It seems not a day goes by without learning someone in our inner circle of family, friends and colleagues has COVID-19. When we ask how unwell our acquaintance is, the responses vary from "they're really crook" to "you wouldn't even know they had it".

This is in line with studies that report moderate to severe illness in a minority of people (usually older with other risk factors) and that up to one in three positive people exhibit no symptoms.

Given the ubiquitous presence of this highly infectious coronavirus in our community and the high rate of asymptomatic illness, those who have not been diagnosed with COVID might wonder, "how would I know if I had been infected?" And, "does it matter if I have?".

How COVID-19 is diagnosed
Most people know they've had COVID because they had a fever or upper respiratory tract symptoms and/or were exposed to an infected person AND had a swab test (PCR or rapid antigen) that detected the COVID virus (SARS-CoV-2) in the upper airway.

At the beginning of 2022, many people with consistent symptoms or high-risk exposures were not able to access PCRs or RATs to confirm their diagnosis, but instead presumed themselves positive and quarantined.

It is possible to diagnose past infection in those who never tested positive. A blood test can look for SARS-CoV-2 antibodies (also known as immunoglobulins). When we are infected with SARS-CoV-2, our immune system launches a precision counter strike by producing antibodies against viral targets, specifically the Spike (S) and Nucleocapsid (N) proteins. COVID vaccination induces a similar immune response against the S protein only. The S antibody "neutralises" the invader by preventing the virus from attaching to human cells.

These antibodies can be detected within one to three weeks after infection and persist for at least six months – potentially much longer. A blood test that shows antibodies to S and N proteins indicates someone has been previously infected. Detection of antibodies to the S protein only indicates vaccination (but not infection).

The problem with antibody tests
Before you rush off to get a COVID antibody test, there are a few notes of caution. There is still much to learn about the characteristics of the immune response to COVID infection. Not everyone mounts a detectable antibody response following infection and levels can decline to undetectable levels after several months in some people.

Because there are other circulating seasonal coronaviruses (such as those that cause the common cold), tests may also pick up antibodies to non-SARS-CoV-2 strains, leading to "false positive" results.

Commercial and public hospital pathology labs can perform SARS-CoV-2 antibody testing, but the interpretation of results should be undertaken carefully.

So, antibody testing should really only be done when there's a good reason to: say, when confirming past infection or effectiveness of vaccination is important for the current care of an individual. Diagnosing a post-infectious complication or eligibility for a specific treatment, for example. It could also be useful for contact tracing or for assessing the background population rate of infection.