Antigen excess in modern immunoassays: To anticipate on the unexpected

•Falsely low test-results caused by antigen excess still occur in modern immunoassays.•Nephelometry and turbidimetry are most prone to antigen excess.•Antigen excess also occurs in solid phase immunoassays.•Discrepant test-results or abnormal assay kinetics may indicate antigen excess.•Awareness and good communication with clinicians avoid assay misinterpretations.

Immunoassays measuring sera with high analyte concentration may be prone to an artifact that causes underestimation of the analyte concentration. This phenomenon is generally described as antigen excess or the prozone effect. Characteristically, serum with high concentrations of a certain analyte can give a false negative/low result when tested at the recommended dilution, but reacts strongly positive upon further dilution. Increased insight of the antigen excess mechanisms and tools to prevent it has reduced the analytical problems caused by prozone effects in daily laboratory practice. However, misinterpretation of laboratory results caused by antigen excess does still occur, in virtually any type of immunoassay. Awareness by the laboratory specialist of the mechanisms underlying antigen excess in the different immunoassays, strategies to detect it, and adequate communication with clinicians can help to avoid reporting false negative test-results.