A-099 Case Study of a Patient With Heterophile Antibodies Interfering with Both Competitive and Immunometric Assays

Abstract

Abstract Background Immunoassay results may be affected by the presence of interfering substances including heterophile antibodies. These heterophile antibodies can cause either an increase or decrease in the result based on assay design and antibody target. We report a case study wherein we identified several analytes whose results were either increased or decreased by heterophile antibodies in a patient. Methods 74 year male presented to Endocrinology with history of hypocalcemia and his previous lab tests were suggestive for idiopathic hypoparathyroidism, Hashimoto’s disease and Addison’s disease raising physician concern for a possible Type 1 autoimmune endocrinopathy. Cortisol was ordered (for an ACTH stimulation test for adrenal insufficiency); upon testing the post-stimulation cortisol result was higher than the analytical measurement range (AMR), and so the sample was diluted 1:2 and repeated. The repeat cortisol result after correction for the dilution factor was well within the AMR and prompted an investigation for an interfering substance. Serial dilution of analytes was performed and send-out testing for the same with another immunoassay from a different manufacturer and/or mass spectrometry method where available were done. Results The results for most analytes did not match with the reference lab results. Serial dilution showed non-parallelism in the results. For most analytes, after dilution of the sample by 1:4 or 1:8 times, the interfering substance was diluted enough to obtain a result matching the reference lab result. Conclusions Based on non-parallelism in serial dilution results, discordant results with other methods, and loss of interference after dilution it was determined that the most likely cause was a heterophile antibody. Formal testing using blocking antibodies was not done as it seemed that the reference lab used did not run our method and their assays were not affected by this particular heterophile antibody. Analysis of the assay designs and analyte results showed that competitive assays showed falsely high results, while immunometric assays showed falsely low results. The physician was contacted and the case discussed in detail and it was recommended that further testing on this individual be sent out to a reference lab using the alternate method or to use mass spectrometry based assays for further testing if needed. Today, the patient continues to have low calcium, high phosphate and an undetectable PTH suggestive of idiopathic hypoparathyroidism but his other endocrine functions are within normal limits.