COI vs Ct: comparing automated antigen tests cut-off index (COI) to PCR cycle threshold (Ct)

The objective of this study was to compare the numeric cut-off index (COI) of automated antigen tests with the cycle-threshold (Ct) value. COI values of all samples processed with the Elecsys® SARS-CoV-2 Antigen (Roche, Switzerland) from January to February 2022 were retrieved and the positive were compared to RdRP Cts of Allplex Variants I (Seegene, Korea). COI between 0.6 and 1 were considered indeterminate and overproved by RT-PCR. From 13,937 samples 7944(57%) were positive and 189(1.35%) indeterminate. There was a strong correlation between Cts and COI values at the positive samples, but that was not the case for indeterminates. COI values of the positive samples (COI >1) are comparable to Ct values and may therefore be used as proxy viral loads, however this is not the case for indeterminate samples.