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Blood culture in community acquired pneumonia

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In the USA there are about four million cases of community acquired pneumonia (CAP) per year with an annual mortality of 45 000. There has been controversy about the value of routine blood culture and researchers at a hospital in New York City have provided evidence against.

The retrospective cohort study included 355 patients out of a total of 821 patients with CAP admitted to hospital from the emergency department between January 1999 and March 2001. For inclusion, patients should not have been in hospital in the last two weeks or live in a nursing home, should have been immunocompetent, and should have had x ray evidence of pneumonia and at least one set of blood cultures taken. The average age of the 355 study patients was 60 (range 18–94) and 187 (53%) were women. Blood cultures from 70 patients were positive. They were deemed false positive (growth considered to be contaminant and patient treated accordingly) in 37. The growths in these 37 cases were of Staphylococcus epidermidis in 14 cases, coagulase negative staphylococci (18), diphtheroids (2), S epidermidis plus diphtheroids (1), and coagulase negative staphylococci plus diphtheroids (2). The blood cultures from the 33 patients regarded as having true positive (non-contaminant) cultures grew Streptococcus pneumoniae (30), Staphylococcus aureus (2), and Staphylococcus haemolyticus (1). Two thirds of the patients (238) had their antibiotic treatment changed whilst in hospital. These included 187 (65%) of the 285 patients with negative blood cultures, 25 (76%) of 33 with true positive blood cultures, and 26 (70%) of 37 with false positive blood cultures. Eighteen patients (5%) had their antibiotic treatment changed in response to the blood culture result. No patient had a change of antibiotic treatment because of antibiotic resistance.

About half of positive blood cultures in this series grew presumed contaminants. Overall, 18 patients (5%) had their antibiotic treatment changed in response to the blood culture results (10 of 33 true positives, six of 37 false positives, and two of 285 negatives). The response was a broadening of antibiotic coverage in seven cases and a narrowing in 11. No pathogen isolated was resistant to the antibiotic or antibiotics used as initial empirical treatment. In 246 cases (69%) initial treatment was with either a cephalosporin alone (76) or a cephalosporin plus a macrolide (170).

The authors of this paper question the use of routine blood cultures for immunocompetent patients with CAP. Restricted use of blood cultures would save on the direct costs of blood culture and might also reduce the duration of hospital stay.