Major method-specific differences in the measurement of intact parathyroid hormone: studies in patients with and without chronic renal failure.

Background: Following the introduction of two-site immunometric assays for parathyroid hormone (PTH), the expectation of good inter-assay agreement has not been fulfilled. The reasons for this may include differences in standardization as well as fragment recognition between the assays.

Methods: PTH values for healthy individuals, patients with renal failure and patients with normal renal function and elevated parathyroid hormone (hPTH) were compared using two commercial two-site immunochemiluminometric assays (Bayer Magic-lite and DPC Immulite 2000).

Results: Immulite results had a mean value 50.4% greater than the corresponding Magic-lite values for the whole study population with individual values ranging from 17.5% below to 118.3% above the corresponding Magic-lite value. There was no significant difference in inter-assay bias between patients with renal failure and those with normal renal function, suggesting that variable cross-reactivity with circulating disease-specific PTH fragments was not the primary cause of the observed discrepancy. Cross-reactivity with the synthetic fragment hPTH (7-84) was 34+/-5% for Magic-lite and 62+/-2% for Immulite. We also studied the stability of synthetic hPTH on storage.

Conclusion: The instability of synthetic hPTH over extended storage periods may affect primary standard material. The consistent inter-assay differences and the over-recovery observed in external quality assessment programmes for the Immulite assay may have best been explained by differences in calibration and the relative cross-reactivities and/or kinetics of the two assay systems for specific parathyroid fragments.