Renal and hepatic dose adjustment in children — branching viva

Station map

Branch A — Renal dose calculation

Examiner: An 8-year-old is 128 cm tall with a serum creatinine of 1.5 mg per dL on a modern enzymatic assay. Calculate the estimated GFR, stage it, and tell me what it means for a renally cleared drug. [1]

Strong answer should include:

• State the bedside Schwartz equation: eGFR equals 0.413 times height in cm divided by serum creatinine in mg per dL, indexed to 1.73 m2. [1]
• Calculate: 0.413 times 128 divided by 1.5, about 35 mL per min per 1.73 m2. [1]
• Stage as KDIGO G3b (30 to 59), and state that most renally cleared drugs need a dose reduction from category G3a (under 60). [4]
• Give the loading dose in full (depends on volume of distribution), then reduce the maintenance dose or extend the interval per the formulary. [5]

Trap: quoting the original 0.55 constant, or forgetting that the loading dose still applies in organ impairment. [1]

Branch B — Cirrhotic child, grade the liver

Examiner: A 10-year-old with biliary atresia has bilirubin 58 micromol per L, albumin 27 g per L, INR 1.9, moderate ascites and grade 1 encephalopathy. Grade the hepatic impairment and tell me why the INR matters more than the ALT for dosing. [6]

Strong answer should include:

• Name the five Child-Pugh variables: bilirubin, albumin, INR, ascites, encephalopathy. [6]
• Score roughly 11 to 12, Child-Pugh class C (severe). [6]
• Explain the INR reflects hepatic synthetic function, which is what clears many drugs, whereas the ALT reflects injury not clearance. [6]
• Plan: reduce hepatically cleared maintenance doses by around half, prefer renally cleared antibiotics guided by the eGFR, avoid sedatives and NSAIDs. [5] [6]

Trap: treating a near-normal ALT as proof of preserved hepatic clearance, and missing failing synthetic function. [6]

Branch C — Vancomycin level

Examiner: A child on vancomycin every six hours for a central-line infection has a trough drawn one hour after the infusion of 22 mg per L. How do you interpret this and what do you do next? [7]

Strong answer should include:

• Recognise the timing is wrong: a trough is taken just before the next dose, so a level drawn one hour after the infusion is a post-peak value, not a trough, and is uninterpretable as a trough. [7]
• State the 2020 consensus preference for area-under-the-curve guided dosing targeting 400 to 600 mg per h per L for serious MRSA infection. [7]
• Order a correctly timed level before acting on the number, and check the creatinine trend. [7]
• Distinguish a rate-related infusion reaction from anaphylaxis if the child is also flushed. [7]

Trap: acting on a mistimed level and either over- or under-dosing the child. [7]

Branch D — Adolescent at transition

Examiner: A 17-year-old with stable CKD transfers to adult care. The paediatric Schwartz equation is in use. What should change at handover? [3]

Strong answer should include:

• Agree the estimating equation with the adult team to avoid a jump in the GFR estimate at handover. [3]
• Reconcile weight-capped adult doses with the paediatric weight-based doses. [5]
• Use a continuity equation such as the Full Age Spectrum creatinine equation across the transition. [3]
• Hand over the reason for any non-standard dose, the last levels, and the monitoring plan. [5]

Trap: carrying a paediatric equation and dose into adult care without confirming continuity, producing a silent dosing error. [3]