Session: SAT 449-497-Thyroid Neoplasia & Case Reports
Poster Board SAT-487
Human anti-mouse antibodies (HAMA) are one of the most common types of heterophile antibodies and are known to interfere in immunoassays. HAMA represent human immunoglobulins that cross react with mouse immunoglobulins frequently utilized in immunoassays. We present here the case of a patient with a consistently falsely elevated TSH due to interference of HAMA.
A 60 year old man was evaluated for weight gain. The physical examination was unremarkable. Using the Tosoh assay, his TSH was 61.5 uIU/mL (normal range, 0.38-4.31), leading to a diagnosis of primary hypothyroidism. The Free T4 was not checked at the time of diagnosis. The follow up TSH ranged from 14 – 33 uIU/mL by Tosoh assay, despite titration of levothyroxine and exclusion of noncompliance and malabsorption. He did not have any symptoms of hyperthyroidism with titration of levothyroxine. Subsequently, he was referred to endocrinologist for elevated TSH and Free T4 on levothyroxine. The TSH was 0.05 uIU/mL (0.34-5.6) by Beckman Coulter DXi assay, and his Free T4 was 1.83 ng/dl (0.47-1.41). The thyroid antibodies and rheumatoid factor were negative in the patient. The marked difference in the TSH values by two different assays raised the suspicion of laboratory error due to antibody interference. These immunoassays utilize different antibody formulation and thus are likely to exhibit differential susceptibility to potential HAMA. A assay designed to detect the presence of HAMA yielded a concentration of 290 ng/mL (0-188) in this patient. The levothyroxine was discontinued.
TSH is commonly measured by two site immunoassay, frequently utilizing mouse antibodies. Cross-reaction of HAMA with the mouse monoclonal antibodies used to measure TSH by immunoassays can result in falsely high or low TSH levels. The degree of interference in the presence of different HAMA antibodies is not predictable in different immunoassays. Potentially clinically significant HAMA is present in serum of up to 0.5% of patients and often goes unrecognized. These antibodies may be induced through vaccines containing animal immunoglobulins, environmental contact with animals and/or infusion of murine monoclonal antibodies for therapeutic and diagnostic purposes. The other sources of antibody interference in TSH assay include thyroid antibodies and rheumatoid factor. In addition to switching to an alternate assay formulation to alleviate interference, there are commercially available HAMA blocking reagents, commercial kits which detect HAMA positive patient and use of nonimmune mouse sera. The heterophile antibodies often go unnoticed leading to inappropriate diagnostic testing and treatment.
It is important for clinicians to be aware of heterophile antibody interference with TSH assay since the mouse monoclonal antibodies are increasing been used for diagnostic and therapeutic purposes
Nothing to Disclose: AJ, JAB, GLW, MA
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