37.2.3. Freelite and Hevylite antigen excess detection on the Optilite
Selected assays on the Optilite include automated antigen excess checks (prozone detection). The Optilite has three methods of identifying samples that may be in antigen excess: control addition, sample addition and reaction kinetics monitoring.
Freelite and IgM Hevylite assays use reaction kinetics monitoring to detect antigen excess (Sections 7.5 and 11.4). The Optilite measures the rate of the reaction at 3 different time windows in order to identify samples that have a relatively high initial rate of reaction, and therefore may be at risk of being in antigen excess. The ratio between the change in absorbance for time windows 2 and 3 is compared with that of the other time windows, which is then compared to prozone limits defined within the assay parameters. If one or both of the calculated values is lower than defined limits, the sample may be in antigen excess and so the software applies a “High Activity” error and automatically retests the sample at a higher dilution.
Hevylite IgG assays use a control addition method, and the kits are supplied with a vial of IgG antigen excess control fluid, which must remain on the analyser during testing. In this method, a set amount of the antigen excess control fluid is added at the end of the reaction. Samples that are not in antigen excess demonstrate an increase in absorbance following the addition of extra antigen.The efficacy of the antigen excess checks employed by Freelite Optilite assays was analysed by Coley et al., who reported that antigen excess was correctly flagged for Freelite κ and λ in 98% and 99% cases, respectively, and that there were no examples of undetected antigen excess. This indicates that the antigen excess detection protocols employed by the Optilite are robust and the occurrence of undetected antigen excess experienced on this platform is rare (Sections 7.5.3 and 11.4).