1. PCR tests
These are based on a standard laboratory technique called polymerase chain reaction (PCR), which is used to detect small amounts of genetic material within samples – in this case, SARS-CoV-2. It is an extremely sensitive test, with high accuracy rates, which is why the PCR test is widely accepted as the gold standard for the detection of COVID-19. However, it requires specialized equipment and training, not to mention specific reagents, to function. If any of these are in short supply, delays may occur.
Most PCR tests for COVID-19 involve taking swabs from your nose and throat and sending them to a lab for analysis, which means it can take several days to receive the result. If the result is positive, it is very likely that you have the virus, although it is difficult to know if it is still infectious, because a PCR test can also detect dead viruses. To address this problem, some researchers have proposed quantifying the amount of virus in the sample, using something called a cycle threshold value, to assess the likelihood that someone who tests positive for the virus will pass it on to others.
False-negative results, when a person tests negative for SARS-CoV-2 despite being infected, is another problem, estimated to occur in 2-29% of cases. This can happen because the swab is not done correctly or the swab occurs too early in the infection. It is risky because that person can pass the virus to other people without knowing it.
PCR tests are also being developed that use saliva, rather than rubbing the nose and throat, which is uncomfortable for some people. These tests appear to be less sensitive than conventional swabs, but maybe more appropriate for certain groups, such as children.
Rapid PCR tests
Unlike traditional PCR, these tests can be performed with minimal training using portable benchtop machines, and they promise to deliver a result in 90 minutes. One such test has been developed by an affiliate of Imperial College London called DnaNudge. In August, the UK government placed an order for 5.8 million of its kits, which it hopes to roll out in hospitals and residences, to rapidly diagnose and isolate COVID-19 patients. A recent study suggested that the diagnostic accuracy of this test is equivalent to the conventional PCR test for COVID-19. However, because each machine can only process a maximum of 16 tests in 24 hours, such tests likely only play a limited role in detecting coronavirus infection in the broader population.
This is another proposed solution to try to increase the testing capacity of countries and speed up the response time of viral tests. Previously, the Red Cross had used it to test donated blood for HIV, Zika virus, and hepatitis, and it was tested in the San Francisco Bay Area in the context of SARS-CoV-2. Rather than having labs run a PCR test on every sample they receive, samples from multiple people are pooled and tested as one. If the results of the pooled samples are negative for SARS-CoV-2, it is safe to assume that each of the individual samples is negative.
If the pooled sample is positive, each individual sample from that pool is individually retested to identify the source of the virus. Mixing samples can also reduce the cost of viral testing by reducing the number of tests performed. How much depends on the proportion of people infected. Researchers have calculated that if 9% of a population were infected, joint testing would reduce test use by about 50%, but if only 0.1% of a population were infected, test use would decrease by almost 90%.
2. Antigen testing
Instead of detecting genetic material from the virus, antigen tests identify proteins present on the surface of SARS-CoV-2. They generally still require a nose or throat swab, although saliva tests are also being developed. Antigen tests are much faster and cheaper to perform than PCR tests. For example, the US Food and Drug Administration recently granted emergency use approval to a credit card-sized test device developed by Abbott Laboratories, which does not require a laboratory or machine for processing, gives results in 15 minutes, and costs just the US $ 5. The company aims to produce 500 million such tests per month by October.
However, this speed and scale can come at a cost of precision. Antigen testing is less sensitive than PCR, which may mean that more people infected with the virus are missed, resulting in unnecessary outbreaks. On the other hand, antigen tests are more likely to identify people who carry higher amounts of virus, so they can help the most infectious. They are also more likely to detect viruses when people are at their most infectious, and they are less likely to detect dead viruses that persist after the infection has ended.
Rather than detecting current infection, antibody tests aim to identify previous exposure to SARS-CoV-2, by detecting one type of immune response: the production of antibodies by B cells. Their presence in the blood is a good indication that someone has been exposed to the virus and may indicate that they are protected against future infections.
Antibodies take a long time to appear and in some cases can decrease over time, so a negative antibody test result also doesn’t necessarily mean that you haven’t had COVID-19 or that you don’t have any immunity against it, because antibodies may appear later or other types of immune response may also be involved in fighting the virus.
However, antibody tests still have an important role to play in helping inform public health organizations about what proportion of the population has been exposed to the virus. They are also useful for researchers hoping to learn more about how immunity to SARS-CoV-2 develops.