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Hughes RG, editor. Patient Safety and Quality: An Evidence-Based Handbook for Nurses. Rockville (MD): Agency for Healthcare Research and Quality (US); 2008 Apr.

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

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Chapter 23Patient Acuity


Bonnie M. Jennings, D.N.Sc., R.N., F.A.A.N., colonel, U.S. Army (Retired), and health care consultant; e-mail: ten.xoc@sgninnejmb


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 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

Table 1

Table 1

Evidence on Patient Acuity

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.

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.


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.


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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