The following is the summary of ‘Comparison of Rapid Antigen Tests’ Performance Between Delta and Omicron Variants of SARS-CoV-2’ published in the December 2022 issue of Internal Medicine by  Soni, et al.

There needs to be evidence that  antigen rapid diagnostic tests (Ag-RDTs) can detect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants. The purpose of this study is to evaluate the accuracy of Ag-RDTs in detecting SARS-CoV-2 Delta (B.1.617.2) and Omicron (B.1.1.529) strains. Secondary analysis of a prospective cohort study with a recruitment period of 18 October 2021–24 January 2022. Every 48 hours for 15 days, participants performed Ag-RDTs and collected samples for reverse transcriptase polymerase chain reaction (RT-PCR) analysis. The original study recruited people from all around the continental United States via an online portal. Nasal swabs were taken from the front of each participant’s nose for RT-PCR and fast antigen testing. Although nasal samples were sent to a central lab for RT-PCR, all Ag-RDTs were conducted at home.

This substudy included 5,779 asymptomatic people who tested negative for SARS-CoV-2 on day 1 of the study, out of a total of 7,349 participants recruited in the primary trial. The sensitivity of Ag-RDTs was evaluated on the day of the first positive (index) RT-PCR result and again 48 hours later. The RT-PCR results for 207 subjects were positive (58 Delta, 149 Omicron). On the same day, Delta had a sensitivity of 15.5% (95% CI, 6.2% to 24.8%), while Omicron had a sensitivity of 22.1% (95% CI, 15.5% to 28.8%). After 48 hours, Delta had a sensitivity of 44.8% (95% CI, 32.0% to 57.6%), while Omicron had a sensitivity of 49.7% (CI, 41.6% to 57.6%). Rapid antigen sensitivity did not substantially differ between Delta- and Omicron-infected subjects (48-hour sensitivity: Delta, 81.5% [CI, 66.8% to 96.1%] vs. Omicron, 78.0% [CI, 69.1% to 87.0%]). There were 109 participants with RT-PCR-positive results after 48 hours. Within a week, only 7 of the 69 people with positive RT-PCR findings for less than 48 hours also tested positive with an Ag-RDT; others with Delta infections continued to test negative.

Ag-RDT results are based on self-report, and a testing frequency of 48 hours does not allow for finer temporal precision of the examination of test performance. SARS-CoV-2 Omicron infection does not reduce the efficacy of Ag-RDTs compared to SARS-CoV-2 Delta infection, according to the study’s findings. Ag-RDTs showed increased sensitivity after being tested for both types serially. Rapid antigen testing results can be affected by how long an RT-PCR test remains positive.