Testing is important but many are falsely reassured after a negative test. An area of concern is that individuals are being tested without clinical input and an explanation of the relevant test and limitations.
Here’s a summary of the 2 main tests available:
PCR tests – throat / nose swab that test for presence of the actual virus. They are usually accurate when you first start developing symptoms (likely withing the first 7 days) but this is dependent on taking the swab correctly. An incorrect swabbing technique can certainly give a false negative result.
This test can also be positive in those with no symptoms – so called asymptomatic carriers who are still able to spread the virus.
The PCR swab may show as positive in the incubation period, possibly 1 or 2 days before developing symptoms in some cases but this is not completely reliable and therefore despite a negative test you may still be a carrier and able to infect others.
We are unsure how long the test will remain positive for, as once symptoms develop individuals will clear the virus at different rates. Another interesting development is that some people test positive even 4-5 weeks after becoming symptomatic – currently the consensus is they are not infectious at this stage but the swab is being overly sensitive and picking up inactive viral genetic material.
Serology – generally a blood test which looks at the immune response specifically to the SARS-CoV2 virus. Serological tests can look for for both short term (IgM) and long-term (IgG) antibodies. In essence it looks for previous COVID19 infection but is not ideal for assessing acute current symptoms.
Timing of the test is important. The antibodies may not show up as positive in the first few days of having the illness so if tested too early you may get a false result. Most manufacturers state tests are accurate after day 7 – 14 of having symptoms.
The IgG antibody is likely to remain positive for at least a few months. It can show if you had COVID19 in the past but we are unsure if this means future protection and immunity. As with most tests there are also limitations (discussed in next update) – this is especially the case with point of care / home tests.