Clinical and laboratory evaluation of a radioimmunoassay kit for measuring the mid-molecule fraction of parathyrin.

We report the results of a laboratory and clinical evaluation of a commercial kit procedure (Immuno Nuclear Corp.) for measuring the mid-region of the parathyrin molecule. The estimated dose at 50% binding averaged 130 pmol/L, and the minimum detectable concentration was 14 pmol/L. The within-assay CV was less than or equal to 6.6%, the between-assay CV less than or equal to 12.5%. Relative analytical recoveries of various parathyrin fragments averaged 93% (intact, amino acid residues 1-84), 100% (midregion, 44-68), less than 0.01% (N-terminal, 1-34), and less than 0.01% (C-terminal, 69-84). Correlation of results obtained for 59 patients' samples with a radioimmunoassay for midregion/C-terminal parathyrin performed by the Mayo Medical Laboratory yielded the equation INC = 1.16 (Mayo)-1.94 pmol/L (r = 0.971). Clinical evaluation indicates that the INC results for parathyrin correlate with the diagnoses of patients at least as well as the results obtained with the Mayo assay; in some cases, the INC procedure better distinguishes hyperfunction from normal parathyroid function. The INC procedure can be easily performed in hospital laboratories, with a 4-h turnaround time for results.