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]

Closing synthesis the candidate should say

"I measure the child's height and estimate GFR with the bedside Schwartz equation using the 0.413 constant, stage the kidney with KDIGO and the liver with Child-Pugh, look up the drug-specific dose in a current formulary, give the loading dose in full and reduce the maintenance dose or extend the interval, monitor high-risk drugs with levels drawn at the right time, and document and hand over the reason for every non-standard dose." [1] [4] [7]