Determination of serum calcitonin by immunometric two-site assays in normal subjects and patients with medullary thyroid carcinoma.

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/l. 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/l to 50 ng/l. Basal calcitonin concentrations in 30% of the men (< 2-48 ng/l) and 51% of the women (< 2-10 ng/l) were undetectable. Stimulation of calcitonin with pentagastrin (0.5 microgram/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/l (defined as the 95th percentile): 79 ng/l for men, and 50 ng/l 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 pathological 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 the limits of the established reference range. In conclusion, reference ranges for basal and pentagastrin-stimulated calcitonin concentrations have been established for an immunometric two-site assay.(ABSTRACT TRUNCATED AT 250 WORDS)