The CRB-65 score is a clinical prediction rule that grades the severity of community-acquired pneumonia in terms of 30-day mortality. AimThe study sought to validate CRB-65 and assess its clinical value in community and hospital settings. Design of studySystematic review and meta-analysis of validation studies of CRB-65. Medline (1966 to June 2009), Embase (1988 to November 2008, British Nursing Index (BNI) and PsychINFO were searched, using a diagnostic accuracy search filter combined with subject-specific terms. The derived (index) rule was used as a predictive model and applied to all validation studies. Comparison was made between the observed and predicted number of deaths stratified by risk group (low, intermediate, and high) and setting of care (community or hospital). Pooled results are presented as risk ratios (RRs) in terms of over-prediction (RR>1) or under-prediction (RR<1) of 30-day mortality. Method ResultsFourteen validation studies totalling 397 875 patients are included. CRB-65 performs well in hospitalised patients, particularly in those classified as intermediate (RR 0.91, 95% confidence interval [CI] = 0.71 to 1.17) or high risk (RR 1.01, 95% CI = 0.87 to 1.16). In community settings, CRB-65 over-predicts the probability of 30-day mortality across all strata of predicted risk, low (RR 9.41, 95% CI = 1.75 to 50.66), intermediate (RR 4.84, 95% CI = 2.61 to 8.69), and high (RR 1.58, 95% CI = 0.59 to 4.19). ConclusionCRB-65 performs well in stratifying severity of pneumonia and resultant 30-day mortality in hospital settings. In community settings, CRB-65 appears to over-predict the probability of 30-day mortality across all strata of predicted risk. Caution is needed when applying CRB-65 to patients in general practice. Keywordsgeneral practice; meta-analysis; pneumonia; prognosis; severity of illness index. INTRODUCTIONCommunity-acquired pneumonia (CAP) has an annual incidence of 5-11 per 1000, accounting for 5-12% of all cases of adult lower respiratory tract infection managed by GPs.1 Appropriate management for patients with suspected CAP must be determined by GPs at initial presentation, particularly with regard to whether or not to manage a patient in the community or refer them to hospital.2 The proportion of adults with CAP who require hospital admission in the UK varies between 22% and 42%.1 This proportion varies in other countries, possibly due to differing structures of primary and secondary healthcare systems.