During a titration procedure, if the first three tubes are negative and the later tubes are positive, this pattern is referred to as which phenomenon?

• A. Postzone
• B. Prozone
• C. Zone Phenomenon
• D. Antibody Excess

Answer: (B)

This pattern is the prozone phenomenon. It happens when there is an excess of antibody relative to antigen during the titration, so early tubes fail to show agglutination because antibodies saturate the binding sites and cannot cross-link particles. As you dilute the sample and antibody concentration falls toward the zone of equivalence, cross-linking can occur and agglutination appears in the later tubes. This is different from the postzone, where antigen is in excess and lattice formation is also hindered, leading to false negatives even at higher dilutions. The term “antibody excess” describes the cause, but the recognized name for this pattern is the prozone phenomenon.