[PTH measurement: where do we stand?]

PTH measurements are widely used by nephrologists because parathyroid function is frequently altered in uremic patients, with clinical implications for bone and the cardiovascular system. This is why both national and international guidelines recommend target values for PTH. However, the reliability of PTH assays is hampered by the presence of many circulating molecular types of the hormone, which are known to have different biological effects. The so-called first-generation methods measuring all C-term fragments were replaced by second-generation ones based on the double-antibody technique; the latter were shown to be more reliable and easy to use. These methods have been widely adopted, proving helpful for diagnosis, prognosis and treatment in clinical settings. However, when different second-generation methods were compared, inconsistent values were obtained. Moreover, it was shown that they cross-reacted with N-truncated fragments, including C-term 7-84 PTH, which do not display PTH activity. The more recently introduced third-generation methods exhibit higher specificity for the 1-84 whole molecule and are not liable to interference by N-truncated fragments. When compared to intact PTH, the whole-PTH methods yield about 50% lower values, but the difference remains constant through the entire range of PTH values. Indeed, despite different absolute results either between whole and intact PTH or within identical-generation methods, there are very close correlations among them, with coefficients above 0.95. Thus, most assays can be considered reliable but the different results, if not correctly interpreted, may give rise to misinterpretation on clinical grounds. It is agreed that these differences depend on the use of both different calibration standards and antibody specificity. We conclude that, irrespective of the method used, one should clearly know what PTH is being measured, using specific reference ranges and applying specific targets.