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Estimating Glom..

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Although serum creatinine assays are routinely used for estimating glomerular filtration rate (GFR), overdiagnosis of chronic kidney diseases can occur. Cystatin C is an alternative marker for estimating GFR.1

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Inker and colleagues2 report the development and validation of two new equations for estimating GFR using standardized cystatin C alone and using cystatin C combined with standardized creatinine in a diverse population. The authors also compared existing equations and tested their usefulness in improving the classification of patients with chronic kidney disease. 

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Estimating equations using a cross-sectional analysis were developed using cystatin C alone and in combination with creatinine in 5352 participants from 13 studies. These equations were then validated in 1119 participants from 5 different studies in which GFR had been measured.   

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The authors demonstrated that the mean measured GFRs were 68 and 70 ml per minute per 1.73 m2 of body surface area in the development and validation data sets, respectively. In the validation data set, the creatinine–cystatin C equation performed better than equations that used creatinine or cystatin C alone.

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Precision was improved with the combined equation [P = 0.001 and P<0.001]), and the results were more accurate [P<0.001 for both comparisons]. In participants whose estimated GFR based on creatinine was 45 to 74 ml per minute per 1.73 m2, the combined equation improved the classification of measured GFR [P<0.001] and correctly reclassified 16.9% who were classified as having an estimated GFR of 45 to 59 ml per minute per 1.73 m2 to having a GFR of 60 ml or higher per minute per 1.73 m2.

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The authors concluded creatinine–cystatin C equation performed better than equations based on either of these markers alone and may be useful as a confirmatory test for chronic kidney disease.

1. Levey AS, Stevens LA, Schmid CH, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med 2009;150:604-12.   [PubMed: 19414839]
2. Inker LA, Schmid CH, Tighiouart H, et al. Estimating glomerular filtration rate from serum creatinine and cystatin C. N Engl J Med. 2012 Jul 5;367(1):20-9.   [PubMed: 22762315]

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