Dual site antibody-base immunoassays are commonly used in clinical laboratories to quantify the CT serum concentrations as a specific and sensitive marker of medullary thyroid carcinoma (MTC). Heterophile antibodies can interfere with these assays, however, and cause erroneous results. In order to avoid this interference, immobilized and conjugated antibodies from two different animal species or immunoreactive antibody fragments, as well as the addition of non-immune globulins, are generally included among the assay reagents. We describe the case of a 73-year-old man affected by a multinodular goiter, who showed high basal CT plasma levels as measured by a monoclonal antibody based IRMA. The finding of negative results for the presence of MTC at fine needle aspiration (FNA) and the mild increase observed in plasma CT during a pentagastrin (Pg) stimulation test, suggested that the high CT levels might depend on a cross-reaction with heterophilic antibodies. In fact, after the addition of the heterophilic blocking tube (HBT) to each specimen, the CT levels markedly decreased by more than 80 (average decrease+/-SE= 87.6+/-2.668). Such a decrease strongly suggests that in our case the routinely used F(ab')2 fragments were unable to eliminate all of the interference and that the elevated serum CT levels might have been caused by human heterophilic antibodies. In conclusion, these results indicate a novel cause of CT false positivity, suggesting that high serum CT levels, when combined with a slight increase during Pg stimulation, should be critically interpreted in view of the possible presence of heterophilic antibodies in the specimens.
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