Calcitonin levels by ECLIA correlate well with RIA values in higher range but are affected by sex, TgAb, and renal function in lower range.

Calcitonin (CT) is a marker for both initial diagnosis and monitoring of patients with residual or recurrent medullary thyroid carcinoma (MTC). In Japan, serum CT had been measured by radioimmunoassay (RIA) until recently. Electrochemiluminescence immunoassay (ECLIA) became commercially available in 2014, and this technique is now the only method used to examine CT concentration. The purposes of this study were to investigate the correlations between the CT concentration measured with ECLIA (ECLIA-CT) and RIA (RIA-CT) and to explore the clinical characteristics of patients with elevated ECLIA-CT. CT concentrations of 348 sera samples from 334 patients with various thyroid disorders including nine MTC were measured using both assays. The correlation analysis revealed an excellent correlation between ECLIA-CT and RIA-CT among the cases with CT level >150 pg/mL by both assays (r = 0.991, p < 0.001). However, 63% of all samples exhibited undetectable ECLIA-CT, while their RIA-CTs were measured between 15 and 152 pg/mL. The ECLIA-CTs in all patients who underwent total thyroidectomy for non-MTC showed low concentrations. High ECLIA-CT was observed in patients with MTC or pancreas neuroendocrine tumor. ECLIA-CT was also increased in 14 other male patients with non-MTC, including four with renal failure. Multivariate logistic regression analysis showed that male sex, negative TgAb, and lower estimated glomerular filtration rate were independent factors to predict detectable ECLIA-CT (≥0.500 pg/mL). These results indicate that ECLIA-CT correlates well with RIA-CT in higher range and is affected by sex, TgAb, and renal function.