Evaluation of a New Automated Chemiluminescence Immunoassay for FGF23

Fibroblast growth factor 23 (FGF23) is a hormone regulating phosphate and vitamin D metabolism. We have previously established a sandwich enzyme-linked immunosorbent assay (ELISA) for FGF23 and reported that FGF23 values are useful for the differential diagnosis of chronic hypophosphatemia. However, this ELISA has a rather narrow assay range of 3–800 pg/ml, and it was pointed out that the assay performance is not satisfactory when automatic washing is used. Here we evaluated a new automated chemiluminescence immunoassay for FGF23. This assay uses 10 μl sera or plasma samples and requires 20 min to obtain the first result. The assay was linear up to about 15,000 pg/ml and had a detection limit of 1 pg/ml. In addition, this assay showed coefficients of variation of less than 5% using samples with average FGF23 levels of 43.2–2,454.0 pg/ml. When FGF23 levels in 210 samples from chronic hypophosphatemic patients were evaluated by both the previous ELISA and this new assay, there was a good correlation of R 2 = 0.96. However, FGF23 levels by the new assay showed lower values, especially in samples with high FGF23 levels. Given that the lowest FGF23 level in patients with FGF23-related hypophosphatemia was 30.8 pg/ml and that the highest FGF23 levels in patients with non-FGF23-related hypophosphatemia was 20.8 pg/ml by this novel assay, the sensitivity and specificity were 100% when the cutoff was set between 20.8 and 30.8 pg/ml. From the aspect of convenience and the coefficients of variation of this assay, we propose that the cutoff be 25 pg/ml. There results indicate that this new assay is ideal for both clinical use and clinical studies, especially when measuring many samples with high FGF23 levels.