PERFORMANCE-CHARACTERISTICS OF DIFFERENT IMMUNOASSAYS FOR DETERMINATION OF PARATHYRIN (1-84) IN HUMAN PLASMA SAMPLES

The performance characteristics of four radioisotopically and non-radi oisotopically labelled two-site immunoassays for the determination of 'intact' parathyrin in plasma samples are reported. Within-run as well as between-assay imprecision was characterized by coefficients of var iation usually < 10%. Assessment of the linearity of dilution in plasm a samples from patients with severe secondary hyperparathyroidism (obt ained from patients with chronic renal failure prior to dialysis) reve aled that an assay with N-terminal capture antibodies showed an increa se of the values after dilution (p < 0.05) whereas another assay with C-terminal capture. antibodies was characterized by a decrease of the values after dilution (p < 0.05). Correlation between the data obtaine d by the four assays and our currently used routine method (N-tact(R) PTH from INCSTAR) revealed correlation coefficients of r > + 0.96 and slope values between 0.83 and 1.34. Determination of the analytical re covery of parathyrin (1-84) from two reference materials [First Intern ational Reference Preparation 79/500 and synthetic human parathyrin (1 - 84)] revealed that the recovery rates were strongly influenced by ( a) the assay employed for determination of parathyrin concentrations, (b) the matrix of the diluent and (c) the reference material used. The se results as well as systematic differences between the assays we exa mined (employing plasma samples from unselected nephrological patients ) require further efforts towards a more rigorous standardization of ' intact' parathyrin assays.