Determination of Serum Calcitonin by Immunometric Two-Site Assays in Normal Subjects and Patients with Medullary Thyroid Carcinoma

Summary: We evaluated the clinical usefulness of a commercial two-site immunoassay for calcitonin, which is based on two monoclonal antibodies against distinct epitopes of calcitonin. The potential lower detection limit of the assay was 2 ng/1. Inter-assay and intra-assay variation were both < 11%. No cross reaction with calcitonins from different species (salmon and eel calcitonin) was found. Of the various fragments of human calcitonin, only the 11 — 32 fragment could be detected. Basal calcitonin concentrations in normals (n = 69) ranged from < 2 ng/1 to 50 ng/1. Basal calcitonin concentrations in 30 % of the men (< 2—48 ng/1) and 51 % of the women (< 2 — 10 ng/1) were undetectable. Stimulation of calcitonin with pentagastrin (0.5 g/kg body weight) in normal subjects was followed by an increase in calcitonin in all men (n = 17) and in 60 % of the women (n =12). The upper reference value for pentagastrin-stimulated calcitonin concentrations for all 29 normals was 78 ng/1 (defined as the 95th percentile): 79 ng/1 for men, and 50 ng/1 for women. Using a different immunoradiometric two-site assay, a similar percentage of "positive " responses to pentagastrin in normals was obtained (20 out of 29). Four previously thyroidectomized patients with medullary thyroid carcinoma had normal basal, but patho-logical stimulated calcitonin values at follow-up. At reoperation, cervical lymph nodes with tumour tissue were removed. Postoperatively, the patients had basal and stimulated calcitonin concentrations within th