Clinical Usefulness of Cystatin C for the Estimation of Glomerular Filtration Rate in Type 1 Diabetes
Reproducibility and accuracy compared with standard measures and iohexol clearance
- G.D. Tan, MRCP, DTM&H1,
- A.V. Lewis, MSC2,
- T.J. James, MSC, FIBMS2,
- P. Altmann, MD, FRCP3,
- R.P. Taylor, PHD, FRCPATH2 and
- J.C. Levy, MD, FRCP1
- 1Oxford Centre for Diabetes, Endocrinology & Metabolism, Oxford, U.K
- 2Department of Clinical Biochemistry, Oxford Radcliffe Hospitals, Oxford, U.K
- 3Oxford Kidney Unit, Oxford Radcliffe Hospitals, Oxford, U.K
Abstract
OBJECTIVE—Assessment and follow-up of early renal dysfunction is important in diabetic nephropathy. Plasma creatinine is insensitive for a glomerular filtration rate (GFR) >50 ml/min and creatinine clearance is unwieldy and subject to collection inaccuracies. We aimed to assess the reproducibility, reliability, and accuracy of plasma cystatin C as a measure of GFR ranging from normal to moderate impairment due to type 1 diabetes in the presence of a normal plasma creatinine concentration.
RESEARCH DESIGN AND METHODS—A sensitive immunoturbidimetric cystatin C assay was examined in 29 subjects with type 1 diabetes and 11 nondiabetic subjects. Duplicate measurements of the following were collected from each subject, 2 weeks apart: cystatin C, enzymatic plasma creatinine, 24-h creatinine clearance, GFR estimated from plasma creatinine by the Cockcroft-Gault equation, and iohexol clearance as a gold standard.
RESULTS—Iohexol clearance ranged from 35 to 132 ml · min−1 · 1.73 m−2. Plasma cystatin C compared well with the other clinically used tests. The reliability of cystatin C, as assessed by the discriminant ratio, was superior to creatinine clearance (3.4 vs. 1.5, P < 0.001) and the correlation of cystatin C with iohexol clearance (Rs −0.80) was similar to that of creatinine clearance (Rs −0.74) and superior to that of plasma creatinine and the Cockcroft-Gault estimate (Rs −0.54 and 0.66, respectively). Duplicate estimations were used to provide an unbiased equation to convert plasma cystatin C to GFR.
CONCLUSIONS—Based on this study, cystatin C is a more reliable measure of GFR than creatinine clearance, is more highly correlated with iohexol clearance than plasma creatinine, and is worthy of further investigation as a clinical measure of GFR in type 1 diabetes.
- CV, coefficient of variation
- DR, discriminant ratio
- GFR, glomerular filtration rate
- GFR-CG, GFR calculated by the Cockcroft-Gault method
- GFR-IO, GFR as measured by iohexol clearance
- SDU, underlying between-subject SD
- SDW, within-subject SD
Footnotes
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Address correspondence and reprint requests to Dr. J. C. Levy, Oxford Centre for Diabetes, Endocrinology and Metabolism, The Radcliffe Infirmary, Oxford OX2 6HE, U.K. E-mail: jonathan.levy{at}drl.ox.ac.uk.
Received for publication 11 December 2001 and accepted in revised form 13 July 2002.
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances.
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