Diagnosis:
Heterophile antibodies and competitive immunoassays
• Competitive binding immunoassays are based on reactions in which the number of antigen-binding sites is limited. The analyte that is being measured and a labeled derivative of the analyte compete for binding to the capture antibody.
• There is an inverse relationship between the amount of labeled analyte that binds to the capture antibody and the concentration of analyte in the sample. At low analyte concentrations, more of the labeled analyte binds to the capture antibody and a high signal is observed.
• If a human anti-mouse antibody (HAMA) is present and binds to the capture antibody, it prevents formation of the labeled analyte–capture antibody complex, thereby resulting in a lower than expected signal and a higher than expected analytical result.
• The interference of HAMA with competitive binding assays is not as frequent as with two-site immunometric assays because of the greater affinity of the analyte for the antibody binding sites.
• Falsely elevated results have been observed for competitive binding assays for free T4, T4, and inhibin A. Addition of blocking agents (e.g., mouse or animal immunoglobulins) is used to prevent binding of HAMA to the capture antibody.
