Performance evaluation of the new chemiluminescent intact FGF23 assay relative to the existing assay system

Introduction This study assessed the performance of a new fully automated immunoassay for fibroblast growth factor (FGF) 23 (Determinar CL FGF23 CL) among healthy individuals and those with chronic hypophosphatemia compared with the previous assay (Kainos FGF23 KI). Materials and methods A total of 380 serum samples from healthy participants were collected to determine the reference range of FGF23 levels with CL. A total of 200 serum samples from 22 hypophosphatemic patients were collected simultaneously to compare the difference in FGF23 levels between CL and KI. The Mann-Whitney U test and linear regression analysis were adopted to assess the differences and linearity between the two assays. Results The median FGF23 levels among healthy individuals was 31.7 (interquartile: 26.4-37.5) pg/mL. When the reference range was calculated as the mean +/- 2 standard deviation (2SD), it was 16.1-49.3 pg/mL. A total of 363 individuals (96%) among normal cases fell in this range. Among 200 samples from patients with chronic hypophosphatemic disorder, the median FGF23 levels analyzed by CL and KI were 123.0 (90.2-237.7) and 172.5 (115.8-290.7) pg/mL. KI yielded significantly higher FGF23 values than CL (p < 0.001). A linear regression model revealed the correlation between KI (x) and CL (y), which had a slope of 0.76 with a y-intercept of -0.32 and high linearity (R-2 = 0.99). Conclusion The new measurement kit yielded lower FGF23 values when compared with the previous assay. Clinicians should consider this discrepancy when they assay intact FGF23 values with CL.

Automated summary

The study evaluated a new fully automated immunoassay for FGF23 and compared it with a previous assay. The new assay yielded lower FGF23 values compared to the previous one, which clinicians should consider when assessing intact FGF23 values.