A patient in the oncology ward is neutropenic and has a catheter-related bloodstream infection. The team needs to know whether an isolate is rapidly killed by an agent (bactericidal) and what MIC is to optimize dosing. Which testing sequence would you recommend and why?

• A. Run only a Kirby-Bauer disk diffusion because categorical susceptibility is sufficient and the urgency in neutropenic patients means lab precision is less important than quick interpretation, so the simpler test is both faster and clinically acceptable.
• B. Perform broth microdilution to obtain a precise MIC and then subculture clear wells to determine the MBC because this sequence yields both quantitative MIC data for dosing and definitive bactericidal information required in neutropenic patients, despite greater lab effort and time.
• C. Delay any susceptibility testing until the patient improves clinically to minimize unnecessary lab work; changes in neutrophil count will make any current test results irrelevant and therefore no test should be performed while the patient is neutropenic.
• D. Start with an E-test for an approximate MIC and if the MIC is above clinical breakpoints, perform no further testing because E-test results automatically infer bactericidal activity when they exceed breakpoints and therefore MBC testing is unnecessary.

Answer: (B)