Mesure de la parathormone : facteurs de variation et problèmes de standardisation

The parathyroid hormone (PTH) assay still involves some problems, considering the molecular heterogeneity and the different specificity of the available sandwich immunometric methods. A number of preanalytical variation factors can affect the PTH measurement: the intraindividual biological variability (pulsatile secretion, circadian rhythm) and the interindividual variability (sex, age, genetics factors), as well as the sample typology and way of storage, can be mentioned. Beside the molecular heterogeneity (C-terminal fragments, included the recently identified non-(1-84)) molecular forms which may interfere with "intact molecule" PTH methods), the factors affecting the analytical variability concern the antibody interference (anti-PTH autoandibodies and heterophilic antibodies) and particularly the calibration differences existing among the commercially available assays. In order to evaluate these differences, we performed recovery tests with both synthetic hPTH(1-84) from Bachem and recombinant hPTH(1-84) 95/646 from NIBSC. The results are similar to those recently obtained by UK-NEQAS and highlight remarkable between-method calibration differences, which are the main reason for the disagreements among PTH values obtained with different kits. On the contrary, no relationship apparently exists between the cross-reactivity with synthetic hPTH(7-84) fragment and the method bias (UK-NEQAS, 2001). Therefore the lack of an International Standard is the main obstacle to solving the problems related to PTH measurement?