CAP survey of parathyroid hormone assays

Parathyroid hormone (PTH) analysis was recently added to the College of American Pathologists ligand assay survey. Synthetic whole-molecule PTH was used as the test hormone. This material was found to be stable in lyophilized serum for at least 10.7 days at 35°C and 3.9 days at 45° C. The survey results showed wide ranges of answers yielding within-assay coefficients of variation from 12% to 93%. The median percentage of the spiked hormone that was recovered by the various assay methods ranged from 24% to 390%. Separation of the spiked and endogenous measured PTH concentrations ranged from a twofold separation to an approximate 50% overlap of results. The survey may not be a valid indicator of PTH assay efficacy since whole-molecule human PTH may not be the most clinically important form of circulating PTH; however, the survey shows that there is a large heterogeneity in commercial PTH assays.