The FDA has granted emergency use authorisation to an entirely new kind of Covid-19 assay – one that detects viral antigens found on or within particles of the novel coronavirus. The Sofia 2 Sars Antigen FIA assay can provide results in 15 minutes, according to its manufacturer, Quidel.
Antigen testing is the third type of test to gain the FDA’s authorisation, after viral RNA tests and assays that identify former Covid patients by detecting antibodies to the virus in their blood. The advantages of antigen testing make this approach a useful new diagnostic method for US healthcare staff – the technique’s relatively low accuracy means it will have to be used with care.
Antigen tests are similar to the sequencing-based assays designed to detect viral RNA, in that both kinds of tests are used to diagnose active Covid-19 infections, and both are used with similar samples. Quidel’s test is used with nasal swabs; molecular tests may be used with specimens taken from the back of the nasal cavity, the throat and in some cases the lungs. Additionally, an academically-developed molecular test that uses saliva samples was granted EUA on Friday.
Both antigen and molecular approaches are used to test people already suspected to have Covid-19 infections. They therefore fit a different niche to antibody tests. Antibody tests are used with blood samples, and are intended – eventually – to be used to screen the general population to identify people who have built up adaptive immunity to a prior Covid-19 infection.
The two great advantages antigen tests have over viral RNA tests are that they generally provide results faster and are cheaper to produce.
But they are not, generally speaking, as accurate. The FDA warns that while antigen tests are very specific for the virus, helping guard against false positives, they are not as sensitive as molecular tests. Antigen tests “may not detect all active infections”, the agency says, adding that negative results do not rule out infection. This means that negative results from an antigen test may need to be confirmed with a viral RNA test prior to making treatment decisions or to prevent the possible spread of the virus due to a false negative.
It is therefore likely that this new technology will be used as a sort of triage, picking up the patients who definitely do have the virus but being unable to rule anyone out.
Quidel is already shipping the test to its customers, but has not said how many it will be able to produce over the next few months. The FDA believes that once “multiple manufacturers” of antigen tests enter the market, these tests could have the potential to test millions of US patients per day owing to their simpler design.
It is not clear how many companies have requested an EUA for an antigen test, but the number is surely set to grow.