Under the current emergency measures in force in different parts of the world, and with the fourth wave rising or feared in many countries, the time it takes to process samples is crucial to triage patients, making rapid antigen detection tests can be a very useful assay. But all efforts should be made to ensure the tests used are as sensitive as possible.
When comparing the different tests examined in this study and using the Ct of the q(RT)-PCR as validation, a large range of sensitivity was found, from 42.10% using the tests by the nal von minden and Tody Laboratories to 84.78% with the COVID-19 Ag Gold Saliva from PCL.
As would be expected, we found the correlation between sensitivities on rapid antigen detection test and q(RT)-PCR for Cts under 23 to be practically complete (only the Operon test was not 100% accurate, although it did correctly identify over 90% of positives). However, as soon as Ct is higher, sensitivity decreases notably. With Cts over 23, four of the 14 tests correctly identified only 50% of positives, and just five were accurate in 80% or more of cases. The situation is even more marked when Ct is over 30, where just 2 tests correctly identified 50% or more of positives (PCL with 63.63% and Beright with 57.14%).
When the accuracy of the various tests was compared on the basis of the quantification of human β-globin, which allows the true measurement of viral load and validates the quality of the sample extraction (13–14), the results are somewhat different.
In this case, not all tests have a 100% correlation for samples with over 5 log/103 cells, as might be expected. This is very important, both clinically and epidemiologically, because it implies a not insignificant percentage of false negatives which correspond to contagious patients (15–16). This indicates, therefore, that these rapid antigen detection tests are not recommended for massive screenings.
For samples with VL of below 4 log, only two methods can detect SARS-CoV-2 antigens, and both at low rates: 31% (Operon) and 20% (PCL). This is to be expected, and is not of great epidemiological significance given that patients with low viral load are not considered to be transmitters (12), even though they may in fact be at the beginning of the infection, so could become transmitters at a relatively short later date.
To our knowledge this is the first study which compares so many rapid antigen detection tests for SARS-CoV2. We counteracted any potential bias of the low number of samples used by checking each sample with each commercial test and, despite the variance derived from the manual procedure of this kind of probes, our results for specific tests are similar to those obtained in other studies (4–5, 10–11, 17). They are, though, far from the promising results published by the manufacturers themselves, or claimed by certain authors (17) and this emphasises that special care should be taken with the general lack of sensitivity with higher Cts (or low viral load), as results shows that the reliability of obtaining positive PCR and contagion capacity in this range are as yet not well known (15–16).
This concern is confirmed when a normalized measure of viral load is taken into account. These results also confirm that sampling procedures are very important and rapid tests using easily recovered samples can have compromised sensitivity.
In conclusion, this kind of immunoassay for antigen detection can be useful to ensure the quick isolation of positive patients, but the lack of sensitivity of some tests, even in patients with high viral load, means they miss identifying patients who are positive for SARS-CoV2 who might be infectious, so they must be used with great care.