Urinalysis, renal function and paediatric kidney assessment — formative SAQs

SAQ 1 — Bedside Schwartz eGFR and staging (10 marks)

A 6-year-old boy is 115 cm tall. His serum creatinine is 0.9 mg per dL, reported on a modern enzymatic assay. He is well, and the test was done as follow-up for an isolated abnormality found at screening. ^{[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. Explain why a measured height is essential, and what happens if the original 0.55 constant is used instead of 0.413. (3 marks) ^[1]

3. State one reason the estimate might be inaccurate despite correct arithmetic. (2 marks) ^[1]

Model answer

Calculation (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 115 divided by 0.9, equal to 0.413 times 127.8, about 53 mL per min per 1.73 m2. A stable value of 53 falls in KDIGO category G3a, which covers 45 to 59 mL per min per 1.73 m2, and warrants nephrology referral. ^{[1] [4]}

Height and constant (3). Height replaces muscle mass in the equation, so a guessed height produces a guessed GFR. The original 0.55 constant, or 0.45 for infants, was derived with older creatinine assays that read higher than current enzymatic methods; using 0.55 today would overestimate the GFR and falsely reassure. ^[1]

Inaccuracy (2). Reduced muscle mass, such as in neuromuscular disease or cachexia, lowers creatinine generation and drifts the estimate upward; cystatin C would then be a better cross-check. ^[1]