A non-(1–84) circulating parathyroid hormone (PTH) fragment interferes significantly with intact PTH commercial assay measurements in uremic samples

Abstract

Abstract We have previously shown that the Nichols assay for intact parathyroid hormone (I-PTH) reacts with a non-(1–84) molecular form of PTH. This form behaves as a carboxy-terminal fragment and accumulates in renal failure, accounting for 40–60% of the measured immunoreactivity. We wanted to see whether this was a common event with other commercial two-site I-PTH assays. We thus compared the ability of three commercial kits [Nichols (NL), Incstar (IT), and Diagnostic System Laboratories (DSL)] to measure I-PTH in 112 renal failure patients and to detect hPTH(1–84) and non-(1–84)PTH on HPLC profiles of serum pools from uremic patients with I-PTH concentrations of 10–100 pmol/L. The behavior of synthetic hPTH(7–84), a fragment possibly related to non-(1–84)PTH was also compared with hPTH(1–84) in the three assays. The I-PTH concentrations measured with the three assays in the 112 uremic samples were highly related (r2 ≥ 0.89, P <0.0001), and the values measured with NL were, on average, 23% higher than IT. Values measured with DSL were 23% and 56% higher than IT for values less than and more than 40 pmol/L, respectively. The three assays detected two HPLC peaks on four different profiles corresponding to hPTH(1–84) and non-(1–84)PTH. This last peak represented 36 ± 8.4% of the immunoreactivity with NL, 24 ± 5.5% with IT, and 25 ± 2.8% with DSL (NL vs IT or DSL: P <0.05). These differences were confirmed by a 50% lower immunoreactivity to hPTH(7–84) compared with hPTH(1–84) for IT and DSL but not for NL. These results suggest that most of the two-site I-PTH assays would cross-react with non-(1–84)PTH material, thus explaining about one-half of the 2–2.5 × higher I-PTH concentrations reported in uremic patients without bone involvement than in subjects without uremia.