Method-specific differences in plasma fibroblast growth factor 23 measurement using four commercial ELISAs

Area of Science:

• Biochemistry
• Clinical Chemistry
• Assay Development

Background:

• Growing interest in plasma fibroblast growth factor 23 (FGF23) measurement across clinical settings.
• Lack of comparative data for commercially available intact and C-terminal FGF23 assays.
• FGF23 plays a critical role in mineral and bone metabolism.

Purpose of the Study:

• To compare the analytical performance of four commercial FGF23 assays.
• Evaluate precision, recovery, linearity, and pre-analytical stability of intact and C-terminal FGF23 assays.
• Assess method agreement using Passing-Bablok regression and difference plots.

Main Methods:

• Analysis of plasma samples from healthy adults and chronic hemodialysis patients (n=67).
• Comparison of four commercial FGF23 assays (Kainos, Millipore, Immutopics Inc.).
• Evaluation of assay characteristics including precision, recovery, linearity, and stability.

Main Results:

• Significant negative proportional bias and poor recovery observed for Millipore and Immutopics intact FGF23 kits compared to Kainos.
• Immutopics C-terminal FGF23 showed strong association with intact FGF23 in hemodialysis patients but not at physiological levels.
• Intact FGF23 measurements from the Immutopics assay exhibited instability after 8 hours.

Conclusions:

• Current ELISA kits for plasma intact FGF23 demonstrate poor analytical agreement and are not interchangeable due to calibration differences.
• Harmonization of assays using a common international standard is needed for consistent data interpretation.
• Discordance between intact and C-terminal FGF23 assays is more pronounced at physiological concentrations.