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CHAPTER 13: Assessment of Severity of Illness 91

in individual patients, in order to determine appropriate utilization of rates of critically ill children were higher in the United Kingdom than
intensive care facilities and predict staffing requirements. TISS quan- in Australia. 57
tifies the amount of critical care provided to patients by measuring Severity-of-illness scoring systems can also be used to assess ICU
76 nursing activities, monitoring techniques, resuscitation procedures, performance in different models of organization. For example, Carson
and technology. Each intervention is given 1 to 4 points. Therefore, TISS and coworkers evaluated the effects of changing from an “open” to a
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assesses severity of illness indirectly by the level of services provided to “closed” model of ICU care by dedicated intensivists by using a “before/
the patient (as opposed to measuring physiology and organ function). after” study design. Patient severity of illness as assessed by APACHE
TISS was designed as a descriptor of the intensity of care, and was not II was greater, yet care costs were similar, and the ratio of actual to pre-
designed specifically to predict outcome. dicted mortality was lower after converting a medical ICU from open to
TISS scores have been used to categorize the level of care that patients closed care. Similar studies involving patients with sepsis demonstrated
require. 49,50 Beck and coworkers used TISS scores at ICU discharge as that changing ICU staffing to include physicians formally trained in
an objective assessment of the risk of premature discharge, and inves- critical care medicine reduced mortality. 59,60 Other examples of the
tigated the relationships of discharge time, TISS scores, and discharge use of scoring systems to assess ICU performance include studies of
destination on post-ICU mortality. There was a significant association availability of ICU technology and studies of organizational practices
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between increasing TISS scores and post-ICU mortality at ICU dis- and outcomes. 61
charge (χ for trend = 0.90, p = 0.028). Patients with high TISS scores Rapoport and coworkers described a method to assess cost-
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(>30) who were treated in hospital wards had significantly increased effectiveness of ICUs. A clinical performance index was defined as
severity-adjusted mortality risks compared with a comparable group of the difference between actual and MPM II predicted mortality. The
patients who were discharged to high-dependency units. economic performance (resource use) used a surrogate for costs: the
In addition, acuity of care can be correlated with indices of resource “weighted hospital days,” a length-of-stay index that weights ICU days
utilization. Furthermore, reimbursement can be guided by assessment more heavily than non-ICU days. Predicted resource use was calculated
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of severity of illness. For example, planning for ICU bed allocation, by a regression including severity of illness and percentage of surgical
staffing, and budget can be aided by measures of admission numbers, patients. The actual and predicted survival and actual and predicted
diagnoses (eg, diagnosis-related groups [DRGs] and case-mix groups resource use of hospitals were compared with the mean. A scatterplot
[CMGs]), and severity of illness. illustrated which units were more than one standard deviation off for
■ SCORING SYSTEMS TO ASSESS INTENSIVE CARE UNIT PERFORMANCE clinical and economic performance.
The cost-effectiveness of ICUs should include nonmortality measures
Scoring systems can be used by ICUs to evaluate quality of care (quality of effectiveness such as quality of life, return to independent living, and
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assurance; see Chapters 2, 3), to assess performance of an ICU over time, patient/family satisfaction. These nonmortality measures of outcome
to assess performance of different intensivists, and to assess perfor- need to be adjusted for ICU severity of illness by using severity-of-illness
mances of different ICUs (see Table 13-1). The scoring systems provide scoring systems.
samples of patients. Although quality assurance has largely been sup- ■ SCORING SYSTEMS TO ASSESS INDIVIDUAL
a tool to normalize for differences in severity of illness of different
planted by newer approaches such as continuous quality improvement, PATIENT PROGNOSIS AND TO GUIDE CARE
severity-of-illness scoring systems nonetheless can be used to assess The assessment of individual patient prognosis is complex and remains
predicted and actual mortality. ICUs can review the outcomes of controversial. Moreover, the use of severity-of-illness scoring systems
patients in general, or for specific disease categories, and compare the for assessment and prediction of individual patient prognosis is often
actual outcomes with predicted mortality. The performance of an ICU inaccurate. We believe that management decisions cannot be based
can also be followed over time. Evaluation of new technologies or new solely on prognosis as evaluated by the scoring systems. Assessment
treatment modalities in an ICU can also be the object of continuous of individual patient prognosis influences decisions regarding triage of
quality improvement evaluations. patients (ie, ICU admission), intensity of care, and decisions to withhold
There are potential problems associated with the use of scoring and withdraw care.
systems to compare actual with expected mortality in an ICU. For Theoretically, a very accurate estimate of patient prognosis could
example, biases in the regression techniques used to calculate the risks be used to triage patients who have such a good prognosis that ICU
of mortality can lead to situations in which hospitals providing care to admission would be unnecessary and inappropriate, and to identify
more severely ill patients will tend to have actual mortality rates above patients who are so hopelessly ill that ICU admission would be futile and
predicted, and thus will appear to be giving suboptimal care. This occurs inappropriate. Scoring systems may complement physician judgment
because most scoring systems underestimate mortality of high-risk regarding appropriateness of ICU admission. However, it is important to
patients. Also, medical and nursing interventions can improve physi- emphasize that most scoring systems were derived from patients already
ologic data, leading to a lower estimated risk of mortality for the same admitted to an ICU using data from the first 24 hours of ICU admission.
patient. The outcomes of individual intensivists can be adjusted for The MPM II might be more accurate and appropriate because MPM
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0
severity of illness to better assess performance. This is controversial for used variables available immediately at ICU admission rather than the
several reasons. First, patient sample size of the intensivist may be insuf- worst values of variables over the first 24 hours in the ICU. However,
ficient to draw legitimate conclusions regarding performance. Second, none of the commonly used scoring systems were validated for the
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ICU care is team care, including house officers, nurses, respiratory purpose of triage of ICU patients.
therapists, physiotherapists, and other caregivers, so outcomes are less Scoring systems have been used to assist in triage of patients to inter-
influenced by the behavior of individual physicians. mediate care (monitoring) or to intensive care (life support). Recently,
Scoring systems can be used to compare ICUs in different hospital APACHE III was modified to estimate the probability of need for life
settings (tertiary care, community, academic, etc) and to compare ICUs support of patients admitted for ICU monitoring. Among 8040 ICU
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of different countries. A comparison of New Zealand and US hospitals admissions for monitoring, 79% were predicted to have a low probability
demonstrated different patient selection and fewer admissions to ICUs (<10%) for active treatment during their ICU stay. These patients were
in New Zealand, and yet hospital mortality rates were comparable. admitted to an intermediate care unit and 96% received no subsequent
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Another observational study comparing hospitals in Canada and in the active treatment. The predictive equation had an ROC curve area of
United States revealed similar results. However, important differences 0.74. There are scoring systems designed specifically for triage of trauma
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in mortality have been observed between pediatric ICUs in the United patients. The Triage Index for trauma patients assesses injury severity
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Kingdom and Australia. For comparable severity of illness, the mortality and predicts an outcome using physiologic variables available before

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