Author: Australian GP, Dr Wendy Burton, MBBS, FRACGP (Hon)
THE SHORT ANSWER
When you are being tested for Covid-19, the thing we want to know is if you have the infection and are you infectious. Well, rapid antigen tests certainly can help answer these questions, but you need to understand what they can and can not do (read on if you want to understand why and how)
We know that for most of the rapid antigen tests (RATs) available in Australia, the chances of a positive test being a false positive lessens rapidly if you have symptoms and/or are a close contact of a positive case and a negative test is still reliable
If you test negative, there is still a chance that it is a false negative, especially if you are testing very soon after your exposure (see the visual below, the yellow bit is when your RAT is likely to be positive). If you are in a very high risk situation (e.g. if you are visiting or caring for vulnerable people or are a healthcare worker) you can reduce the risk of having a false negative result by repeating the test on a regular basis. You reduce this risk even further by doing a repeat test with a different brand of RAT
THE LONG ANSWER
Why would a rapid antigen test not correctly confirm that you have a Covid infection?
The RATs are antigen tests
Like every test ordered by medical scientists, there is a false positive and a false negative rate
false positives are when this test says you have Covid, but actually, you don’t. This is not dangerous, but it doesn’t help us to know how many people truly have the infection and has implications for where you go, what you do, how safe you feel and for public health planning
false negatives are when this test says you don’t have Covid, but actually, you do. This is very dangerous if it is being used to identify who is safe to be out and about or in contact with vulnerable people e.g. healthcare workers, family members. It may also lead to delays in seeking appropriate help if you are unwell
The antigen tests, while complete in 10-15 minutes, may not be positive until a few days after you have been exposed to Covid, and they are positive for a shorter period of time compared to the PCR test (see visuals above and below). It is, however, useful as a marker for infectivity
The PCR swab test will be positive earlier in the infection, but lasts longer and so may be positive even when you are no longer infectious. This helps tell us who has or who has had an infection, but not as helpful in knowing if it’s ok to be out and about again (although it is likely that most people are no longer infectious after 7-10 days)
The Therapeutic Goods Administration (TGA) in Australia has approved a number of RATs, with a minimum clinical sensitivity of at least 80% (for specimens collected within 7 days of symptom onset) and a minimum clinical specificity of at least 98%
Sensitivity is defined as how likely is it that the test correctly identifies those who do have the condition (i.e. the true positive rate) and specificity is how likely it is that a test correctly identifies those who do not have the condition (i.e. the true negative rate)
if a test has a sensitivity of 80%, this means that it will find 80% of the actual positive cases but 20% of the positive cases (or 1 person in 5) will test negative (false negative rate)
if a test has a sensitivity of 95%, this means that it will find 95% of the actual positive cases but 5% of the positive cases (or 1 person in 20) will test negative (false negative rate)
if a test has a specificity of 98%, it means that it will correctly identify 98% of those who do not have the infection, but 2% (or 1 person in 50) of those who do not have the infection will test positive (the false positive rate)
Even if you have the best of the available tests, so for the RATs, with 98% specificity and 95% sensitivity, a positive test is less likely to be truly positive if a condition is uncommon in the population you are testing, but the rates of false negatives will be very low
this is true of stress tests for heart disease and ultrasound scans for ovarian cancer and it is definitely true for Covid-19
False positives/false negatives
These have serious implications
The incidence of confirmed Covid-19 cases, since the beginning of the pandemic (Source, WHO and Worldometer) are
USA 15% (Dec 31, 2021); 29% (Oct 3, 2022)
UK 18% (Dec 31, 2021); 34% (Oct 3, 2022)
Australia 1.2% (Dec 31, 2021); 39% (Oct 3, 2022)
IF we make a false assumption that everyone who has ever had Covid is currently positive, the figures play out as above. As you can see from the graph above, a positive RAT in December 2021 was more likely to be a false positive (if you had no symptoms) in Australia, who then had a low incidence, hence the initial advice to have the infection confirmed with a PCR test.The lower the incidence of the condition, the less reliable a positive test result (see graph below)
If you are testing because you have symptoms or had close contact with someone who is Covid positive, your chance of being positive goes up (pretest probability) and the false positive rate will be markedly lower with the false negative rate slightly higher (see visual below)
Sensitivity 95% Specificity 98%
The less sensitive a test, the higher the rate of false negatives, and while it climbs more quickly than when you use a more sensitive test, this number remains low so long as the incidence of the infection within the community is low (see below)
Sensitivity 80% Specificity 98%
To state the obvious…
No test is reliable if it is not used or not used properly
Some of the tests involve multiple steps, mixing of solutions and 15 seconds a side of nasal swabbing to 1.5 - 2 cm inside the nostril. There are many people, particularly young children, who find this difficult to do and so may not do it properly or may not do it at all. The saliva tests are generally simpler to do, can also involve multiple steps, but most have reduced sensitivity, which means there will be more cases missed (false negatives) but as noted above, the number of missed cases remain small while our case numbers remain low
You can read the instructions for each test before you buy by clicking on the PDF beside each brand name on the TGA website
One last thought
As of January 5, 2021, the official Australian National Cabinet advice is that these tests can be used if you are symptomatic - although a PCR is more reliable, the surge in our case numbers is improving the reliability and our PCR testing capacity is finite. They can also be used if you are asymptomatic, especially if you are in a high risk environment (healthcare worker, visiting/caring for vulnerable people) and repeated on a regular basis, ideally just prior to your work shift/contact with vulnerable people
The ABC have also written about this topic
Disclaimer: This is not individual medical advice. Please follow your local, state or national guidelines and talk to your GP about your particular situation and healthcare needs
This work is licensed under a Creative Commons Attribution 4.0 International License