Figure.

Spurious decline of anti-SARS-CoV-2 spike protein antibody titres due to the prozone effect

(A) Serial results for a selected patient. Orange arrow indicates administration of the third dose of the BNT162b2 mRNA vaccine (Pfizer–BioNTech). The February, 2022, sample was probably more than 250 U/mL and underwent autodilution as per analyser programming. Samples from March and April, 2022, and the two samples from May, 2022, were less than 250 U/mL; therefore, they did not undergo autodilution. Manual dilution of the two samples in May, 2022, (red markers) gave expected results. Results of 25 000 or more were reported as more than 24 999. The dotted line indicates y axis not in scale. (B) Under normal conditions, the reagent antigens are in excess. Antibodies in the patient's sample bind one capture antibody and one signal antibody, forming a double antigen sandwich. This produces a signal that is proportional to the antibody concentration in the sample. If the antibodies are in extreme excess, few double antigen sandwiches are formed, yielding a signal that is falsely low.