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Enzyme-linked immunosorbent assay (ELISA) represents a quick, reproducible, and easy assay that allows a single technician to measure antibodies to a defined target in a large number of samples. Obviously, its limitations must be included in the interpretation of results. Specifically, false negativity and false positivity are common problems, and the mechanisms underlying these phenomena must be understood before ELISA results can be incorporated into the clinical decision-making process. In many diseases, e.g.,
Lyme disease and human immunodeficiency virus (HIV) infection, Western blot analysis is necessary to corroborate the ELISA reactivity, to differentiate between true positive and false positive results, and occasionally to determine whether equivocal ELISA results are of any significance. ELISA has revolutionized the practice of medicine related to certain diseases, and its applications will doubtless expand. However, ELISA and all serologic techniques measure only the immune response to an infection, not the presence of the infectious agent itself, which can be detected by culture, immunohistochemical staining, and polymerase chain reaction (the subject of an upcoming article in this series).