Stevens, Ryan W

Abstract Purpose A case of vancomycin enzyme immunoassay (EIA) interference confirmed by high-performance liquid chromatography (HPLC) is described. Summary Therapeutic drug monitoring is standard of practice in vancomycin dosing and monitoring in order to maximize the pharmacodynamic effects and minimize toxicity. After a 52-year-old woman received 5 doses of vancomycin, serum concentrations continued to rise for several days in the absence of ongoing vancomycin administration. Despite persistently elevated vancomycin concentrations, the patient clinically deteriorated and required treatment with an alternative agent. Subsequently, serum concentrations were processed via HPLC and analyzed for percent protein binding. Confirmatory analysis revealed substantially lower concentrations by HPLC than were obtained by EIA and an abnormal elevation in protein binding. After discharge from the index admission, the patient returned 11 months later and had a dectectable vancomycin concentration by EIA prior to receipt of vancomycin. HPLC analysis confirmed the true concentration was undetectable. Though the exact interfering substance was not identified, the above discrepancy in concentrations between the two assay methods indicates the presence of assay interference, and adds to the available literature suggesting similar occurrences. This case is particularly troubling given that the level of interference was not such that it would lead a clinician to immediately suspect interference, and the patient experienced treatment failure. Conclusion Falsely elevated values for serum vancomycin concentration, measured by EIA, contributed to treatment failure in a patient. The substance presumably responsible for EIA interferences was not identified.

Multidrug‐resistant (MDR) Pseudomonas aeruginosa infections often represent a therapeutic challenge requiring utilization of older, more toxic antibiotics, or new agents with limited data. Once susceptibility to β‐lactam and fluoroquinolone antibiotics has been lost, other therapeutic options such as aminoglycoside or polymyxin antibiotics carry significant adverse effects such as nephrotoxicity, neurotoxicity, and ototoxicity. A novel cephalosporin, cefiderocol, lacks gram‐positive and anaerobic activity but offers broad coverage of gram‐negative bacteria, including P. aeruginosa . A unique catechol side chain gives it activity as a siderophore, thereby increasing bacterial uptake and decreasing efflux. Additionally, cefiderocol is stable against a wide array of β‐lactamases. Despite these promising characteristics, there are minimal data currently available in the literature detailing the use of cefiderocol in the treatment of MDR P. aeruginosa infections. We present a case of a 46‐year‐old man who developed an MDR P. aeruginosa intraabdominal infection where serious and life‐threatening toxicities to aminoglycosides and polymyxin antibiotics led to the utilization of cefiderocol on compassionate use approval. The isolate was susceptible to cefiderocol, and the patient was treated for 28 days of therapy. He demonstrated clinical and radiographic resolution of his infection and was discharged to home.