Therapeutic drug monitoring — formative SAQs

SAQ 1 — Vancomycin monitoring in a child with serious MRSA infection (10 marks, 15 minutes)

Stem: A six-year-old, 20 kg child is receiving intravenous vancomycin for a complicated methicillin-resistant Staphylococcus aureus bacteraemia. The team asks you to design the therapeutic drug monitoring plan, state the target, and interpret an unexpectedly low trough. ^[1]

Model answer

Target (2 marks). The therapeutic target for serious MRSA infection is the 24-hour area under the concentration–time curve over the minimum inhibitory concentration — an AUC₂₄/MIC of 400 or more — per the 2020 consensus guideline from ASHP, IDSA, PIDS and SIDP. State that AUC-guided monitoring is preferred over a trough-only strategy because it achieves target exposure while reducing nephrotoxicity. ^[1]

Sampling (3 marks). Wait for steady state — about four to five half-lives, which in this child is around the fourth dose — before interpreting a maintenance trough. Draw the true trough within 30 to 60 minutes before the next dose, from a clean line, not during the infusion. AUC can be calculated from two timed levels (for example a peak after the infusion ends and a trough before the next dose) or estimated by Bayesian forecasting from a single level. Always document the dose, the dosing interval, the time of the last dose, and time sample was drawn. ^[1]

Interpreting an unexpectedly low trough (3 marks). Before changing the dose, exclude a sampling error (drawn at the wrong time or from the infusing line) and confirm the dose and interval were given as written. If the sample is valid, a low trough in a critically ill child often reflects augmented renal clearance — a state of high glomerular filtration driven by inflammation, fever and vasopressors — which clears vancomycin faster than standard dosing assumes. The response is not to assume underdosing but to raise the dose, shorten the interval, or move to Bayesian AUC dosing, then recheck. ^[1]

Daily review and safety (2 marks). Each day ask whether the drug is still needed, plan de-escalation once the organism and sensitivities return, and avoid co-administered nephrotoxins. Treat the child as well as the number: a low level in an improving child still needs correction of exposure, while a high level with a rising creatinine means withhold the next dose and recheck. ^[1]