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Clinical Case Studies |
Department of Pathology & Laboratory Medicine, University of Pennsylvania Medical Center, Philadelphia, PA.
Address correspondence to the author at: Department of Pathology & Laboratory Medicine, 7.103 Founders Pavilion, University of Pennsylvania Medical Center, 3400 Spruce St., Philadelphia, PA 19104. E-mail kricka{at}mail.med.upenn.edu
Interferences in laboratory tests, immunoassays in particular, continue to be a significant challenge in clinical laboratory practice (1). A raised awareness to this type of analytical problem is important if laboratories are to avoid reporting incorrect test results. In spite of all efforts by the diagnostic community, interferences still occur, sometimes with disastrous consequences for the patient.
These 2 cases illustrate 2 different circumstances that led to the suspicion of assay interference. In the case reported by van der Watt et al., it was discordance in thyroid test results that alerted the laboratory, and in the case reported by Kellogg et al., investigation of a possible laboratory artifact was a component in the standard of care for the suspected rare syndrome of inappropriate thyroid-stimulating hormone (TSH) secretion.
Both cases illustrate the investigations that are required to establish the presence of an interference. These investigations include repeat analysis, reanalysis using another method, dilution, blocking, fractionation of the sample by (for example) precipitation, and investigating possible exposure of the patient to animals or animal products (e.g., monoclonal antibody preparations for therapy or imaging). The manufacturer of a test kit can also be an ally in such investigations. Kellogg et al. collaborated with the test kit manufacturer in some of the blocking studies that used a Ru-interference blocking agent designed to reveal an interference with the ruthenium complex used in the free thyroxine (T4) assay.
In its own way, each case is notable. The case reported by Kellogg et al. is of special interest, as it is one of the relatively few reported cases of a negative interference in a sandwich immunoassay caused by a circulating antibody (2)(3). The more common finding is a false-positive result due to an interfering antibody [e.g., human antimouse antibody (HAMA)]. This case also illustrates that often it is not possible to verify the exact identity of an interferent. These types of studies are also frustrated by the limited amount of specimen available and patients who are lost to follow-up, thus precluding extensive and exhaustive studies. The van der Watt et al. case represents an example of false-positive interference in a competitive assay (due to binding of a putative antithyroxine antibody to the conjugate) and illustrates the differing effects of the interferent in a 1-step vs a 2-step analog-based free hormone assay.
In light of the continuing problems with interferences, the question remains whether the Holy Grail of immunoassays, the interference-free immunoassay, will ever be within reach. Incremental improvements to the efficacy of blocking agents have not eradicated the problem. Alternative strategies, such as incubating all samples in the Scantibody-type blocking tubes, are prohibitive owing to cost and adverse impact on operational efficiency of the laboratory. Some believe that the answer lies with chickens (4)! Chicken antibodies are not susceptible to common interferences such as rheumatoid factors and HAMA. Despite this beneficial feature, chicken antibodies have not made serious inroads into the dominant position held by mouse monoclonal and other animal antibodies as immunoassay reagents. A more ambitious notion is that the immunoassay, and its attendant problems, will be replaced by some superior non–antibody-based analytical technique. It is not clear what technology might sweep away a firmly entrenched method such as immunoassay with its >50-year history of development and application. In 2008, in what seems the heyday of the immunoassay, it is difficult to predict such a sea change. However, we should not forget that massive disturbances to the status quo are rarely predicted with any accuracy, and hence immunoassay is not necessarily guaranteed a role as the method of choice for its current range of analytes.
Acknowledgments
Grant/Funding Support: None declared.
Financial Disclosures: None declared.
References
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