A stated earlier, the presence of a Test lLine, no matter how faint, indicates the presence of SARS-CoV-2 antigen and thus a positive COVID-19 test result. But how able are rapid antigen tests to detect the presence of SARS-CoV-2 antigen?
The Liverpool School of Tropical Medicine evaluated the limit of detection of the Excalibur Rapid SARS-CoV-2 Antigen Test. In other words, to see how little antigen or live virus the test can detect.
SARS-CoV-2 N protein antigen was diluted from 500 pg/ml, representing a high level of antigen, through decreasing amounts down to 0 pg/ml, or no virus antigen present. Each dilution was applied to Excalibur Rapid SARS-CoV-2 Antigen Test cards and read by two observers, each blinded to the results of the other. A third observer was available to adjudicate discrepant tests.
The Excalibur test had 100% detection down to a level of 75 pg/ml, dropping to 30% at 50 pg/ml. Positive test results were seen down to 30 pg/ml.
Similar tests were carried out using live virus diluted down in stages from 10,000 plaque-forming units (PFUs)/ml to zero. Plaque forming units is a measure of the number of infectious virus particles per unit volume. The Excalibur test was able to detect 100% of replicates down to a level of 250 PFU/ml, dropping to 50% at 125 PFU/ml.
The Excalibur Rapid SARS-CoV-2 Antigen Test had a limit of detection of 250 PFU/ml, which exceeds the World Health Organisation recommendation of 500 PFU/ml.
There have been over 150 million confirmed cases of COVID-19 worldwide with new variants of the virus arising all the time. The most significant of these for the UK is the B.1.1.7 variant causing over 95% of new infections as of April 2021. It is therefore important to know if there is any reduction in sensitivity of currently available rapid antigen tests when exposed to the B.1.1.7 variant.
Researchers at the Liverpool School of Tropical Medicine have determined the limit of detection of several currently available rapid antigen tests1.
A clinically isolated SARS-CoV-2 strain from the B.1.1.7 lineage was serially diluted from 1,000,000 PFU/ml to 100 PFU/ml. Each dilution was tested in triplicate and applied to Excalibur Rapid SARS-CoV-2 Antigen Test cards and read by two observers, each blinded to the results of the other. A third observer was available to adjudicate discrepant tests. The limit of detection was defined as the lowest dilution at which all three replicates were deemed to be positive.
The Excalibur Rapid SARS-CoV-2 Antigen Test had a limit of detection of ≤ 500 PFU/ml, meeting or exceeding the British Department of Health and Social Care (DHSC) acceptable criteria.
Similar tests performed using the B.1 2020 strain taken from previously published work show the Excalibur Rapid SARS-CoV-2 Antigen Test to have a limit of detection of 250 PFU/ml.
This study demonstrates equivalent performance with SARS-CoV-2 B.1.1.7 as with the original dominant strain from the beginning of the pandemic.
Reference: 1. Kontogianni K, Cubas-Atienzar AI, Wooding D et al. Liverpool School of Tropical Medicine 2021. Unpublished.
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