Supplementary Material for: Combining Cystatin C and Creatinine Yields a Reliable Glomerular Filtration Rate Estimation in Older Adults in Contrast to β-Trace Protein and β2-Microglobulin

<p><b><i>Background:</i></b> The glomerular filtration rate (GFR) is the most important measure of kidney function and chronic kidney disease (CKD). This study aims to validate commonly used equations for estimated GFR (eGFR) based on creatinine (cr), cystatin C (cys), β-trace protein (BTP), and β2-microglobulin (B2M) in older adults. <b><i>Method:</i></b> We conducted a validation study with 126 participants aged between 72 and 98 with a mean measured GFR (mGFR) by iohexol clearance of 54 mL/min/1.73 m². The eGFR equations (CKD-Epidemiology collaboration [CKD-EPI], Berlin Initiative Study [BIS], Full Age Spectrum [FAS], Modification of Diet in Renal Disease [MDRD]_cr, Caucasian-Asian-Pediatric-Adult [CAPA]_cys, Lund-Malmö Revised [LM-REV]_cr, and MEAN-LM-CAPA_cr-cys), were assessed in terms of bias (median difference: eGFR-mGFR), precision (interquartile range of the differences), and accuracy (P30: percentage of estimates ±30% of mGFR). The equations were compared to a benchmark equation: CKD-EPI_cr-cys. <b><i>Results:</i></b> All cystatin C-based equations underestimated the GFR compared to mGFR, whereas bias was mixed for the equations based only on creatinine. Accuracy was the highest for CKD-EPI_cr-cys (98%) and lowest for MDRD (82%). Below mGFR 45 mL/min/1.73 m² only equations incorporating cystatin C reached P30 accuracy >90%. CKD-EPI_cr-cys was not significantly more accurate than the other cystatin C-based equations. In contrast, CKD-EPI_cr-cys was significantly more accurate than all creatinine-based equations except LM-REV_cr. <b><i>Conclusion:</i></b> This study confirms that it is reasonable to use equations incorporating cystatin C and creatinine in older patients across a wide spectrum of GFR. However, the results call into question the use of creatinine alone below mGFR 45 mL/min/1.73 m². B2M and BTP do not demonstrate additional value in eGFR determination in older adults.