Renal and Genitourinary Disorders
Estimating function and adjusting doses
The bedside Schwartz equation estimates GFR: 0.413 × height in cm ÷ serum creatinine. Neonates and small infants normally have lower GFR, so a creatinine that looks normal for an adult may still reflect reduced clearance. Adjust renally cleared drugs by interval extension first (aminoglycosides, vancomycin) and dose reduction when necessary. Avoid NSAIDs and aminoglycosides where possible, and hold ACE inhibitors or ARBs during acute illness with dehydration because of acute kidney injury risk. Combining trimethoprim-sulfamethoxazole with an ACE inhibitor or ARB raises hyperkalemia and creatinine concerns.
Nephrotic syndrome
First episode: prednisone 60 mg/m2/day (max 60 mg/day) for 6 weeks, then 40 mg/m2 on alternate days for 6 weeks. Relapses are treated with a shorter high-dose course. Counsel on sodium restriction, infection risk, and, during remission, that live vaccines should be deferred until off steroids.
UTIs and enuresis
Treat febrile UTIs according to local resistance: cephalexin, amoxicillin-clavulanate, or trimethoprim-sulfamethoxazole. Avoid nitrofurantoin in G6PD deficiency, in infants under 1 month, and when fever suggests pyelonephritis, because tissue penetration is inadequate. Nocturnal enuresis: desmopressin can cause dilutional hyponatremia, so restrict fluids after dosing; oxybutynin manages overactive bladder.
Hyperkalemia
Sequence: calcium gluconate for cardiac membrane stabilization, then insulin with glucose and nebulized albuterol to shift potassium intracellularly, then a binder or dialysis for elimination.
Worked example
A 100 cm child with serum creatinine 1.2 mg/dL: eGFR = 0.413 × 100 ÷ 1.2 = approximately 34 mL/min/1.73 m2, which is stage 3 chronic kidney disease. For a renally cleared drug, extend the interval rather than simply reducing the dose, and recheck levels.