An automated 'bio-intact' PTH assay: a step towards standardisation and improved correlation with parathyroid...

Most methods for the determination of parathyroid hormone (PTH) show cross-reactivity with N-truncated forms of PTH. The analytic and diagnostic value of a recently developed automated PTH test without this cross-reactivity was examined.

The PTH levels of 73 patients undergoing hemodialysis were compared using the 'bio-intact' PTH (Nichols Institute Diagnostics) and 3 'intact' PTH tests (from Nichols, Roche Elecsys and Diagnostics Corporation DPC). Further, the (non 1-84) PTH fragment and the PTH ratio (bio-intact PTH/(non 1-84) PTH) were calculated. All results were then correlated with biochemical bone markers (bone-specific alkaline phosphatase and bone collagen C-terminal telopeptides in serum).

'Bio-intact' PTH values were lower than the PTH results generated by the 'intact' PTH assays. Results of all PTH tests were closely correlated (r=0.96-0.98, p<0.01). Correlations with biochemical bone markers were high (r=0.31-0.63, p<0.01), but no significant association between the PTH ratio and all other tests (r=-0.2 to 0.03) was found.

In stable hemodialysis patients, the different PTH tests show a similar correlation with the bone markers. It is however desirable to measure PTH with assays devoid of any cross-reaction for a better comparability. In this study, the PTH ratio was not correlated with biochemical bone markers; the use of this ratio requires further investigation.