Blood cultures are recommended for patients admitted to the hospital for community-acquired pneumonia (CAP). While awaiting culture results, most patients are routinely treated with a second- or third-generation cephalosporin, with or without a macrolide antibiotic. Because of the low yield in identifying a pathogen and its limited impact on antibiotic selection, some authorities have questioned the usefulness of blood cultures (and sputum cultures) in patients admitted with CAP. An advantage to identifying an organism such as Streptococcus pneumoniae would be in determining its sensitivity to penicillin, thus allowing therapy to be narrowed to penicillin if the minimum inhibitory concentration of the organism was acceptably low. If the strain was penicillin resistant, appropriate adjustment in therapy could then be made. Waterer and colleagues retrospectively reviewed the use of blood culture results and their effect on management of patients admitted to their institution with a diagnosis of CAP.
The medical records of patients admitted to the hospital with a diagnosis of CAP and at least one blood culture positive for S. pneumoniae were reviewed. Patient demographics, initial antibiotic choices, culture results in regards to antibiotic sensitivity, modifications in treatment based on blood culture results, complications and outcomes were noted in the charts. The Pneumonia Severity Index (PSI) score determined illness severity. Isolates of S. pneumoniae were considered to be penicillin nonsusceptible if the minimum inhibitory concentration was ≥0.1 μg per mL and penicillin-resistant if the minimum inhibitory concentration was ≥2.0 μg per mL.
Over a two-year period, 1,805 patients were admitted with CAP, and 118 had blood cultures positive for S. pneumoniae. After eliminating patients whose charts were not available for review, patients who had other diagnoses related to Pneumococcal sepsis (e.g. endocarditis, meningitis) and those allergic to penicillin, 74 patients made up the study group. Empiric antibiotic choice was a third-generation cephalosporin in 67.6 percent of the patients, alone or in combination with another drug; 51.4 percent received only a macrolide, and 25 percent received a quinolone. Isolates from 11 patients were nonsusceptible to penicillin, and four were fully penicillin-resistant. These four were also resistant to cephalosporin. Seven of the S. pneumoniae isolates were resistant to erythromycin. One death occurred in the penicillin-resistant group, and five deaths occurred in the patients whose bacteria were penicillin-sensitive.
Positive pneumococcal blood culture results altered management in only 31 patients—even among those who could have been switched to penicillin monotherapy. Only 11 patients were changed to penicillin therapy after the culture results were obtained. Only two antibiotic changes were made in patients who had a penicillin-resistant strain. Other changes included 13 patients who discontinued macrolide therapy after antibiotic sensitivities were determined.
On the basis of this study, the authors conclude that physicians are hesitant to change antibiotic therapy for CAP even when a penicillin-sensitive organism is identified. In this study, penicillin was used in only 22 percent of eligible patients, and coverage for atypical organisms was discontinued in only 37 percent of patients. The continued indiscriminate use of broad-spectrum antibiotics contributes to the growing problem of resistance and substantially increases costs. Although it seems apparent that blood cultures are helpful in establishing an etiologic diagnosis for a patient with CAP, if the results do not affect management, obtaining blood cultures is not cost effective. The authors recommend further prospective studies to examine this problematic issue.
editor's note: This is a provocative study that illustrates a common practice of physicians—ordering tests and then failing to change management or act on the results. Perhaps many of the physicians who managed these patients trained in the post-penicillin era and simply are not comfortable using this “weaker” antibiotic. The key question remains (and one that has produced dissenting opinions from the Infectious Disease Society of America and the American Thoracic Society): is it necessary to determine the causative agent in patients diagnosed with community-acquired pneumonia?—j.t.k.