Analytical quality of calcitonin determination and its effect on the adequacy of screening for medullary carcinoma of the thyroid.

Calcitonin, a 32–amino acid calcium-lowering peptide secreted by the C cells (parafollicular cells) of the thyroid, is used as a marker for medullary carcinoma of the thyroid (MCT). However, calcitonin is not specific for MCT, because it is also secreted by other neoplasms, including breast cancer and small cell lung cancer. Secretion of calcitonin is regulated primarily by the concentration of extracellular calcium, but can also be stimulated by gastrin. Although calcitonin concentrations are higher in men than in women and tend to decline with age, many laboratories use a cutoff value of 10 ng/L instead of a population-based reference interval. Prior studies have shown that basal calcitonin concentrations are below this threshold in a normal population (1, 2 ) and in 90% of patients suffering from other nodular thyroid diseases (3 ). In clinical practice, a patient with a calcitonin concentration higher than 10 ng/L should undergo a pentagastrin test to exclude MCT. It is important to note, however, that these interpretive guidelines have been developed using the Cisbio international reagent set for calcitonin measurement. This immunoradiometric assay uses 2 antibodies, one directed against the 11–17 and the other against the 24 –32 sequence of the peptide. IRMA reagent sets, and particularly the Cisbio international kit, have been recommended for calcitonin measurement in the past and are still recommended by some scientific societies. However, in recent years, most medical laboratories have moved from radioisotopic methods to fully automated methods, and such a change may pose problems, because some chemiluminescent methods lack adequate sensitivity and specificity (4 ). In our laboratory, we planned to move from an immunoradiometric method (Cisbio) to an automated method (Liaison, Diasorin). When both reagent sets were calibrated against the 2nd International Standard 89/620, the Cisbio reagent set clearly indicated that 1 IU of the international standard corresponded to 3.6 pg of Cisbio calcitonin. Because this information was not provided by Diasorin, we created our own standard curve for Liaison. For that purpose, we used a sample with a very low calcitonin concentration, to which we added different amounts of the international standard; then we performed calcitonin determination on Liaison. We thus observed that 1 IU of the international standard corresponded to 3.2 pg of Liaison calcitonin. These results demonstrate that 10 pg of Cisbio calcitonin corresponds approximately to 8 pg of Liaison calcitonin. During the analytical validation of the Liaison reagent set, we established the limit of quantification (LOQ), defined as the lowest concentration giving a CV smaller than 20% for interassay variation (5 ). Our results showed the LOQ was approximately 12 ng/L. This observation was confirmed on a second set of samples (Table 1). With the Cisbio reagent set, we obtained a much lower LOQ (7 ng/L). Basing the lowest reportable limit for a test on the determined LOQ (5 ), we reported any results obtained with the Liaison reagent set below 12 ng/L as “ 12 ng/L.” However, with the application of a 10 ng/L threshold for pentagastrin testing as described above, the LOQ of 12 ng/L meant that patients with calcitonin concentrations between 9 and 12 ng/L (2% of women and 10% of men, in our experience) would potentially be missed with assays performed using the Liason reagent set. Such an outcome is problematic for a screening test. We suggest that the “old” 10 ng/L cutoff obtained with the Cisbio reagent set should be confirmed by a multicenter study involving several different calcitonin assays.