Antibody Interference in Thyroid Assays: A Potential for Clinical Misinformation (Clinical Report‪)‬

The repertoire of clinical tests for thyroid function evaluation includes measurement of thyrotropin [(TSH).sup.1], free thyroxine ([FT.sub.4]), free thyroxine index, free triiodothyronine ([FT.sub.3]), thyroxine ([T.sub.4]), and triiodothyronine ([T.sub.3]). In the past 20 years, there have been numerous reports of interferences in thyroid hormone immunoassays. In highly sensitive single- or double-antibody immunoassays, the presence of circulating endogenous antibodies directed against different antigens may cause either falsely depressed or falsely increased values of thyroid hormones, depending on the nature of the interfering antibody or the assay design. The importance of interference on clinical laboratory analyses may be estimated by frequency and impact on patient care. Because these abnormal values may influence the clinical decisions, they have important clinical consequence and may lead to unnecessary clinical investigations as well as inappropriate treatments. The three major possible sources of antibody interference in thyroid hormone immunoassays are autoantibodies, heterophile antibodies, and rheumatoid factors (RF). Autoantibodies can cause an analyte-specific interference in thyroid assays [1, 2], in contrast to heterophile antibodies and rheumatoid factors, which may be responsible for method-specific disturbances in a wide range of immunoassays, including thyroid hormone measurement techniques [1, 3-5]. After considering the nature of endogenous factors that may interfere in thyroid function evaluation, their prevalence, and their detection, we will focus on their clinical consequences, if not recognized, and on the methods to overcome these interferences. This review can be used as a guide to clinical chemists and physicians in cases where thyroid function test results that are inappropriate to a patient's clinical state could be attributable to antibody interference. An excellent general overview of interfering endogenous and exogenous factors that may affect clinical chemistry tests has been presented previously [6].