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

• Published in: Annals of clinical biochemistry 2004-03, Vol.41 (2), p.149-154
• Main Authors: Worth, G K, Vasikaran, S D, Retallack, R W, Musk, A A, Gutteridge, D H
• Format: Article
• Language: English
• Subjects: Humans; Immunoassay - methods; Kidney Failure, Chronic - blood; Parathyroid Hormone - blood; Reference Standards; Reference Values; Reproducibility of Results; Research Design - standards; Sensitivity and Specificity
• ISSN: 0004-5632; 1758-1001
• DOI: 10.1258/000456304322880050

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.