Chapter 23. Patient Acuity 

Bonnie M. Jennings 

Background 

For more than 50 years, researchers have worked to develop staffing methodologies to 

accurately indicate the number of nurses needed to give good care to patients.1

 By the 1980s, 

patient classification systems (PCSs) were in common use to predict patient requirements for 

nursing care. These requirements, or patient acuity, could then be used to manage nursing 

personnel resources, costs, and quality.2, 3 

PCSs have numerous limitations, however. Paramount among these are (a) validity and 

reliability are infrequently monitored;4, 5 (b) the tools are often complex and require considerable 

time to complete;4

 (c) they lack credibility among staff nurses and administrators;5, 6 (d) they are 

not designed to detect census variability throughout the day from patient movement due to 

admissions, discharges, transfers, and short-stays;7, 8 and (e) their focus on tasks shortchanges the 

cognitive work and knowledge inherent to expert nursing care and sophisticated surveillance.9, 10 

As restructuring and mergers escalated in the 1990s, issues of patient acuity once again 

moved to the foreground. Patients were said to be sicker and leaving health care facilities more 

quickly. Concerns about rising patient acuity continue into the new millennium because of the 

relentless change that is now common in health care. Moreover, acuity is one of many elements 

that comprise the often used but not yet well specified concept of workload.11, 12 

Research Evidence 

In assessing the research conducted between 1995 and 2005 about patient acuity, three things 

stand out. First, most of the research reports are about developing or comparing instruments to 

measure patient acuity. Unlike early PCSs that were designed for medical-surgical patients in 

acute care facilities, these instruments are tapping into other care settings such as long-term 

care,13–17 home care,18, 19 emergency departments (EDs),20–28 and neurological rehabilitation 

centers,29–33 to name but a few. There is little evidence, however, regarding the extent to which 

these tools are being used. 

Second, most reports simply mention that patient acuity is increasing without supporting 

data. Only four studies actually examined trends in patient acuity to empirically substantiate 

perceptions that acuity is rising. Interestingly, these investigations were all conducted outside the 

United States. PCS scores were compared over 3 months in 1996 and the same period in 1999 for 

critical care patients in one Australian hospital.34 Acuity varied by shift (day, evening, night), 

with the evening shift demonstrating the highest patient acuity. Although the PCS scores 

followed similar patterns in 1996 and 1999, the PCS scores were higher for all shifts in 1999. 

Monthly PCS data from 17 units in a Swedish hospital indicated that average scores in each 

of four acuity categories increased from 1995 to 1996.35 The investigators concluded that 

patients were sicker and their treatments more time consuming. However, they also demonstrated 

discrepancies between actual and required staff, with the actual staff consistently lower than 

required. This gap has also been observed in U.S. hospitals.6

 In a Canadian study from Ontario, 

case-mix data were examined for all acute care hospitals from 1997 to 2002.36 After controlling 

1 
Patient Safety and Quality: An Evidence-Based Handbook for Nurses

for age, it was evident that the average inpatient case-mix index (CMI) increased by 17 percent 

over the 5 years of data that were examined. The least complex patients declined by 24 percent, 

and the most complex patients increased by 144 percent, representing an overall increase of 211 

percent for the most complex patients. The fourth study examined care needs for long-term-care 

(LTC) residents in Alberta, Canada, between 1988 and 1999.37 The data demonstrated an 

increase in residents needing greater help with activities of daily living and more intervention for 

difficult behaviors such as dementia. 

Finally, studies were rarely designed to assess patient acuity in relation to patient outcomes. 

