Comparison of Calcitonin Determinations by Polyclonal and

To the Editor: Serum calcitonin (CT) has important diagnostic and prognostic value in patients with medullary thyroid car-cinoma (MTC). Although CT can be routinely quantified by a variety of 2-site immunoassays, different as-says may yield very different results. This variability, particularly at low CT concentrations, led to the recom-mendation to use reliable and sensi-tive assays in the screening of MTC and C-cell hyperplasia (1). To highlight CT variability at high concentrations, we measured 43 se-rum samples by use of a polyclonal IRMA and by a monoclonal IRMA. The single-step polyclonal assay used 2 different goat antibodies (as the capture and labeled antibodies), which were specific for well-defined regions of the CT molecule, with no cross-reactivity with parathyroid hormone, thyroid-stimulating hor-mone, CT gene–related peptide, por-cine CT, or salmon CT. The reagent set was used according to the manu-facturer’s instructions (Calcitonin Assay, Scantibodies Laboratory, Inc.). The concentrations of calibra-tors ranged from 10 to 1000 ng/L. Expected values were 9.9 ng/L for women and17.0 ng/L for men. The single-step monoclonal assay uses 2 antibodies with no cross-reactivity with procalcitonin. The reagent set was used according to manufactur-er’s instructions (CIS Biointerna-tional). The concentrations of calibra-tors ranged from 10 to 1500 ng/L, with a reference interval of 10 ng/L. Both assays were calibrate