Serum bactericidal test as a prognostic indicator in acute pulmonary exacerbations of cystic fibrosis.

The serum bactericidal test has been used for many years for optimal assessment of the efficacy of antibiotic therapy in patients with infective endocarditis and other bacterial infections. Its capacity to predict the bacteriological outcome of acute pulmonary exacerbations in patients with cystic fibrosis was evaluated. A total of 54 courses of intravenous antibiotic therapy were analyzed in 22 patients, whose ages ranged from 4 months to 24 years (mean age: 10 years). The serum bactericidal activity of blood samples, taken at expected peak and trough antibiotic levels on day 4 of therapy, were determined against the potentially pathogenic strains isolated in sputum at the time of admission. For 104 isolates (64 Pseudomonas aeruginosa, 28 Staphylococcus aureus, and 12 Haemophilus influenzae strains), the peak and trough bactericidal titers were compared to bacteriological outcome. Bacteriological success was defined as a decrease of 2 log10 units or more in the bacterial density in sputum between days 0 and 7 of therapy. At peak antibiotic levels, serum bactericidal titers of 1:128 or more were 96% (all isolates) and 89% (P aeruginosa isolates), predictive of cure, whereas serum bactericidal titers of less than 1:16 were 100% predictive of failure for all infecting bacteria. In patients aged less than 18 years, the best peak titer for predicting success was 1:64, with a predictive value of 96% for titers of 1:64 or greater. The peak titer that best predicted success in patients aged 18 years or more was 1:128, with a predictive value of only 83% for titers of 1:128 or greater.(ABSTRACT TRUNCATED AT 250 WORDS)