Supplementary Material for: A Risk Score to Guide Cystatin C Testing to Detect Occult-Reduced Estimated Glomerular Filtration Rate

<strong><em>Background/Aims:</em></strong> Persons with occult-reduced estimated glomerular filtration rate (eGFR <60 ml/min/1.73 m<sup>2</sup> detected by serum cystatin C but missed by creatinine) have high risk for complications. Among persons with preserved kidney function by creatinine-based eGFR (eGFRcreat >60 ml/min/1.73 m<sup>2</sup>), tools to guide cystatin C testing are needed. <b><i>Methods:</i></b> We developed a risk score to estimate an individual's probability of reduced eGFR by cystatin C (eGFRcys <60 ml/min/1.73 m<sup>2</sup>) in The Reasons for Geographic and Racial Differences in Stroke (REGARDS) study and externally validated in the Third National Health and Nutrition Examination Survey (NHANES III). We used logistic regression with Bayesian model averaging and variables available in practice. We assessed performance characteristics using calibration and discrimination measures. <b><i>Results:</i></b> Among 24,877 adults with preserved kidney function by creatinine, 13.5% had reduced eGFRcys. Older and Black participants, current smokers and those with higher body mass index, lower eGFRcreat, diabetes, hypertension and history of cardiovascular disease were more likely to have occult-reduced eGFR (p < 0.001). The final risk function had a c-statistic of 0.87 in REGARDS and 0.84 in NHANES. By risk score, 72% of occult-reduced eGFR cases were detected by screening only 22% of participants. <b><i>Conclusions:</i></b> A risk score using characteristics readily accessible in clinical practice can identify the majority of persons with reduced eGFRcys, which is missed by creatinine.<br>