Of those shown in Table 1, three evaluated heterogeneous groupings of patients in acute care 

settings.38–40 An additional three studies examined acuity in more homogeneous patient 

populations. One study focused exclusively on critical care patients,41 and another considered 

only obstetrical care for teenagers.42 Acuity was also examined in relation to patient outcomes in 

the ED.43 

Evidence-Based Practice Implications 

There is little empirical evidence about the relationship between acuity and patient safety, 

making the practice implications from these studies modest. Although three studies showed a 

positive association between acuity and adult mortality,38, 40, 41 findings were more equivocal for 

the relationship between acuity and neonatal mortality rates.42 This latter study illustrated that 

factors other than acuity were more predictive of outcomes, particularly weekend births and 

ethnicity or race. The investigators who studied critical care patients concluded that variations in 

mortality might be partially explained by excess workload.40

Findings from the studies involving a variety of inpatients were not consistent. As expected, 

the two studies using the same dataset38, 40 both showed similar results—a positive relationship 

between acuity and adverse outcomes such as infections and decubiti, but not medication errors 

and falls. The third study was conducted on 32 units in a different hospital.39 Data were collected 

for a full year. Although the association between hours of nursing per patient day was 

statistically significant (r = .60; P <.05), the relationship between acuity and adverse outcomes 

was not examined. Rather, acuity was a significant predictor of various self-care measures such 

as symptom management. 

The ED study assessed patient satisfaction as the outcome measure.43 Although this work did 

not provide evidence about outcomes related to patient safety, it did illustrate how patient 

perceptions come into play regarding features of care delivery. Patients whose acuity placed 

them in the ‘emergent’ category were more satisfied with their care than patients in either the 

‘urgent’ or ‘routine’ acuity groups. However, when perceived throughput time was controlled, 

acuity did not predict satisfaction with ED care. The importance of patient perceptions was 

clearly in effect in determining satisfaction. 

Research Implications 

At present, very little is known about the relationship between acuity and outcomes. The lack 

of a standardized approach to measuring acuity has broad research implications. For 

investigations using PCSs, reports need to include information about the psychometric properties 

of the tools. It would also be helpful to examine the relationship of PCS acuity to clinical 

outcomes using more homogeneous patient groupings. 

2
Patient Acuity

Perhaps the most important research issues concern greater clarity about the larger concept—

workload. There is an urgent need to develop a conceptual model illustrating the relationships of 

the various elements comprising workload as well as a standardized definition of workload. 

Empirical testing of the model might then better elucidate how acuity, as one aspect of workload, 

relates to patient safety. 

It would also be very helpful if U.S. studies were conducted to ascertain whether the 

perceptions of increased acuity are verifiable. It would be most beneficial if these studies looked 

not just at acuity in the aggregate, but also at acuity for homogeneous patient populations. This 

could help clarify whether acuity for medical-surgical patients has escalated. Finally, it would be 

useful to have a sense of acuity in the outpatient setting, given how patient care has shifted. 

Although outpatient acuity is particularly difficult to capture, it remains a research challenge for 

the future. 

Conclusion 

Patient acuity is a concept that is very important to patient safety. Presumably, as acuity rises, 

more nursing resources are needed to provide safe care. Very little research has actually been 

conducted, however, to verify this premise. Moreover, findings from the research that has been 

conducted are largely inconsistent. Design issues account for these differences. In addition, it is 

possible that factors other than patient acuity may contribute more to patient outcomes. It 

remains important to derive a much better grasp of the relationship between patient acuity, 

outcomes, and patient safety. At present, little can be said with confidence about this association. 

Search Strategy 

The literature for this review was identified by searching the MEDLINE® and CINAHL®

databases from 1995 to 2005 for research-based articles published in the English language. A 

reference librarian assisted in choosing the search terms. In the CINAHL® search, the terms were 

“patient acuity” or “patient classification.” This yielded 345 citations. The MEDLINE® search 

was tried four times using various combinations of terms such as “patient acuity,” “patient 

classification,” “severity of illness index,” “acute disease classification” and “diagnosis related 

groups.” The combined efforts of the four searches resulted in identifying 98 references. 

The abstracts for all 443 citations were reviewed. Of these, 104 were considered to be 

potential candidates for use in this review. The references that were excluded from this 

assessment included a wide array of topics that were irrelevant to patient acuity. The diversity of 

these articles is too great to provide a complete view of them, but a few examples include quality 

of life, menstrual cycle abnormalities, blood pressure variability, and fever management for 

children. 

