Are Blood Cultures Necessary in Community-Acquired Pneumonia?

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Blood cultures are included in all current community-acquired pneumonia (CAP) management guidelines. However, recent studies have cast doubt on the utility of blood cultures to affect patient outcomes. To be an effective tool, a test needs to have sufficient sensitivity and specificity in the patient group of interest to alter physician prescribing habits, and when made, those changes need to impact positively on outcome. The yield of blood cultures varies but averages 7% to 8%, with a significant (4%–5%) false positive rate. Blood cultures are more likely to be positive in patients with comorbidities (particularly liver disease, chronic obstructive pulmonary disease, alcoholism and diabetes), in patients admitted to the intensive care unit, and in patients with more abnormal vital signs at presentation. Antibiotics prior to presentation markedly reduce the likelihood of a positive blood culture. When blood culture–guided changes in treatment are made, there is little other than anecdotal evidence that outcomes are improved, possibly due to the time period between the administration of effective antibiotics and a positive impact on outcome. Clinicians’ reluctance to alter antibiotic therapy based on positive blood cultures further reduces their ability to affect outcomes. Given these limitations, blood cultures are unlikely to influence outcomes in previously well patients with less severe CAP, particularly if they have received previous antibiotic therapy. Despite these significant limitations, blood cultures are probably still advisable in patients with severe CAP due to the high mortality in this group.

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