Discrepant Results of Serum Creatinine and Cystatin C in a Urological Patient

A 3-month-old boy was seen for routine follow-up at the pediatric nephrology outpatient clinic. He had been diagnosed as having Sotos syndrome manifesting with craniofacial dysmorphism, feeding difficulties, pulmonary artery stenosis, and atrial septal defect, as well as complex urological abnormalities. He had bilateral hydronephrosis with megaureter and grade V vesicoureteral reflux to the left and grade I to the right kidney. At the age of 6 weeks, static renal scintigraphy using DMSA (99mTc-dimercaptosuccinic acid) to assess renal morphology, structure, and function had demonstrated almost symmetrical kidney function (split kidney function left 44% vs right 56%) without cortical scarring. While his baseline serum creatinine had been 40 μmol/L (0.45 mg/dL), a sudden rise to 69 μmol/L (0.79 mg/dL) was noted. Urinary tract infection was ruled out, as was dehydration. On renal ultrasound, dilation of the right collecting system and ureter had increased significantly and a novel fluid collection at the upper pole was noted, which prompted an MRI study (Fig. 1). In addition to serum creatinine, cystatin C measurement was ordered and was within the reference interval for age (1.13 mg/L). Fig. 1. Abdominal MRI. Bilateral hydronephrosis and fluid collection at right upper pole (asterisk). Creatinine, the most commonly used endogenous marker of glomerular filtration rate (GFR),4 has in recent years been supplemented by cystatin C. In fact, today's most reliable equations for estimating GFR both in adults (1) and in children (2) make use of a combination of the 2. Still, in certain conditions the 2 markers can give very discordant findings, which may be diagnostic, as in the case presented here. Discordant findings may occur …