Development and Validation of Residual Kidney Function Estimating Equations in Dialysis Patients

RATIONALE & OBJECTIVE: Measurement of residual kidney function is recommended for the adjustment of the dialysis prescription, but timed urine collections are difficult and prone to errors. Equations to calculate residual kidney function from serum concentrations of endogenous filtration markers and demographic parameters would simplify monitoring of residual kidney function. However, few equations to estimate residual kidney function using serum concentrations of small solutes and low-molecular-weight proteins have been developed and externally validated. STUDY DESIGN: Study of diagnostic test accuracy. SETTING & PARTICIPANTS: 823 Chinese peritoneal dialysis (PD) patients (development cohort) and 826 PD and hemodialysis patients from the Netherlands NECOSAD study (validation cohort). TESTS COMPARED: Equations to estimate residual kidney function (estimated clearance [eCl]) using serum creatinine, urea nitrogen, cystatin C, β(2)-microglobulin (B2M), β-trace protein (BTP), and combinations, as well as demographic variables (age, sex, height, and weight). Equations were developed using multivariable linear regression analysis in the development cohort and then tested in the validation cohort. Equations were compared with published validated equations. OUTCOMES: Residual kidney function measured as urinary clearance (mCl) of urea nitrogen (mCl(UN)) and average of creatinine and urea nitrogen clearance (mCl(UN-cr)). RESULTS: In external validation, bias (difference between mCl and eCl) was within ± 1.0 unit for all equations. Accuracy (percent of differences within ± 2.0 units) was significantly better for eCl(BTP), eCl(B2M), and eCl(BTP-B2M) than eCl(UN-cr) for both mCl(UN) (78%, 80%, and 81% vs 72%; P < 0.05 for all) and mCl(UN-cr) (72%, 78%, and 79% vs 68%; P < 0.05 for all). The area under the curve for predicting mCl(UN) > 2.0 mL/min was highest for eCl(B2M) (0.853) and eCl(BTP-B2M) (0.848). Results were similar for other validated equations. LIMITATIONS: Development cohort only consisted of PD patients, no gold-standard method for residual kidney function measurement. CONCLUSIONS: These results confirm the validity and extend the generalizability of residual kidney function estimating equations from serum concentrations of low-molecular-weight proteins without urine collection.