Vancomycin dosing during CRRT — FFICM MCQ

Correct answer: C — Measure vancomycin concentration and redose using therapeutic drug monitoring

The correct answer is C, measure vancomycin concentration and redose using therapeutic drug monitoring. In CVVHDF, vancomycin clearance is highly variable and unpredictable because it depends on filter type, effluent (dialysate/ultrafiltrate) flow rate, time since filter change, and protein binding, which is reduced when albumin is low, increasing the free (active) fraction available for clearance. The BNF and UK critical care practice recommend that vancomycin dosing beyond the loading dose be guided by measured plasma concentrations rather than fixed intervals or renal function estimates, aiming for a trough (or steady-state) level typically 15 to 20 mg/L in severe infection such as bacteraemia. Given that the effluent dose has increased and the filter has been changed twice, clearance is likely to have risen further, making empirical redosing without a level highly likely to result in subtherapeutic exposure and treatment failure of an MRSA bacteraemia. Measuring a level directly captures the net effect of all these variables and allows individualised, safe redosing. Why the other options are wrong: B. Withhold further doses until CRRT stops: this risks prolonged subtherapeutic vancomycin exposure in an ongoing bacteraemia, promoting treatment failure and resistance; CRRT may continue for many days. A. Give a fixed dose every 7 days: CRRT clearance changes with filter age and effluent rate, so a rigid interval ignores the dynamic pharmacokinetics and risks both toxicity and underdosing. D. Use serum creatinine alone to estimate clearance: creatinine clearance formulae are invalid during extracorporeal therapy because CRRT, not the native kidneys, is the dominant route of drug removal. E. Switch to oral vancomycin for bacteraemia: oral vancomycin has negligible systemic absorption and is used only for Clostridioides difficile infection, not for bloodstream infection. Key point: In CRRT, vancomycin redosing should always be guided by measured plasma concentrations because filter and effluent changes make clearance too unpredictable for fixed-interval or creatinine-based dosing.

Reference: BNF (NICE/British National Formulary), Vancomycin monograph, Medicines guidance: 'Therapeutic drug monitoring for aminoglycosides and vancomycin' section, accessed via bnf.nice.org.uk; supported by UK Kidney Association Clinical Practice Guideline: Renal Replacement Therapy in Acute Kidney Injury (drug dosing in RRT chapter)