Experimental Study of Interference in Calcitonin Testing: A Case Report and Literature Review

Background:: Calcitonin (CT) is a sensitive serum marker of medullary thyroid carcinoma usually detected via immunoassays; however, its levels are easily disturbed by several endogenous factors. Objective:: The study aimed to discuss a case of suspected interference resulting in aberrant CT values and review previous reports of CT interference Methods:: A female patient visited our clinic with a physical ultrasound examination showing a slightly enlarged thyroid gland with small nodules. She had elevated CT levels, inconsistent with the clinical presentation and other findings. We evaluated the results by retesting using the same platform, platform validation, multiplex dilution, Polyethylene Glycol (PEG) precipitation, heterophilic blocking tubes, and RET gene analysis. Results:: Retesting CT using the same platform confirmed the high value obtained. However, serial dilution of the sample produced nonlinear results, suggesting some interference. While PEG precipitation did not significantly reduce the CT level, incubating the sample in HBTs normalized the CT value, indicating interference from heterophilic antibodies. Gene sequencing revealed no RET mutations. Conclusion:: In cases where elevated CT levels are inconsistent with clinical presentations and other findings, the laboratory technicians should communicate with clinicians, analyze the reasons for the inconsistent results, and use different methods to verify the results. Accurate testing provides realistic and reliable data for doctors and patients and helps to avoid unnecessary procedures.