Comparison of intact and 'whole molecule' parathyroid hormone assays in patients with histologically confirmed post-renal transplant osteodystrophy

Background Fragments of parathyroid hormone (PTH) have been identified (amino acids 7-84) which may interfere with commercially available 'intact molecule' PTH assays. Novel assays which employ an antibody directed to the first seven amino acids of the N-terminus of PTH are thought to be free from cross-reactivity with the 7-84 fragments, and therefore measure true 'whole molecule' PTH. Transplant recipients (as well as those in end-stage renal failure) have been reported to have elevated levels of 'intact' in comparison with 'whole molecule' PTH. Methods PTH concentrations were assessed in serum samples obtained from female renal transplant recipients previously classified as either having hyperparathyroid (n = 14) or adynamic bone disease (n = 14) by transiliac crest bone biopsy. PTH was measured as 'whole molecule' (Scantibodies 'whole molecule' PTH) and 'intact' (DPC Immulite 2000 intact PTH and Scantibodies total PTH). Results Scantibodies 'whole molecule' PTH (all-subject mean 48.7 ng/L, +/- 53.0) were significantly lower than DPC intact (83.5 ng/L, +/-88.1; P less than or equal to 0.0001) and Scantibodies total PTH (80.5 ng/L, +/-92.4; P less than or equal to 0.0001). However, the differences between the 'whole molecule' and 'intact' measurements were similar across the two patient groups, and reflected the lower reference range employed by the 'whole molecule' assay. Conclusion The 'whole molecule' PTH assay was unable to discriminate between the two patient populations and provided very little additional clinical information to that obtained from the intact PTH assays.