CLINICAL-PERFORMANCE OF AN EXTRACTION CALCITONIN RADIOIMMUNOASSAY

An extraction radioimmunoassay of calcitonin (CT) was used to replace the direct radioimmunoassay, in which false-negative results (normal b asal CT values but above normal [''positive''] results after pentagast rin stimulation) occurred in 26 to 30% of cases. During the past 6 yea rs, 170 pentagastrin stimulation tests were performed in 135 patients at our institution. We reviewed the results to determine whether extra ction radioimmunoassay yielded improvement. Of the 135 patients, 6 (4. 4%) had indeterminate results of pentagastrin stimulation tests that c onsisted of normal basal levels but increased values after pentagastri n stimulation, a considerable improvement over the results with direct assay. Six other patients had indeterminate results of pentagastrin t esting-increased basal CT levels but normal values after pentagastrin stimulation. Indeterminate results of pentagastrin tests occurred prim arily in postoperative or preoperative patients with a family history of medullary thyroid carcinoma (MTC). The reasons for the indeterminat e results were unclear, as was their importance as a prelude to diseas e, but two preoperative patients with indeterminate results eventually had positive results and were found to have MTC at operation. We conc lude that extraction radioimmunoassay is more reliable than direct rad ioimmunoassay for the diagnosis of MTC. Determination of a basal CT va lue and of the CT value after pentagastrin stimulation is sufficient f or diagnosing MTC.