After reading the 104 candidate articles in their entirety, an additional 72 papers were 

omitted from the remainder of the analysis. Papers were excluded because they were more 

tangentially related to patient acuity (e.g., indicators of patient dependency), they were reviews 

of literature, or they did not focus on patients per se (e.g., a way to classify school-age children 

with disabilities). As a result, this review was based on findings from 32 research reports. 

3 
Patient Safety and Quality: An Evidence-Based Handbook for Nurses

Author Affiliation 

Bonnie M. Jennings, D.N.Sc., R.N., F.A.A.N., colonel, U.S. Army (Retired), and health care 

consultant; e-mail: bmjennings@cox.net. 

Acknowledgment 

Tremendous gratitude is expressed to the staff of the Armed Forces Medical Library, Falls 

Church, VA, for their considerable support of this work. They conducted the database searches 

and assisted in acquiring numerous papers considered in this review. 

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21. Gorelick MH, Lee C, Cronan K, et al. Pediatric 

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5 
Table 1. Evidence on Patient Acuity 

Source Safety Issue Related to Clinical Practice Design Type Study Design, Study Outcome Measure(s) Study Setting & Study Population Study IntervenBlegen 199838 Inpatient outcomes (acuity was measured by the monthly reports derived from the hospital’s acuity system) Cross-sectional Design: Level 3 Patient outcomes: deaths, rates of falls, medication errors, decubiti, urinary tract and respiratory infections (Level 1), complaints 42 inpatient units in an 880-bed hospital during FY 1993 (prior to restructuring); 21,783 discharged patients; 1,074 full- time equivalent nursing staff members, 832 of whom were RNs Boudreaux 200443 ED patients (acuity was measured by triage categories assigned by trained nurses—emergent, urgent, routine) Cross-sectional Design: Level 3 Patient outcomes: satisfaction (Level 4) 1,865 patients over 1 month at a large inner-city hospital ED Patients: average age 30 years; 53% female 

6
Source Safety Issue 

Related to Clinical Practice

Design Type

Study Design, Study Outcome Measure(s)

Study Setting & Study Population 

Study Interven

7

Hamilton and Restrepo 200342

Neonatal deaths preventable in the perinatal period (acuity was measured using maternal medical risk factors such as hypertension, anemia, previous birth of an infant who was preterm or small for gestational age) 

Cross-sectional Design: Level 3 

Patient outcomes: mortality (Level 1) 

All births to teenage 

mothers (< 20 years of age) in Texas in 1999 and 2000 (N = 11,749) with a focus on neonatal deaths (prior to the 28th day 

of life) (n = 397); mean neonatal mortality/1,000 live births = 3.6; Hispanic (56%), White (27%), African American (15%), other (2%) 

Potter 200339 Patient outcomes 

(after adjusting for acuity) 

Cross-sectional Design: Level 3 

Patient outcomes: fall and medication 

error index (Level 1); self-reports of symptom management, self-care, health status; patient satisfaction 

32 inpatient units in one hospital from 2000 to 2001; 3,418 patients 
8

Source Safety Issue Related to Clinical Practice Design Type Study Design, Study Outcome Measure(s) Study Setting & Study Population Study IntervenReed 199840 Adverse occurrences among inpatients (acuity was measured by an institutionally standardized measure; average daily acuity/month was used) Cross-sectional Design: Level 3 Patient outcomes: deaths, pressure ulcers, nosocomial infections, medication administration errors (Level 1), complaints 42 inpatient units in an 880-bed hospital during FY 1993 (prior to restructuring). This was a secondary data analysis of a mix of units: 5 surgical; 10 medical; 3 obstetrical; 8 pediatric; 4 critical care; 4 psychiatric; 2 eye, ear, nose; 6 orthopedic and neuroscience. Tarnow-Mordi 200041 Staff workload (after adjusting for risk using the APACHE II; workload was defined by average nursing 

requirement per occupied bed and peak occupancy) 

Prospective cohort Design: Level 3 

Patient outcome: 

hospital mortality (Level 1) 

1,050 patients 

admitted to an adult ICU in Scotland between January 1, 1992, and December 31, 1995. Patients: Age, 16 to >70, with 43% of the 

patients in the > 70 

age group58% male