Soluble alpha klotho - impact of biological variables and reference intervals for adults- Supplemental File

We confirmed good linearity for samples with soluble alpha klotho concentrations below 3000 pg/mL (recovery rate (% mean (range)): 96 (92-107)) but found that samples with concentrations exceeding 3000 pg/mL must be diluted at least 1:4 to allow further dilutions in a linear manner. In the majority of samples from healthy subjects, soluble alpha klotho remains above the limit of quantification even when diluted 1:4. Pre-analytical stability was excellent, with no significant differences in concentrations observed between samples stored at room temperature for up to 120 h before (whole blood), for up to 72 h after centrifugation (serum), submitted to up to 4 freeze/thaw cycles, or after long-term storage at -20oC (31 months). Soluble alpha klotho also did not differ between serum and EDTA plasma taken in parallel from the same individual. Soluble alpha klotho concentrations correlated mainly with age and estimated glomerular filtration rate, and might be higher in females. However, the impact of other biological variables is negligible. Our detailed reference intervals for soluble alpha klotho from a large population-based sample might facilitate its use in clinical practice.