C-reactive protein and community-acquired pneumonia in ambulatory care: systematic review of diagnostic accuracy studies

Background. There is uncertainty regarding the diagnostic value of C-reactive protein (CRP) in patients presenting with symptoms suggestive of community-acquired pneumonia (CAP) in community or ambulatory settings. Objective. We assessed the diagnostic value of CRP in primary care and accident and emergency departments in terms of ruling in or ruling out CAP. Methods. Diagnostic accuracy systematic review, we searched PubMed from January 1966 to September 2008 and EMBASE from January 1980 to September 2008 using a diagnostic accuracy search filter. We included cross-sectional or cohort studies that assess the diagnostic utility of CRP at different cut-points against a reference standard of chest X-ray. We calculated pooled positive and negative likelihood ratios (LRs) and assessed heterogeneity using the I-2 index. Results. Eight studies incorporating 2194 patients were included. The median prevalence of CAP was 14.6% (range 5%-89%). At a CRP cut-point of <= 20 mg/l, the pooled positive LR+ was 2.1 [95% confidence interval (CI) 1.8-2.4] and the pooled negative LR- was 0.33 (95% CI 0.25-0.43). At the two other CRP cut-points (<= 50, > 100 mg/l), the results were heterogeneous, so the pooled results should be interpreted with caution. Conclusions. CRP may be of value in ruling out a diagnosis of CAP in situations where the probability of CAP > 10%, typically accident and emergency departments. In primary care, additional diagnostic testing with CRP is unlikely to alter the probability of CAP sufficiently to change subsequent management decisions such as antibiotic prescribing or referral to hospital.