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.
