Development of blood culture time-to-positivity as a pharmacodynamic indicator for monitoring antibiotic therapy against MDR Gram-negative bacteria.

Objectives: We investigated the use of time-to-positivity (Tpos), a readily available parameter measured from automated blood culture incubators, as a surrogate marker for serum antimicrobial activity during antibiotic treatment against ESBL- and carbapenemase-producing Klebsiella pneumoniae.

Methods: Banked human serum spiked with clinically representative concentrations of different antibiotics was injected into aerobic blood culture bottles containing 1 × 104 mL inoculum of K. pneumoniae isolates. The time required for the test inoculum to 'grow through' serum antibacterial activity (Tpos) was then measured over 24 h. Serum containing combinations of antibiotics were also tested using an 8 × 8 combination array to assess synergistic or antagonistic interactions.

Results: Tpos increased from <10 to >24 h with increasing serum antibiotic concentrations for all drug-isolate combinations tested with the EC50 Tpos of ∼17 h for all isolates. The EC50 for ceftazidime/avibactam (4:1) ranged from 5.73 mg/L (95% CI, 5.60-5.37) for a WT strain (MIC 0.125 mg/L) to 377 mg/L (95% CI, 356.26-398.41) for a resistant KPC-3-producing isolate (MIC 16 mg/L). Variation in observed Tpos was linked to isolate MIC (R2 = 0.94). Antibiotic combinations marginally increased serum Tpos (1-3 h) for most isolates with the exception of ceftazidime/avibactam + meropenem or aztreonam against KPC-3-producing isolates (increase of 10.36-11.61 h) or ceftazidime/avibactam + aztreonam against an NDM-2 isolate (increase of 10.33 h).

Conclusions: Tpos is a simple, reproducible marker of serum antibiotic activity against ESBL- and carbapenemase-producing K. pneumoniae. With further clinical validation, Tpos could be developed as a complementary surrogate marker for early detection of inadequate antimicrobial therapy in patients receiving antibiotic therapy for bloodstream infections caused by MDR Gram-negative bacteria.