Abstract
IntroductionCommonly used free thyroxine (FT4) immunoassays can be falsely elevated due to interference causing misinterpreted thyroid function. We present two cases with high FT4 concentrations due to antibody interference. This study’s aim was to investigate the source of the FT4 immunoassay interference and possibility of its removal by two different techniques in order to correct the discrepancy between obtained FT4 values and the patient’s clinical status.Materials and methodsTwo patients presented at their general practitioners’ with elevated FT4 concentrations in combination with a normal and increased thyroid stimulating hormone (TSH) concentrations. Clinical symptoms differed between patients but did not correspond with the hyperthyroid status suggested by the laboratory results. FT4 concentrations from both patients were measured on four common commercial immunoassays and the dialysis method before and after treatment with heterophilic blocking tubes and protein A/G.ResultsRemoval of interfering antibodies using protein A/G resulted in normal FT4 concentrations.ConclusionThis report illustrates falsely elevated FT4 concentrations due to assay interference on the Immulite immunoassay analyser caused by heterophilic antibodies, which were eliminated by protein A/G treatment. We point out the importance of a close collaboration between doctors and the laboratory to avoid unnecessary clinical intervention.
Highlights
Used free thyroxine (FT4) immunoassays can be falsely elevated due to interference causing misinterpreted thyroid function
Removal of interfering antibodies using protein A/G resulted in normal FT4 concentrations
This report illustrates falsely elevated FT4 concentrations due to assay interference on the Immulite immunoassay analyser caused by heterophilic antibodies, which were eliminated by protein A/G treatment
Summary
Used free thyroxine (FT4) immunoassays can be falsely elevated due to interference causing misinterpreted thyroid function. A number of factors, including concurrent (non-thyroidal) illness, medication (e.g. amiodarone, thyroxine) and immunoassay interference, can cause these discrepancies [1,2,3]. These factors should be excluded in order to prevent unnecessary diagnostic and therapeutic interventions [4]. The competitive immunoassay is a common method to determine FT4 concentrations in diagnostic laboratories. These assays measure the unbound thyroxine fraction whereby the vast excess is protein-bound (> 99.5%). Immunoassay interference can cause false (positive or negative) FT4 results that can lead to misdiagnosis and potentially harmful therapy [5]
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