The need for accurate Ddimer reporting in COVID19: Communication from the ISTH SSC on fibrinolysis



## | introduc ti on
The coronavirus disease 2019 (COVID-19) pandemic continues to claim many lives across the world. In the attempts to identify a reliable prognostic indicator, marked elevation of D-dimer has been a strong contender. [bib_ref] Abnormal coagulation parameters are associated with poor prognosis in patients with novel..., Tang [/bib_ref] [bib_ref] Anticoagulant treatment is associated with decreased mortality in severe coronavirus disease 2019..., Tang [/bib_ref] In many studies, D-dimers have consistently been shown to be the most significant marker for illness severity and death risk prediction. [bib_ref] D-dimer levels on admission to predict in-hospital mortality in patients with Covid-19, Zhang [/bib_ref] [bib_ref] Acute pulmonary embolism in COVID-19 patients on CT angiography and relationship to..., Leonard-Lorant [/bib_ref] Despite the usefulness of this fibrinolytic marker, along with a recent letter by Gris et al, [bib_ref] Uncertainties on the prognostic value of D-dimers in COVID-19 patients, Gris [/bib_ref] we note several problems across the medical literature with D-dimer reporting creating confusion and potentially misleading data interpretation.
Since the arrival of the first monoclonal antibody-based assays in the 1980s, D-dimer measurements have consistently proven to be a reliable diagnostic tool. [bib_ref] Characterisation of a soluble D dimer-E complex in crosslinked fibrin digests, Gaffney [/bib_ref] Recent heightened awareness among the health care professionals and public about the risk of venous thromboembolism and disseminated intravascular coagulation, coupled with ease of use and accessibility of the D-dimer tests led to its increased popularity. The increased demand has led to proliferation of commercially available assays, but the manufacturers, and consequently workers, publishing their work have not been consistent with the reporting mechanisms of laboratory data, thus leading to confusion and causing outright errors. These issues have been recurring themes in the D-dimer saga. In over two dozen COVID-19 related papers published involving D-dimer levels this year, we noted the following problems:
- Most failed to identify the manufacturer or type of D-dimer assay used - Most did not clearly report the analytical performance of the assay (ie, variations in sensitivity, specificity, and linearity of the There are also issues with the specificity and sensitivity of the D-dimer assay based on the diagnostic purpose. A meta-analysis including 97 studies of patients with suspected deep vein thrombosis (DVT) reported an overall estimated sensitivity and specificity of D-dimer of 90.5% and 54.7%, respectively, but both estimates were subject to significant heterogeneity. [bib_ref] Variation in the diagnostic performance of D-dimer for suspected deep vein thrombosis, Goodacre [/bib_ref] Sensitivity and specificity also varied across the different types of tests and the clinical probability for DVT. [bib_ref] Variation in the diagnostic performance of D-dimer for suspected deep vein thrombosis, Goodacre [/bib_ref] For the diagnosis of DIC, D-dimer quantification is more crucial, where the variability can pose significant issues. [bib_ref] Harmonisation of D-dimera call for action, Longstaff [/bib_ref] FDPs can also affect D-dimer causing over-or underestimation of D-dimer. [bib_ref] D-dimer: standardization versus harmonization, Dempfle [/bib_ref] The poor specificity of D-dimer tests leads to high rates of false positive results but reliable negative test results, and explains why D-dimer screening is commonly used to exclude DVT.

## | in con s is ten cie s of units
D-dimer can be reported as fibrinogen equivalent units (FEU) or D-dimer units (DDU). [bib_ref] D-dimer: an overview of hemostasis and fibrinolysis, assays, and clinical applications, Olson [/bib_ref] One FEU compares the mass of the D-dimer to that of fibrinogen with a calibrator prepared from plasmin degradation of purified fibrinogen. [bib_ref] D-dimer: simple test, tough problems, Olson [/bib_ref] [bib_ref] Plasma D-dimer and venous thromboembolic disease, Righini [/bib_ref] DDU is an estimated mass of the D-dimer unit with purified D-dimer used as the calibrator. [bib_ref] D-dimer: an overview of hemostasis and fibrinolysis, assays, and clinical applications, Olson [/bib_ref] [bib_ref] Plasma D-dimer and venous thromboembolic disease, Righini [/bib_ref] FEU is approximately two-fold higher than that of DDU. If laboratory personnel or clinicians are not aware of this distinction, results interpretation can be inaccurate. Olson et al performed a survey among several US laboratories and noted that almost one-third of the laboratories changed the units from that recommended by the manufacturer. [bib_ref] D-dimer: simple test, tough problems, Olson [/bib_ref] Another web survey revealed that 28 different combinations of measure units are currently used for reporting D-dimer test results. [bib_ref] International survey on D-dimer test reporting: a call for standardization, Lippi [/bib_ref] Second, there is tremendous variability in the magnitude of units reported. Different publications use ng/mL, g/mL, mg/L, and g/L to report D-dimer results, which can cause considerable confusion among non-laboratory health care personnel. This heterogeneity in the reporting units was identified in the web survey, in which one-third each was using "ng/mL" and "mg/L," whereas 18% used "g/L". 21

