Renal and hepatic dose adjustment in children — formative SAQs

SAQ 1 — Bedside Schwartz eGFR, KDIGO staging and a renal dose-adjustment plan (10 marks)

A 9-year-old boy, 132 cm tall, has CKD follow-up. His serum creatinine is 1.7 mg per dL on a modern enzymatic assay. He is well and stable. He now needs a course of a renally cleared antibiotic for a urinary tract infection. [1] [4]

Questions

1. State the bedside Schwartz equation in full with units, calculate this child's estimated GFR, and state the KDIGO category. (5 marks) [1] [4]
2. Outline the stepwise dose-adjustment plan for his antibiotic, including dose versus interval, the role of the loading dose, and how you would monitor and document. (5 marks) [5]

Model answer

Calculation and staging (5). The bedside Schwartz equation gives eGFR in mL per min per 1.73 m2 equal to 0.413 times height in cm divided by serum creatinine in mg per dL. That is 0.413 times 132 divided by 1.7, which equals 0.413 times 77.6, about 32 mL per min per 1.73 m2. A stable value of 32 falls in KDIGO category G3b, which covers 30 to 59 mL per min per 1.73 m2, and at this level most renally cleared drugs need a dose reduction. [1] [4]

Plan (5). First, identify that the drug is renally cleared, so the kidney matters most. Estimate organ function with a measured height (already done). Look up the operative G3b dose in the BNFc or the local paediatric formulary rather than recalling it. Give the loading dose in full, because it depends on volume of distribution rather than clearance; then reduce the maintenance dose or extend the interval per the formulary recommendation. If the drug is high-risk (for example an aminoglycoside or vancomycin), order a level drawn at the correct time and tailor the dose to it. Document the reason the prescription is not standard on the chart, counsel the family, and recheck the dose if organ function changes. [5]

SAQ 2 — Child-Pugh grading, hepatic dose adjustment and a high-risk drug-monitoring plan (10 marks)

A 12-year-old with biliary atresia and a Kasai portoenterostomy is admitted with cholangitis. She has bilirubin 65 micromol per L, albumin 26 g per L, INR 2.1, moderate ascites, and grade 1 encephalopathy. She needs antibiotics and pain relief. [6]

Questions

1. Grade her hepatic impairment using the Child-Pugh score, and explain why the INR matters more than the ALT for dosing. (5 marks) [6]
2. Outline the hepatic dose-adjustment principles for her medicines, including which drug classes to avoid, and how you would monitor a high-risk drug such as vancomycin if it is added. (5 marks) [6] [7]

Model answer

Grading and the INR (5). The Child-Pugh score sums five variables, each scored 1 to 3: bilirubin, albumin, INR, ascites and encephalopathy. This child scores 3 for bilirubin over 51 micromol per L, 3 for albumin under 28 g per L, 2 for INR 1.7 to 2.3, 2 for moderate ascites, and 2 for grade 1 to 2 encephalopathy, giving about 12 points — Child-Pugh class C, severe hepatic impairment. The INR tracks hepatic synthetic function, which is what clears many drugs, far better than the ALT, which reflects hepatocellular injury but not clearance. A child can have a near-normal ALT and failing synthetic function, and the INR is the bedside marker that catches it. [6]

Dose-adjustment and monitoring plan (5). Treat every hepatically cleared drug as substantially impaired in Child-Pugh C. Reduce maintenance doses by around half for drugs the formulary flags, prefer renally cleared antibiotics guided by her (likely falling) eGFR, and avoid sedatives and NSAIDs unless essential. If vancomycin is added for resistant infection, monitor with area-under-the-curve guided dosing targeting 400 to 600 mg per h per L where tools are available, or a trough drawn just before the dose as a pragmatic surrogate, and recheck her renal function daily because nephrotoxin stacking in a child with multi-organ impairment is a major risk. Document the non-standard doses and involve the pharmacist. [6] [7]