Two Novel Equations to Estimate Kidney Function in Persons Aged 70 Years or Older

Background: In older adults, current equations to estimate glomerular filtration rate (GFR) are not validated and may misclassify elderly persons in terms of their stage of chronic kidney disease. Objective: To derive the Berlin Initiative Study (BIS) equation, a novel estimator of GFR in elderly participants. Design: Cross-sectional. Data were split for analysis into 2 sets for equation development and internal validation. Setting: Random community-based population of a large insurance company. Participants: 610 participants aged 70 years or older (mean age, 78.5 years). Intervention: Iohexol plasma clearance measurement as gold standard. Measurements: GFR, measured as the plasma clearance of the endogenous marker iohexol, to compare performance of existing equations of estimated GFR with measured GFR of the gold standard; estimation of measured GFR from standardized creatinine and cystatin C levels, sex, and age in the learning sample; and comparison of the BIS equations (BIS1: creatinine-based; BIS2: creatinine- and cystatin C-based) with other estimating equations and determination of bias, precision, and accuracy in the validation sample. Results: The new BIS2 equation yielded the smallest bias followed by the creatinine- based BIS1 and Cockcroft-Gault equations. All other equations considerably overestimated GFR. The BIS equations confirmed a high prevalence of persons older than 70 years with a GFR less than 60 mL/min per 1.73 m(2) (BIS1, 50.4%; BIS2, 47.4%; measured GFR, 47.9%). The total misclassification rate for this criterion was smallest for the BIS2 equation (11.6%), followed by the cystatin C equation 2 (15.1%) proposed by the Chronic Kidney Disease Epidemiology Collaboration. Among the creatinine- based equations, BIS1 had the smallest misclassification rate (17.2%), followed by the Chronic Kidney Disease Epidemiology Collaboration equation (20.4%). Limitation: There was no validation by an external data set. Conclusion: The BIS2 equation should be used to estimate GFR in persons aged 70 years or older with normal or mild to moderately reduced kidney function. If cystatin C is not available, the BIS1 equation is an acceptable alternative.