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Renal osteodystrophy may present with low, normal, or high bone turnover. An ideal parathyroid hormone (PTH) assay should discriminate between the bioactive whole PTH-(1-84) molecule and PTH fragments, including the PTH-(7-84) fragment. Most dialysis patients have "intact" PTH (iPTH) levels between 65 and 450 pg/ml, which are poorly predictive of bone turnover state, making the iPTH test of limited value for bone turnover prediction. iPTH levels higher than 500 pg/ml can be observed in some dialysis patients with low bone turnover, while iPTH levels as low as 100 pg/ml have been found in patients with bone turnover above normal, indicating the standard second generation iPTH assay is not a reliable sole indicator of bone turnover. The whole PTH immunoradiometric assay, a third generation assay, uses a detection antibody that recognizes antigenic determinants at the extreme amino-terminal (1-4) end of the PTH molecule, making the assay specific for biologically active whole PTH-(1-84). Comparing results using the whole PTH and iPTH assays, the PTH-(7-84) level is indirectly determined and the PTH-(1-84)/PTH-(7-84) ratio can be calculated. It was shown that PTH-(7-84) inhibits the calcemic effect of PTH-(1-84) and its stimulatory effect on bone turnover. In the interpretation of results using the PTH-(1-84)/PTH-(7-84) ratio, it must be taken into consideration that second generation "intact" PTH assays have different cross-reactivity with PTH-(7-84). Therefore, when comparing or analyzing PTH-(1-84)/PTH-(7-84) ratios, the employed PTH assays must be identical. The whole PTH assay and the PTH-(1-84)/PTH-(7-84) ratio allow more meaningful interpretation of PTH trends, and offer a non-invasive means to more accurately diagnose bone disease in this population.
Malluche et al. (Thu,) studied this question.