Scoring Systems for Comparison of Disease Severity in Intensive Care Unit Patients

Published on 10/03/2015 by admin

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Chapter 89 Scoring Systems for Comparison of Disease Severity in Intensive Care Unit Patients

Benoit Misset, MD , Islem Ouanes, MD

1 What are severity scores?

Scoring systems have been developed to compare the severity of disease among patients in intensive care units (ICUs). These scores are established at admission or during the ICU stay, and they are widely used in the ICU; other scores were recently developed to be used at ICU discharge to predict worse outcome after discharge (early death or ICU readmission). The scores include the assessment of several physiologic parameters that have been documented to play an independent role in predicting hospital death. The scores presented in this chapter are only those used to assess general disease severity. Other scoring systems are used to assess particular organ function (i.e., Acute Lung Injury Score for evaluation of acute respiratory distress syndrome and Sequential Organ Failure Assessment [SOFA] or to assess resource and workload use (i.e., Therapeutic Intervention Scoring System [TISS]).

Scores at ICU admission

2 Which scores are used for assessing the general severity of disease at ICU admission?

The three most frequently used systems are the:

image Acute Physiology and Chronic Health Evaluation (APACHE)

image Simplified Acute Physiology Score (SAPS)

image Mortality Predictive Model (MPM)

The most recent versions of each system are the APACHE IV (2006), the SAPS III (2005), and the MPM III (2007).

3 Why were scores to assess general disease severity at ICU admission developed?

image To assess performance of the ICU. The ICU patient is a medical or a surgical patient who has either acute failure of one major vital function or a high risk for development of such failure. Because the mortality rate of ICU populations is usually high and varies widely depending on patient admission policies, an objective assessment of the patients’ general disease severity is necessary to ensure that the mortality rate in an ICU is consistent with the overall severity of its patient population at admission. The ratio between observed and predicted mortality, called the standardized mortality ratio, is the simplest way to assess the performance of an ICU. It allows comparisons among mortality rates of various ICUs or the mortality rates documented in one ICU over time.

image To assess the patient’s risk for death. The scores give an objective evaluation that helps the clinician confirm the severity of the patient’s illness. However, these scores cannot be used to make decisions about individual patients (e.g., withdrawal of support).

image To compare or match populations in clinical studies. In randomized, controlled studies the scores have been used to confirm that the populations obtained by randomization had a similar disease severity at admission to the ICU. In case-control studies, the scores have been used to match the control to the case patients.

4 How were scores assessing general severity at ICU admission constructed?

Scores were constructed in large, multicenter, prospective populations. The variables were selected and weighed by consensus of panels of experts (first version of SAPS in 1984, first version of APACHE in 1981, and APACHE II in 1985) or through multiple logistic regression analyses used for most recently developed scores (SAPS II and III, APACHE III and IV, and MPM II and III) to determine whether the parameters were independent predictors of hospital death. The tested variables include age, worst values over the first 24 hours of ICU admission for certain acute physiologic abnormalities (e.g., sodium, potassium, partial arterial oxygen tension, urine output, Glasgow Coma Scale), category at admission (medical or surgical patient), and several underlying diseases (e.g., metastatic cancer, acquired immunodeficiency syndrome). The MPM system also includes several therapeutic items (e.g., number of venous lines).

The SAPS III is based on a more complex methodology. It has the advantage of being based on a worldwide population and of giving a larger place to prior health status and to circumstances of admission in addition to the physiologic imbalance at ICU admission. MPM0, MPM III, and SAPS III are collected entirely at admission to the ICU (i.e., within 1 hour), which reduces potential suboptimal care in the first day of the ICU in the assessment of severity.

The most recent version of APACHE score is the APACHE IV developed in 2006 with use of a database of more than 100,000 patients admitted to 104 ICUs in 45 hospitals in the United States in 2002-2003, and remodeling the APACHE III score.

MPM III is a recent update of MPM0 (2007) with use of a database of 124,885 patients from 135 ICUs. This recent score uses 16 variables including three physiologic parameters obtained within 1 hour of ICU admission to estimate mortality probability at hospital discharge.

5 How were scores assessing general disease severity at ICU admission validated?

All of these models were validated in the initial studies in a subset of patients that was not used for construction of the scoring system. The performance of the scoring systems was considered adequate if they showed good discrimination in predicting hospital mortality and had a good calibration for the entire population under investigation. Discrimination in predicting hospital mortality is assessed with receiver operating characteristics (ROCs): the higher the area under the ROC curve, the more discriminative the test. The calibration in the entire population is measured with the goodness-of-fit test: the observed mortality must not be statistically different from the expected mortality in population deciles of equal probability intervals. The lower the Hosmer-Lemeshow H statistic value and the higher the corresponding p value, the better the calibration.

6 Which scores have been validated adequately?

The APACHE I and II, SAPS I, and MPM I have not been constructed or validated with the current accepted methodologic standards. The SAPS II, MPM II, and APACHE III scores have been shown to have good discrimination and calibration in large multicenter studies.

image The SAPS II is well validated. The score needs to be updated with more recent ICU populations.

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