Early recognition of patients with severe CAP will aid in the initiation of prompt therapy directed at the likely etiologic pathogens, a strategy associated with reduced mortality, if it leads to a clinical improvement within 72 h . Although there is no uniformly accepted definition of severe CAP, the original ATS guidelines identified nine criteria for severe illness, and the presence of any one was used to define severe CAP. Subsequently, several studies have shown that when only one of these criteria is used, as many as 65 to 68 percent of all admitted patients have "severe CAP," indicating that the original definition was overly sensitive, and not specific .

In one more recent study, the nine criteria for severe CAP were divided into five "minor" criteria that could be present on admission and four "major" criteria that could be present on admission or later in the hospital stay . The minor criteria included respiratory rate ≥30/min, PaO2/FIO2 <250, bilateral pneumonia or multilobar pneumonia, systolic BP ≤90 mm Hg, and diastolic BP ≤60 mm Hg. The major criteria included a need for mechanical ventilation, an increase in the size of infiltrates by >50 percent within 48 h, septic shock or the need for pressors for >4 h, and acute renal failure (urine output <80 mL in 4 h or serum creatinine >2 mg/dL in the absence of chronic renal failure). In this retrospective study, the need for ICU admission could be defined by using a rule that required the presence of either two of three minor criteria (systolic BP <90 mm Hg, multilobar disease, PaO2/FIO2 ratio <250) or one of two major criteria (need for mechanical ventilation or septic shock). When the other criteria for severe illness were evaluated, they did not add to the accuracy of predicting the need for ICU admission. With this rule the sensitivity was 78 percent, the specificity was 94 percent, the positive predictive value was 75 percent, and the negative predictive value was 95 percent .

Recommended definition of severe CAP and need for ICU admission — Although future prospective studies defining the need for ICU admission are still needed, the committee felt that severe CAP could be defined as the presence of two minor criteria (using only the three listed above or the five in the original ATS statement), or one major criterion (using the two listed above or the four in the original ATS statement) (Level II evidence). Other findings suggesting severe illness may also have utility, but have not been formally tested to define the need for ICU admission. These include the two (of the four) other criteria in the BTS rule (in addition to the two included in the definition of severe CAP), namely confusion and BUN >19.6 mg/dL.

Copyright 2001 The American Thoracic Society, Am J Respir Crit Care Med 2001; 163:1703. Reprinted with permission from The American Thoracic Society.