significantly different, and the risk of major bleeding episodes is increased. 

There are no available studies on head-to-head comparisons of these novel anticoagulants. 

Longer clinical experience and direct drug-drug comparisons are needed to better assess the 

risk-to-benefit ratio of newer oral anticoagulants for surgical thromboprophylaxis. Based on 

current evidence, newer anticoagulants—particularly FXa inhibitors—are a reasonable option for 

thromboprophylaxis in patients undergoing total hip replacement or total knee replacement. 
38 

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for Thromboprophylaxis Evidence-based Synthesis Program

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