Performance characteristics of different immunoassays for determination of parathyrin (1-84) in human plasma samples

The performance characteristics of four radioisotopically and non-radioisotopically 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 variation usually < 10%. Assessment of the linearity of dilution in plasma samples from patients with severe secondary hyperparathyroidism (obtained from patients with chronic renal failure prior to dialysis) revealed that an assay with N-terminal capture antibodies showed an increase 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 obtained by the four assays and our currently used routine method (N-tact PTH from INCSTAR) revealed correlation coefficients of r > + 0.96 and slope values between 0.83 and 1.34. Determination of the analytical recovery of parathyrin (1-84) from two reference materials [First International 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. These results as well as systematic differences between the assays we examined (employing plasma samples from unselected nephrological patients) require further efforts towards a more rigorous standardization of 'intact' parathyrin assays.