Where should antibiotic gradient diffusion strips be crossed to assess synergy? A comparison of the standard method with a novel method using steady-state antimicrobial concentrations.

Multi-drug resistance among Pseudomonas aeruginosa in hospitals, and particularly intensive care units, has achieved alarming rates. Some combination antimicrobial therapies have demonstrated promising synergistic effects and an ability to overcome resistance without increasing drug-related toxicities. Nevertheless, rapid and feasible methods to identify synergy have not been routinely implemented in clinical microbiology laboratories. Synergistic activity of meropenem plus tobramycin or levofloxacin against clinical P. aeruginosa isolates (N=21) was assessed by two different methods using gradient diffusion strips (GDSs). A 90° angle was created at the intersection of the minimum inhibitory concentration (MIC) of each drug by the standard method, and by a novel method, the cross was placed at clinically relevant steady-state concentrations (C) based on recommended dosing regimens. Fractional inhibitory concentration indexes were determined to describe antibiotic interactions. Time-kill analyses were performed over 24 h in duplicate for instances of discordance between the standard cross method and the novel method. Synergy between meropenem and tobramycin by the novel method was observed in one (4.8%) isolate and between meropenem and levofloxacin in two (9.5%) isolates. Agreement with the standard method was 86-100% for meropenem plus tobramycin and meropenem plus levofloxacin combinations, respectively. Time-kill studies resulted in agreement with GDSs crossed at C in two of three instances of discordance between GDS methods. This novel method of synergy testing that involves crossing GDSs at steady-state concentrations may be a rapid and feasible tool for routine practice. Further comparisons of this novel procedure with time-kill methods are needed.