## | the prob lems with cutoffs

## | the need for harmonisati on than s tandardiz ation
Prothrombin time used in patients receiving vitamin K antagonists was in similar turmoil as D-dimer. Collaborative efforts in this area led to development and widespread use of the International Normalized Ratio. Similarly, our group has been investigating potential ways to generate standardization of D-dimer to help address current issues.
The presence of different D-dimer fragments and patient characteristics makes preparation of a "universal standard" not easy and straighfoward. [bib_ref] Harmonisation of D-dimera call for action, Longstaff [/bib_ref] [bib_ref] D-dimer: standardization versus harmonization, Dempfle [/bib_ref] [bib_ref] Standardization of D-dimer testing, Reber [/bib_ref] Harmonization rather than standardization of test results may be a possible solution to this conundrum. [bib_ref] D-dimer: standardization versus harmonization, Dempfle [/bib_ref] [bib_ref] A reference material for harmonisation of D-dimer assays. Fibrinogen subcommittee of the..., Nieuwenhuizen [/bib_ref] Harmonization would involve conversion of D-dimer values from different assays to a common scale, by applying a validated conversion factor. [bib_ref] A model for the harmonisation of test results of different quantitative D-dimer..., Meijer [/bib_ref] Major commitment of manufacturers is required to investigate kit performance in COVID-19 patients and develop new or updated methods if appropriate.

## | p otential utilit y in monitoring covid -19 patients
Consistent elevation in D-dimer in all hospitalized patients with COVID-19 has led some clinicians to use this marker to decide on low-intensity or high-intensity anticoagulation based on fold increase in D-dimers. [bib_ref] Diagnosis, prevention, and treatment of thromboembolic complications in COVID-19: report of the..., Oudkerk [/bib_ref] This approach is probably based on the presumption that all the D-dimers are coming from clot breakdown, which may not be the case in all patients. For the purpose of this communication, it is premature to confirm this is a safe strategy without evidence from randomized trials. Another clinical strategy is intensification of anticoagulation in patients who have increase in D-dimers despite prophylactic anticoagulation, which again is presumptive and not yet proved to be a safe approach.Last, some studies have already shown than decrease in D-dimers may signify the patient is improving and could mean downgrading the anticoagulation intensity. [bib_ref] The procoagulant pattern of patients with COVID-19 acute respiratory distress syndrome, Ranucci [/bib_ref] This is certainly a novel use for D-dimer measurements but requires serial monitoring with an accurate method. Currently, there is no evidence to prove that findings with one D-dimer kit will necessarily translate to other D-dimer kits, and harmonization will expectedly improve comparability.

## | recommendations
We can hence summarize here a set of indications that we recommend be used when reporting data on D-dimer testing, with special focus on studies in COVID-19, where D-dimer may evenly influence the clinical decision making.
- The type of the of the D-dimer assay (name and manufacturer) must always be clearly reported - The minimal analytical performance of the assay (including at least the functional sensitivity, total imprecision, linearity, and potential interference from FDPs) should be described
- A standardized measuring unit should be used for reporting data (FEU, as either "g/L" or "mg/L")
- The cutoff value used in the study should be clearly indicated - The statistical analysis should be appropriately selected according to sample size and value distribution (normal or not)

## Co n fli c t o f i nte r e s t
All authors have no conflict of interest to report.

## Auth o r co ntr i b uti o n s
Dr. Thachil wrote the manuscript and Drs. Lonstaff, Favaloro, Lippi, Urano, and Kim participated in discussion and critical editing of the manuscript.

## O rci d

## Emmanuel j. favaloro
https://orcid.org/0000-0002-2103-1661
Paul Y. Kim https://orcid.org/0000-0002-0504-3064

## T wit ter

## Tetsumei urano @tetsuurano
Paul Y. Kim @kimpy79

## R e fe r e n c e s
[fig] D: -dimer is commonly used to exclude venous thromboembolism in patients with low clinical probability. A threshold or cutoff value is important for application of this exclusion. Once again, the -dimer cutoff level is dependent on the different assay methods and calibrators.18 -dimer users should be aware that cutoff values are not transferable between methods and even between institutions. The assay should ideally be validated in prospective studies or at least compared with already validated assays.19 The British Committee for Standards in Haematology guidelines state that testing a minimum of 200 subjects should be done before local approval of a -dimer assay,22 although this may be difficult to achieve in all laboratories. [/fig]