Swab tests for current infection
- A test showing infection (positive result) is very reliable, and means it is very likely that you have COVID-19 infection currently. NAAT is very sensitive, and patients can remain positive for some time after they have recovered. For this reason, the result needs to be interpreted alongside the clinical findings.
- A test not showing infection (negative result) means you are much less likely to have COVID-19 at the time. However it is important to know that sometimes people with coronavirus can be missed by swab tests. The test could be done too early or too late to give a positive result. The technique of taking the swab might not be good enough to get sufficient virus on it to show up on the test. Lateral flow tests are less sensitive than NAAT and therefore need larger amounts of virus to be positive.
When people who have COVID-19 are missed by the test it is called a ‘false negative’. It is not known exactly how many of these false negative results are expected and this is dependent of the types of patients being tested. False negatives with lateral flow tests are far more likely than with NAAT. Patients with symptoms should be tested with NAAT.
However a false negative on either may mean a social care worker could be falsely reassured and return to work where they could pass on infection. Likewise, if a false negative result is obtained from a patient entering a healthcare setting, they may not be segregated from other patients appropriately and this risks transmission of the virus.
This is why it is very important to thoughtfully consider whether we should trust a negative test result when an ill person's symptoms suggest COVID-19 infection.
If you have strongly suggestive symptoms of COVID-19, it is safest to follow stay at home guidelines even if your swab test is negative.
Antibody tests (blood tests for past infection)
- A positive antibody test means that you are likely to have been infected with COVID-19 at some time. It does not necessarily mean that you are immune.
- Presence of anti-nucleocapsid antibody (with or without anti-spike antibody) is suggestive of previous infection.
- Presence of anti-spike antibody (in the absence of anti-nucleocapsid antibody) is suggestive of SARS CoV-2 vaccination.
- A negative antibody test means that your immune system has not produced antibodies, which means it is likely that you have not been infected with COVID-19 previously.
These tests can be done too early or too late. This means that you could think you haven’t had COVID-19, when you have. We don’t yet know how long antibodies will be able to be detected in patients who have had COVD-19 infection. There is therefore a chance that if we do an antibody test far enough in the future after a COVID-19 infection that no antibodies will be detectable. Also, a small number of patients with confirmed COVID-19 infection do not make antibodies at all. These are new tests and we expect better information about how reliable they are in the coming months.
Antibody tests are useful because it allows researchers to track how many COVID-19 infections there have been. Some people have very mild or no symptoms which means that the amount of COVID-19 in the country may be higher than the amount of people who have had symptoms from it.