Renal and hepatic dose adjustment OSCE — child with CKD, a high-risk antibiotic and family counselling

Candidate brief

You have 10 minutes. [1] [5]

1. Calculate the child's estimated GFR using the bedside Schwartz equation and state the KDIGO category. [1] [4]
2. Decide how to adjust the antibiotic dose for her renal function, including the loading dose and the maintenance strategy. [5]
3. Interpret the drug level and state your next action. [7]
4. Explain the non-standard dose to her mother in language she can repeat back. [5]

Scripted clinical data

• 9-year-old girl with CKD under nephrology follow-up, admitted with febrile urinary tract infection. [4]
• Height 130 cm (measured), weight 28 kg, serum creatinine 1.6 mg per dL on a modern enzymatic assay. [1]
• Plan is for gentamicin extended-interval therapy; the first level (taken just before the second dose) returns above the local target threshold. [10]
• Mother is present and anxious; English is her second language and a professional interpreter is available. [5]

Expected performance

Calculation and staging

• State the bedside Schwartz equation as eGFR equals 0.413 times height in cm divided by serum creatinine in mg per dL. [1]
• Calculate 0.413 times 130 divided by 1.6, about 34 mL per min per 1.73 m2, KDIGO category G3b. [1] [4]

Dose-adjustment decision

• Give the loading dose in full because it depends on volume of distribution, not clearance. [5]
• Reduce the maintenance dose or extend the interval per the BNFc or local paediatric formulary for category G3b; for gentamicin this means a level-tailored extended interval. [5] [10]

Level interpretation and action

• Recognise the level is drawn at the correct time (just before the next dose) but is above target, so extend the next interval and recheck the creatinine. [7] [10]
• Avoid nephrotoxin stacking (do not add an NSAID). [11]

Communication

• Use the interpreter, explain in plain language that the dose is changed because her kidneys clear the drug more slowly, and have the mother demonstrate the plan back. [5]

Examiner scoring anchors

Common fails

• Quoting the obsolete 0.55 Schwartz constant and overestimating the GFR. [1]
• Withholding the loading dose and under-treating a febrile UTI. [5]
• Acting on a level without checking the timing, or ignoring a high level. [7]
• Adding an NSAID for fever and stacking a second nephrotoxin. [11]
• Failing to use a professional interpreter or confirm understanding by teach-back. [5]