Antimicrobial Synergy Testing Based on Antibiotic Levels, Minimal Bactericidal Concentration, and Serum Bactericidal Activity

Abstract

The ratio between the concentrations of different antimicrobial agents varies widely during therapy and often bears no resemblance to the ratios assessed by in vitro methods to evaluate the effectiveness of multiple antimicrobial therapy. The authors examined an alternative method using patient serum that reflects the actual antibiotic levels achieved in the patient. Previous investigators have shown that the ratio of the concentration of free drug (drug-f) to the minimal bactericidal concentration (MBC) in broth approximates the serum bactericidal titer (SBT) when pooled normal human serum is used as the diluent. Theoretically, when two antimicrobial agents are administered, the SBT should be equivalent to (drug-f A/MBC drug A) + (drug-f B/MBC drug B). SBT and drug level determinations were performed on peak and trough serum specimens from ten patients with endocarditis or osteomyelitis who were receiving multiple antimicrobial therapy. The serum dilution synergy method predicted additive interactions twice as often as the checkerboard and kill curve, and predicted synergy less frequently than the kill curve. The checkerboard predicted antagonism four times more often than the other methods and provided equivocal results in four of ten cases. The suggested method may offer an alternative procedure to assess antimicrobial interactions, which is based on antibiotic levels actually achieved in vivo instead of the arbitrary concentrations often used in in vitro tests.

Pharmacokinetics and serum bactericidal activities of quinolones in combination with clindamycin, metronidazole, and ornidazole

“…Against E. coli and K. pneumoniae, the combination of enoxacin and metronidazole and, to a lesser extent, the combination of enoxacin and clindamycin resulted in decreases of serum bactericidal activity compared with enoxacin given alone (Table 4). Although this result was somewhat unexpected, we would not interpret these results as an antagonism since a twofold error is inherent in a tube dilution method (36) and since these changes were not statistically significant. As shown with ciprofloxacin and ofloxacin, the SBA against P. aeruginosa was relatively low.…”

“…Urine was collected before administration and at intervals from 0 to 6, 6 to 12, 12 to 24, and 24 to 48 h following administration. In the fleroxacin study, additional blood samples were obtained after 30,36, and 48 h, and the urine was collected for an additional interval (48 to 7A h) after administration. Serum and urine samples were frozen at -70°C and assayed during the following 3 months.…”

Serum bactericidal test

“…This use of the test will probably be more commonly encountered with increased efforts to shorten hospital stay by utilizing home therapy. As an experimental tool, the SBT has been used for evaluating new drugs and drug combinations (6,77,87,108). When the SBT is used for such evaluations, the antimicrobial agent is administered to a healthy person, serum is collected, and serum bactericidal titers are determined for the pathogen under study.…”

Tests for bactericidal effects of antimicrobial agents: technical performance and clinical relevance

“…The appearance of bacteriostatic or bactericidal activity is typically determined for each tube in the set, with the result indicating the optimal concentrations of each drug in combination for inhibitory and bactericidal effects. IN (59,67,78,85,99,104,114,120), the use of cation supplements (41,92,107,120), whether the culture is mixed or otherwise agitated during incubation (48,104,105,108,120), the growth phase of the inoculum (7,39,48,57,70,104,105,108,114,120), the pH of the medium (34,105,114,120), the use of serum (binding) proteins (92,120), the presence of complement (21,34,120), the elimination of antibiotic carryover during subculture (8,25,35,47,93,103,104,119,122), the final density of the bacterial inoculum …”