Sharma S, Katz R, Bullen AL, et al. - In this case-cohort from the Cardiovascular Health Study, researchers ascertained if iron deficiency, inflammation, and kidney function account for differences in the strengths of associations between these two fibroblast growth factor-23 (FGF23) assays with clinical outcomes. Eight hundred forty-four community-dwelling people aged ≥ 65 years with and without chronic kidney disease who were followed for 10 years. Findings suggested an association of FGF23 measured by either assay with mortality in unadjusted analysis. MI risk was associated with neither C-terminal nor intact FGF23. Depending upon the type of FGF23 assay utilized, the relationship of FGF23 with clinical endpoints is markedly different. Kidney disease may be confounded by the interaction of biologically active FGF23 with clinical endpoints, and therefore much weaker than previously believed.