Why RATs are good but not great for confirming Covid
Thursday, 10 March 2022
Gary Payinda is an emergency medicine specialist working in Northland and Auckland.
OPINION: First off, a disclaimer. I’m an emergency doctor. My interest in rapid antigen tests (RATs) is practical, not theoretical.
I need to understand RATs because I have to use them myself, as a critical healthcare worker under a mandatory testing regime, and as a doctor having discussions about RAT tests with my patients.
Second, the take-home point: your RAT test results can be well-trusted if you are having symptoms and the test result comes back positive. You should home isolate, understanding you have Covid.
But in every other case, you must understand their limitations and know that RAT tests are “good” tests, not “great” tests.
**READ MORE:
* Free RATs make economic sense
* Price war beckons as The Warehouse starts selling rapid antigen tests
**
They will never replace the tried and true methods of Covid management: good mask wearing indoors away from home, boosters, and adequate indoor ventilation.
Masks and ventilation have been effective in addressing every respiratory epidemic from tuberculosis to influenza, and continue to be the backbone of any Covid response.
Covid is an airborne infection, caused by breathing in other people's exhaled breath. Good masks are, and will continue to be, highly effective at reducing the amount of Covid we breathe in. They are also consistent, reliable, cheap and easy to use.
Studies on RAT tests, on the other hand, highlight their many vulnerabilities. In the published medical literature, one can find sensitivities for RATs ranging from a miserably low 40 per cent, right up to the 98 cent printed on some manufacturers' advertisements.
What does this really mean? To start with, a quick definition of sensitivity: if 100 people have Covid, a test with 100 per cent sensitivity would pick up all 100. In real life, this is rarely the case. Perfect tests are nearly impossible to find in medicine.
Among the latest and best studies on RATs, sensitivities are around 94-98 per cent if the patient has a high viral load or is symptomatic.
In a group of 100 asymptomatic people with Covid, 20 people will receive a falsely negative test result. They may sally forth into the world thinking they are safe to others, tricked by a false sense of security.
Why is this?
It’s because RAT tests are very good tests at picking up those who are the most infectious. People with symptoms (cough, fever, chills), are more likely to have high viral loads, because they are near the peak of their illness.
But RATs are significantly less good at correctly identifying those who are in the very early days of infection, very late in their infection, those who have had a single test rather than repeated tests over several days, and those who don’t do the test correctly.
There are around a dozen little steps required to perform the test correctly. And myriad ways of getting it wrong.
Not blowing your nose beforehand, not swiping both nostrils for 15 seconds each, forgetting to wait for more than 15, but not more than 20, minutes. Eleven RAT test brands with 11 slightly different sets of instructions. Every minor error increasing the chance of missing a Covid infection, turning a test that may look good when performed in a lab by healthcare workers, into a test that is unreliable when performed by the average Joe.
The Ministry of Health has excellent guidelines on who should get a RAT test: people who are having Covid symptoms, people who live with someone who has tested positive for Covid, and critical workers who will benefit from regular, repeated testing.
But now that RAT tests are available for purchase in shops, there is the risk of irresponsible testing. We’ve already seen that with some anxiety-driven runs on RATs, similar to the illogical runs we’ve witnessed on toilet paper, but with more significant implications for public health.
People who feel well, who are taking a RAT in order to safely visit Nana, or go to a party or pub, need to realise that RAT tests will in fact get it dangerously wrong a significant portion of the time.
RATs or no RATS, masks, booster vaccine, and ventilation recommendations will still apply for the foreseeable future, at least while the world faces significant ongoing losses to the pandemic.
Read more from Gary Payinda at drgarypayinda.com.
This article has been updated to clarify that the sensitivity rate for asymptomatic people applied to those who were infected with Covid but asymptomatic. The change was made at 4.55pm on Thursday, March 10.