Anomalies In The Assay Of Parathyroid-Hormone In Normocalcemic Patients With Renal Stone Disease

A group of 22 normocalcaemic patients with renal stone disease were retrospectively studied using seven different immunoassays for parathyroid hormone (PTH). N-terminal immunoradiometric assays and an intact hormone immunochemiluminometric assay sometimes gave results which fell above the normal range. These results were at variance with values obtained in three region-specific radioimmunoassays and an immunoradiometric assay for the intact hormone. Since these atypical samples contained normal levels of biologically active PTH the elevated immunoassay results were not thought to relate to the normal circulating hormone but rather to interference in the immunometric assays. The interfering substance was thought to be a large molecule which could not be removed by charcoal and did not dilute out in parallel to the PTH standard curve.