Quantification of Serum 1-84 Parathyroid Hormone in Patients with Hyperparathyroidism by Immunocapture In Situ Digestion Liquid Chromatography-Tandem Mass Spectrometry

Immunoassays specific for 1-84 parathyroid hormone (PTH) reportedly reflect the bioactivity of PTH; however, PTH immunoassays can be susceptible to interference by cross-reacting PTH fragments. In addition, these assays currently lack standardization. A methodology using immunocapture purification with liquid chromatography-tandem mass spectrometry (LC-MS/MS) detection, along with a stable isotope-labeled internal standard, may help address these issues. We isolated 1-84 PTH from 1 mL serum by immunocapture on a 6.5-mm polystyrene bead. The immobilized PTH was digested in situ and analyzed by LC-MS/MS. For quantification, we used the selected reaction monitoring response from the N-terminal tryptic peptide 1-13 PTH ((1)SVSEIQLMHNLGK(13)). The linear range of the assay was 39.1-4560 ng/L, and the limit of detection and limit of quantification were 14.5 ng/L and 39.1 ng/L, respectively. The intraassay CVs ranged from 6% to 11%, and the interassay CVs ranged from 7% to 17%. Interference by PTH fragments 1-44 PTH, 7-84 PTH, 43-68 PTH, 52-84 PTH, 64-84 PTH, and PTH-related protein (PTHrP) was