Performance of the Innova SARS-CoV-2 antigen rapid lateral flow test in the Liverpool asymptomatic testing pilot: population based cohort study

Abstract

Objective To assess the performance of the SARS-CoV-2 antigen rapid lateral flow test (LFT) versus polymerase chain reaction testing in the asymptomatic general population attending testing centres. Design Observational cohort study. Setting Community LFT pilot at covid-19 testing sites in Liverpool, UK. Participants 5869 asymptomatic adults (≥18 years) voluntarily attending one of 48 testing sites during 6-29 November 2020. Interventions Participants were tested using both an Innova LFT and a quantitative reverse-transcriptase polymerase chain reaction (RT-qPCR) test based on supervised self-administered swabbing at testing sites. Main outcome measures Sensitivity, specificity, and predictive values of LFT compared with RT-qPCR in an epidemic steady state of covid-19 among adults with no classic symptoms of the disease. Results Of 5869 test results, 22 (0.4%) LFT results and 343 (5.8%) RT-qPCR results were void (that is, when the control line fails to appear within 30 minutes). Excluding the void results, the LFT versus RT-qPCR showed a sensitivity of 40.0% (95% confidence interval 28.5% to 52.4%; 28/70), specificity of 99.9% (99.8% to 99.99%; 5431/5434), positive predictive value of 90.3% (74.2% to 98.0%; 28/31), and negative predictive value of 99.2% (99.0% to 99.4%; 5431/5473). When the void samples were assumed to be negative, a sensitivity was observed for LFT of 37.8% (26.8% to 49.9%; 28/74), specificity of 99.6% (99.4% to 99.8%; 5431/5452), positive predictive value of 84.8% (68.1% to 94.9%; 28/33), and negative predictive value of 93.4% (92.7% to 94.0%; 5431/5814). The sensitivity in participants with an RT-qPCR cycle threshold (Ct) of <18.3 (approximate viral loads >10 6 RNA copies/mL) was 90.9% (58.7% to 99.8%; 10/11), a Ct of <24.4 (>10 4 RNA copies/mL) was 69.4% (51.9% to 83.7%; 25/36), and a Ct of >24.4 (<10 4 RNA copies/mL) was 9.7% (1.9% to 23.7%; 3/34). LFT is likely to detect at least three fifths and at most 998 in every 1000 people with a positive RT-qPCR test result with high viral load. Conclusions The Innova LFT can be useful for identifying infections among adults who report no symptoms of covid-19, particularly those with high viral load who are more likely to infect others. The number of asymptomatic adults with lower Ct (indicating higher viral load) missed by LFT, although small, should be considered when using single LFT in high consequence settings. Clear and accurate communication with the public about how to interpret test results is important, given the chance of missing some cases, even at high viral loads. Further research is needed to understand how infectiousness is reflected in the viral antigen shedding detected by LFT versus the viral loads approximated by RT-qPCR.

Optimized Post-Vaccination Strategies and Preventative Measures for SARS-CoV-2

“…For example, the Innova SARS-CoV-2 antigen rapid lateral flow test that is widely used in England has high sensitivity on samples with higher viral load (91% for samples with Ct < 18.3, 69% for samples with Ct < 24.4) and poor sensitivity otherwise (9.7% for samples with Ct < 24.4). 41 Our model for antigen test matches the sensitivity of this test for moderate to high viral loads as we generally expect for carriers with the delta variant, but may not represent real-world scenarios for other antigen tests. In a low prevalence situation, both PCR and antigen testing can yield a substantial number of false positive results.…”

Feasibility and Effectiveness Assessment of SARS-CoV-2 Antigenic Tests in Mass Screening of a Pediatric Population and Correlation with the Kinetics of Viral Loads

“…Conversely, though lateral flow antigen testing (LFT) for SARS-CoV-2 has been widely commercialized for its use in airports, pharmacies, schools and universities, it has been questioned for its degree of sensitivity, which has shown not to meet the minimal performance requirements of WHO and ECDC [22,23]. For instance, in the Liverpool pilot, LFT is likely to detect around a third and, at most, half of the PCR positive cases [24].…”

Covid-19: Rapid testing cuts cases in pilot but questions remain over use of lateral flow tests

“…The interim results of the national pilot were reported in December and showed that the rapid test kits missed over half of cases 2. The full evaluation of the test’s performance, published in The BMJ ,3 said that when the accuracy of LFTs was assessed against PCR results, the overall sensitivity of the Innova test was 40%. The specificity was found to be 99.9%, the positive predictive value is 90.3%, and the negative predictive value is 99.2%.…”