Reference interval for serum cystatin C in children.

Serum creatinine and creatinine clearance are widely used as measures of glomerular filtration rate (GFR) in clinical medicine. Unfortunately, serum creatinine concentrations are not determined only by glomerular filtration (1 ). Alteration in renal handling and metabolism of creatinine and methodological interferences in its measurement may influence the concentrations of serum creatinine (2 ). Creatinine production is proportional to muscle mass (2 ). In children, the muscle mass increases significantly with linear growth, and serum creatinine concentrations have to be adjusted for body height and body size to reflect the renal function (3, 4). Cystatin C, a nonglycosylated low molecular weight protein (Mr 13.359) (5 ), is a proteinase inhibitor involved in the intracellular catabolism of proteins (6 ). Unlike creatinine, cystatin C is produced in all investigated nucleated cells at a constant rate, freely filtered in the renal glomeruli and almost completely reabsorbed and catabolized in the renal proximal tubular cells (7, 8). Recent studies have indicated that serum cystatin C can be used as an endogenous marker of GFR in adults (9, 10) and is a promising marker in children (11–13). The aim of the study was to establish a reference interval for serum cystatin C in children without evidence of kidney disease. One hundred and thirty-seven children (79 boys and 58 girls) of ages between 7 days and 14.1 years (3.2 6 3.5 years; mean 6 SD) with body weight .1.5 kg and without clinical evidence of kidney diseases were included in the study (Table 1). Twenty-nine children had pneumonia, 25 viral infections, 32 other infectious diseases, 23 bronchial asthma, and 28 other noninfectious diseases. The children’s parents gave their informed consents according to the Declaration of Helsinki, and the study was approved by the local committee of ethics. During a period of 5 months, blood samples were collected randomly from children acutely hospitalized in the Department of Pediatrics or visiting the outpatient pediatric clinic, Viborg Hospital. Leftover serum after routine chemistry measurements was used to analyze cystatin C and creatinine. Serum cystatin C was analyzed using the N Latex Cystatin C assay on the Dade Behring Nephelometer II (BN II; Dade Behring Diagnostics). The BN II was programmed and calibrated according to the instructions from the manufacturer. By using the BN II pediatric sample rack, the assay could be performed with a total sample volume of 70 mL (instrument dead volume, 30 mL; sample volume, 40 mL). The measuring range was 0.25–7.90 mg/L, with the default dilution 1:100 of the sample. The detection limit was 0.05 mg/L, corresponding to a minimum dilution 1:20 of the sample. The coefficient of variation for within-run and between-run imprecision studies was between 1.1% and 1.8% in serum pools with cystatin C concentrations between 0.87 and 4.63 mg/L (14 ). No interferences from hemoglobin, bilirubin, and lipemic samples have been demonstrated (14 ). Serum creatinine was analyzed using the Vitros CREA slide, an enzymatic method, on the Vitros 950 Chemistry System (Ortho-Clinical Diagnostics). The CV for withinrun and between-run imprecision studies was between 0.4% and 1.0% using the Vitros Performance Verifier I and II as controls (15 ). The measuring range was 4–1238 mmol/L. Estimated GFR was calculated using the Morris formula (4 ) in children .2 years: GFR (mL/min per 1.73 m) 5 [40 3 height (cm)]/serum creatinine (mmol/L). All data are expressed as mean 6 SD. Statistical analysis was performed using GraphPad Prism Ver. 2.01 for Windows NT (GraphPad Software, Inc.). Nonparametric reference intervals were calculated using GraphROC for Windows Ver. 2.0 (developed by Veilo Kairisto, Turku, Finland and Allan Poola, Tallin, Estonia). P ,0.05 was considered significant. The serum concentrations of cystatin C were highest after birth, followed by a decrease over the following weeks (Fig. 1a). In the group of children younger than 1 month, the serum concentrations of cystatin C were 1.63 6 0.26 mg/L (mean 6 SD), and in the group of