Non-GFR Determinants of Low-Molecular-Weight Serum Protein Filtration Markers in the Elderly: AGES-Kidney and MESA-Kidney

Background Studies in chronic kidney disease populations suggest that the non–glomerular filtration rate (GFR) determinants of serum levels of the low-molecular-weight protein filtration markers cystatin C, β 2 -microglobulin (B2M), and beta-trace protein (BTP) are less affected by age, sex, and ethnicity than those of creatinine. Study Design Cross-sectional study. Setting & Participants Predominantly elderly participants selected from the Age, Gene/Environment Susceptibility Kidney Study (AGES-Kidney; N=683; mean [SD] age, 79 [4] years; GFR, 62 [17]mL/min/1.73 m 2 ) and from the Multi-Ethnic Study of Atherosclerosis Kidney Study (MESA-Kidney; N=273; mean [SD] age, 70.5 [9] years; GFR, 73 [19]mL/min/1.73 m 2 ). Predictors Demographic and clinical factors hypothesized to be associated with conditions affecting non-GFR determinants of the filtration markers. Outcomes Measured GFRs and estimated GFRs (eGFRs) based on creatinine, cystatin C, B2M, and BTP levels (eGFR cr, eGFR cys , eGFR B2M , and eGFR BTP , respectively). Residual associations of factors with eGFR after accounting for measured GFR as the parameter of interest. Results eGFR cys , eGFR B2M , and eGFR BTP had significantly less strong residual associations with age and sex than eGFR cr in both AGES-Kidney and MESA-Kidney and were not associated with ethnicity (black vs white) in MESA-Kidney. After adjusting for age, sex, and ethnicity, residual associations with most clinical factors were smaller than observed with age and sex. eGFR cys and eGFR B2M , but not eGFR BTP , had significant residual associations with C-reactive protein levels in both studies. Limitations Small sample size may limit power to detect associations. Participants may be healthier than the general population. Conclusions Similar to previous studies in chronic kidney disease, in community-dwelling elders, cystatin C, B2M, and BTP levels are less affected than creatinine level by age and sex and are not affected by ethnicity. Both cystatin C and B2M levels may be affected by inflammation. These findings are important for the development and use of GFR estimating equations based on low-molecular-weight serum proteins throughout the range in GFRs.