Comparison between Whole and Intact Parathyroid Hormone Assays

The standard measurement of parathyroid hormone (PTH) is the intact PTH (iPTH) assay, which is used for approximately 90% of Japanese dialysis patients. The iPTH assay reacts not only with 1-84 PTH, but also with large truncated fragments of non-1-84 PTH, including 7-84 PTH. On the other hand, the whole PTH assay is specific for 1-84 PTH. The aim of the current study was to define the validity of both whole and intact PTH assays. A total of 738 hemodialysis patients were enrolled from twelve dialysis services. The serum PTH level was evaluated by both intact and whole PTH assays simultaneously. Non-1-84 PTH was determined by subtracting the whole PTH value from that of the intact PTH assay. The median level of whole PTH was 121 pg/mL, and that of iPTH was 210 pg/mL. The whole PTH assay had a very high correlation with the iPTH assay (r = 0.870, P < 0.001). For 43 out of 738 patients (5.8%) the value for intact PTH-whole PTH was < 0. Both assays significantly correlated with non-1-84 PTH (P < 0.001), while the iPTH assay, particularly, had a very high correlation with non-1-84 PTH (r = 0.791). As a whole, 18% of the total population was misclassified into a different Japanese guideline category. Stratified by Japanese guideline classifications, 28% of patients within an iPTH target range were misclassified. Using Bland-Altman plot analysis, as the serum PTH level increased, there was a large difference between two assays. Both PTH assays correlate strongly, although the whole PTH assay may be more useful for precise evaluation of PTH function than the iPTH assay.