Real world clinical feasibility of direct-from-specimen antimicrobial susceptibility testing of clinical specimens with unknown microbial load or susceptibility

Abstract Within healthcare settings, physicians use antibiograms, which offer information on local susceptibility rates, as an aid in selecting empirical antibiotic therapy and avoiding the prescription of potentially ineffective drugs. While antibiograms display susceptibility and resistance data at hospital, city, or region-specific levels and ultimately enable the initiation of antibiogram-based empirical antibiotic treatment, AST reports at the individual patient level and guides treatments away from broad-spectrum antibiotics towards narrower-spectrum antibiotics or the removal of antibiotics entirely. Despite these advantages, AST traditionally requires a 48- to 72-h turn-around; this window of time can be critical for some antimicrobial therapeutic interventions. Herein, we present a direct-from-specimen AST to reduce the time between patient sampling and receipt of lab AST results. The biggest challenge of performing AST directly from unprocessed clinical specimens with an unknown microbial load is aligning the categorical susceptibility report with CLSI reference methods, which start from a fixed inoculum of 0.5 McFarland units prepared using colonies from a sub-culture. In this pilot clinical feasibility study using de-identified remnant specimens collected from MCW, we observed the high and low ends of microbial loads, demonstrating a final categorical agreement of 87.5% for ampicillin, 100% for ciprofloxacin, and 100% for sulfamethoxazole-trimethoprim.