The clinical utility and cost impact of cystatin C measurement in the diagnosis and management of chronic kidney disease: A primary care cohort study

Background To reduce over-diagnosis of chronic kidney disease (CKD) resulting from the inaccuracy of creatinine-based estimates of glomerular filtration rate (GFR), UK and international guidelines recommend that cystatin-C-based estimates of GFR be used to confirm or exclude the diagnosis in people with GFR 45-59 ml/min/1.73 m(2) and no albuminuria (CKD G3aA1). Whilst there is good evidence for cystatin C being a marker of GFR and risk in people with CKD, its use to define CKD in this manner has not been evaluated in primary care, the setting in which most people with GFR in this range are managed. Methods and findings A total of 1,741 people with CKD G3a or G3b defined by 2 estimated GFR (eGFR) values more than 90 days apart were recruited to the Renal Risk in Derby study between June 2008 and March 2010. Using Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations, we compared GFR estimated from creatinine (eGFR(creat)), cystatin C (eGFR(cys)), and both (eGFR(creat-cys)) at baseline and over 5 years of follow-up. We analysed the proportion of participants with CKD G3aA1 reclassified to 'no CKD' or more advanced CKD with the latter two equations. We further assessed the impact of using cystatin-C-based eGFR in risk prediction equations for CKD progression and all-cause mortality and investigated non-GFR determinants of eGFR(cys). Finally, we estimated the cost implications of implementing National Institute for Health and Care Excellence (NICE) guidance to use eGFR(cys) to confirm the diagnosis in people classified as CKD G3aA1 by eGFR(creat). Mean eGFR(cys) was significantly lower than mean eGFR(creat) (45.1 ml/min/1.73 m(2), 95% CI 44.4 to 45.9, versus 53.6 ml/min/1.73 m(2), 95% CI 53.0 to 54.1, P < 0.001). eGFR(cys) reclassified 7.7% (50 of 653) of those with CKD G3aA1 by eGFR(creat) to eGFR >= 60 ml/min/1.73 m(2). However, a much greater proportion (59.0%, 385 of 653) were classified to an eGFR category indicating more severe CKD. A similar pattern was seen using eGFR(creat-cys), but lower proportions were reclassified. Change in eGFR(creat) and eGFR(cys) over 5 years were weakly correlated (r = 0.33, P < 0.001), but eGFRcys identified more people as having CKD progression (18.2% versus 10.5%). Multivariable analysis using eGFR(creat) as an independent variable identified age, smoking status, body mass index, haemoglobin, serum uric acid, serum albumin, albuminuria, and C reactive protein as non-GFR determinants of eGFR(cys). Use of eGFR(cys) or eGFR(creat-cys) did not improve discrimination in risk prediction models for CKD progression and all-cause mortality compared to similar models with eGFR(creat). Application of the NICE guidance, which assumed cost savings, to participants with CKD G3aA1 increased the cost of monitoring by 23 pound per patient, which if extrapolated to be applied throughout England would increase the cost of testing and monitoring CKD by approximately 31 pound million per year. Limitations of this study include the lack of a measured GFR and the potential lack of ethnic diversity in the study cohort. Conclusions Implementation of current guidelines on eGFR(cys) testing in our study population of older people in primary care resulted in only a small reduction in diagnosed CKD but classified a greater proportion as having more advanced CKD than eGFR(creat). Use of eGFR(cys) did not improve risk prediction in this population and was associated with increased cost. Our data therefore do not support implementation of these recommendations in primary care. Further studies are warranted to define the most appropriate clinical application of eGFR(cys) and eGFR(creat-cys).