Prospective, protocolized study evaluating effects of antibiotics on sputum culture results in injured patients

Appropriate utilization of antibiotics for critically ill patients involves tailoring the drug to culture results; however, the culture results must be reliable. We hypothesized that antimicrobial agents reduce significantly the reliability of cultures obtained between 1 and 24 h after antibiotic administration.

Patients were eligible for the study if they were ventilated mechanically and were suspected to have pneumonia. After enrollment, sputum cultures were obtained, and broad-spectrum antibiotics were started. Sputum cultures were repeated at 1, 6, 12, and 24 h after delivery of the first dose of antibiotic. Twenty-one patients whose initial culture was positive were included in the analysis. Their average age was 49.4 years, and the average Injury Severity Score was 27.7 points.

The average intensive care unit and hospital lengths of stay were 20.2 days and 24.7 days, respectively. All of the organisms grown from the pre-antibiotic cultures also grew in the cultures obtained 1 h after antibiotics were given. However, a significant number of these organisms were unable to be grown in subsequent cultures. The rate of negative cultures increased to 21%, 32%, and 42% in the 6-, 12-, and 24-h groups (p<0.01), respectively. Gram-positive organisms accounted for 42.9% of infections, with Staphylococcus aureus being the most common. All patients positive for S. aureus prior to antibiotic administration remained positive at each subsequent time. By 6 h, 21.5% of the gram-negative organisms could no longer be cultured. At 12 h, among the gram-positive organisms, 11 of 12 cultures were still positive, whereas only 50% of gram-negative organisms were still recoverable.

Antibiotics have a substantial effect on culture results that is most pronounced in gram-negative organisms and is observed in cultures obtained beginning 1 h after antibiotics are given. As a result, cultures obtained more than 1 h after antibiotics are started cannot be used to tailor antibiotic choice in injured patients with suspected infections.