Playing by the Rules: Using Decision Rules Wisely Part 2, Nontraumatic Conditions

Pneumonia Severity Index

The Pneumonia Severity Index (PSI) was developed as a tool to predict mortality risk from pneumonia, allowing providers to appropriately manage care for these patients in the hospital or as outpatients. A derivation cohort of 14199 patients was utilized to create a prediction rule in two steps meant to parallel a clinician’s decision-making process. The first step identified a population of patients that were at low risk for death, which were assigned to class I. The second step quantified the risk for death in the remaining patients using weighted factors including demographics, comorbidities, exam findings, and clinical data. In all, 20 variables were used and assigned corresponding points, the sum of which would assign a patient to a particular risk for mortality (class II-V).⁵⁶

Mortality risk was relatively low for patients in class I and II (0.4 and 0.7%, respectively). Class III carried a mortality risk of 2.8%. Mortality increased with class IV and class V classification: 8.5% and 31.1%, respectively. These data were replicated with a separate validation cohort of 38039 patients, with similar mortality rates in each class. This study concluded with the recommendation that patients diagnosed with pneumonia falling into class I and II mortality risk should be managed as outpatients, possible brief inpatient observation for class III, and class IV and V managed as inpatients.⁵⁶

Subsequent trials evaluating the utility of the PSI score in the management of patients diagnosed with pneumonia randomized low-risk patients (class I-III PSI) to treatment as outpatients vs inpatients. There were no statistical differences in adverse outcomes (ICU admission, hospital readmission, mortality, complications), with notable improvements in hospital admission rates and patient satisfaction.^57,58 A meta-analysis of 6 studies that used a clinical decision tools to identify low-risk patients to treat pneumonia as outpatients showed no significant difference in mortality, patient readmissions, or patient satisfaction. Low-risk patients that required admission often included comorbid illnesses not included in the PSI, inability to take oral medications, barriers to compliance, or hypoxemia.⁵⁹

Though the PSI has been shown to successfully identify patients at low risk for mortality, it has been less accurate at predicting and stratifying classes of severe pneumonia. A meta-analysis by Loke et al showed that PSI class IV or V had pooled sensitivity of 0.90 and specificity 0.53 for 30-day mortality, which was significantly better than the CURB-65 rule (discussed below).⁶⁰ However, a subsequent large meta-analysis showed that PSI class IV or V had a sensitivity of 75% and specificity 40% for requiring ICU intervention or admission, which are not sufficient to guide disposition decisions.⁶¹

CURB-65

One of the criticisms of PSI included its complexity, with inclusion of 20 factors making it impractical for use as a bedside tool. The CURB-65 score was developed with a similar goal of identifying low-risk patients with pneumonia who would be candidates for outpatient management, but also patients at high risk for mortality or ICU admission. Criteria for severe pneumonia published by the British Thoracic Society include: respiratory rate ≥ 30 breaths/min, diastolic blood pressure ≤60 mmHg, and blood urea nitrogen >7 mmol/L. The presence of 2 criteria was 88% sensitive and 72% specific for mortality or ICU admission.⁶² The CURB-65 tool was based on these criteria, with the addition of age ≥65 years, which was found to be a separate independent predictor of mortality. Thus, the 5 criteria making up the score are as follows (1 point each, 0-5 total):

Confusion, meaning Mental Test Score ≤8, or disorientation to person, place, or time
Urea >7 mmol/L (>19.6 mg/dL)
Respiratory rate ≥ 30 breaths/minute
Blood pressure (systolic < 90 mmHg or diastolic ≤ 60 mmHg)
Age ≥ 65 years