Getting a lot of questions about rapid tests for COVID. Just a short thread about the various types of tests available or in development. #COVID19
Disclaimer: I'm not a lab person and others (@MackayIM @kedzierskalab @drdebwilliamson) may want to correct me
Nucleic acid testing (commonly known as PCR, polymerase chain reaction) detects the SARS-CoV-2 RNA itself. This is therefore a good test for acute COVID-19 infection.
These are based on swabs (ie sampling directly from the upper or lower respiratory tract). Although the sensitivity is dependent on the specimen quality and location (sputum> nasopharyngeal> throat), these tests are quite specific.
When used to monitor recovery or infectiousness, it should be noted that PCR positives don't necessarily mean that there is viable, infectious virus - it can pick up RNA fragments.
Patients with mild infection usually have a positive PCR for about 3-7 days, but it can be longer following severe infection. Reports of "reinfection" (esp within 2-3 weeks) may represent persisting shedding of viral RNA.
Most "rapid tests" are based on serology (these are tests usually done on blood). There are various versions, but many look like a pregnancy test or a dipstick. They are often promoted as tests that could be used at the bedside, but are more commonly still used in labs.
Serological tests answer a different question to PCRs - whether there has been an immune reaction to the virus. Typically, antibodies don't appear for at least a few days, and may not become positive for a week or two after the onset of symptoms.
Therefore, they may not be good tests for the diagnosis of acute infection. There are different types of antibodies - IgM, IgA, IgG (and various subclasses of IgG) with different kinetics - they appear at different times after the onset of infection.
These work by detecting antibodies that bind to a test antigen, which can be different between tests. Their specificity needs to be checked - there are concerns that some tests may cross-react with other coronaviruses.
There are different test technologies - it is likely that each tests will have different performance characteristics (accuracy). When manufacturers claim that their test is accurate, it is important to verify what they are using as the gold standard.
Therefore, false negatives (esp testing too early) and false positives (infection with a different coronavirus) are possible with serological tests.
As an example, one recent study checked for the possibility of false positives by testing US samples and people with other infections. (Declaration: I'm one of a large group of authors). medrxiv.org/content/10.110…
It showed that some antibodies were present as soon as 2 days after onset, but there was a lower level of antibodies present earlier. IgM, IgG3 and IgA were present in the highest concentrations.
Population level serology studies are going to be very important in defining what proportion of the population have been infected even if not diagnosed by PCR. If done serially at different times, then this can provide a indicator of the incidence of infection.
