Soluble IL-15/IL-15Rα protein standard for the heterodimeric IL-15/IL-15Rα ELISA. Standard curve for human heterodimeric IL-15/IL-15Rα ELISA. HPLC-purified human IL-15 and sIL-15Rα were quantified by amino acid mass analysis and mixed in a molar ratio 1:1. Triplicates of 3-fold serial dilutions of IL-15/IL-15Rα complexes were assayed in the heterodimeric IL-15/IL-15Rα ELISA. Circles represent RLU average for each IL-15/IL-15Rα concentration tested from 5 independent assays; SD for the 5 assays is shown. The intra-assay variability was 3% to 12% coefficient of variation (CV), and the interassay variability was 18% to 33% CV. Open squares represent RLU values of the indicated amounts of single-chain IL-15 tested in the heterodimeric IL-15/IL-15Rα ELISA (negative control). Linear regression analysis showed no significant correlation (r² = 0.32, P = .32)