Background: 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.
Methods: 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.
Results: 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.
Conclusion: 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.