In an observational study of 180 healthy adults and 18 adults with stage 3–5 CKD, Smith et al. determined that there was a high variability in FGF23 measures, specifically iFGF23, as a diagnostic tool, suggesting that risk-related thresholds of cFGF23 measures may be more appropriate for clinical decision-making [130]. Here, FGF23 is linked to chronic kidney disease.