SOMETIMES PCR test results don’t come back with the result we were expecting.
Perhaps you get a negative result despite being certain you have been exposed to the virus, or you have a positive lateral flow.
Or, after waiting potentially days for your result, the result simply says “void”.
Dr Alexander Edwards, Associate Professor in Biomedical Technology, Reading School of Pharmacy, University of Reading, said: “When you get a void result, it often means the swab wasn’t taken effectively.”
Taking a Covid PCR test correctly is vital if you want to be confident in its results, experts say.
You should always leave enough time to sit down and read instructions carefully before doing the test.
Dr Edwards told The Sun: “There are things that can definitely affect the test accuracy. In other words, these are changes that really could change the result.
“Things like swab technique are important. If you don’t swab a site where there is virus present, you can’t detect the virus.
“In the end, when people have a lot of virus it’s probably quite easy to detect- it’s everywhere- this is why the virus spreads so well.”
But on the other hand, it’s possible the test will miss the virus if someone has mild or no symptoms, making it even more important to do the test as carefully as possible.
“This is why testing in general is really good for picking up and confirming a Covid-19 infection, but not so good at ‘proving’ someone is ‘safe’”, Dr Edwards said.
He also said it’s vital everyone does the test in the same way, because “it’s hard to know how reliable something is if people keep changing the way they do it”.
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The PCR test is the most accurate test out there. But it’s not perfect.
The probability of detecting Covid comes down to when the test was taken or how carefully the swab was handled, for example.
When performed under lab conditions, the UK Government estimates that PCR tests should never show more than five per cent of “false positives” or five per cent of “false negatives”.
However, in real-life conditions, when people are conducting their tests at home or haphazardly in their car, false results may be more common.
Here are a list of reasons that could alter the true result of a PCR test:
1. Drinking or eating
Covid tests shouldn’t be taken if you have eaten or drunk in the past 30 minutes.
“If you eat or drink or use mouthwash just before testing, there is the chance that some residue could interfere with the test,” Dr Edwards warned.
2. Failing to clean
Before doing a test, you must clean the surface you are using for the test, such as the table.
You also must wash your hands thoroughly for 20 seconds using soap and water or hand sanitiser.
Failing to do these prior to the test may result in contamination, which would void the PCR test.
3. Swabbing around the mouth
The swab should not touch anywhere other than your tonsils (at the back of the throat) and nose.
If the swab touches the tongue, teeth, cheeks or gums, it may ruin the sample.
But the Government says if this does happen, complete and return the test anyway.
4. Touching the swab
For the same reasons, touching the swab with your fingers could ruin the sample by contamination.
The swab should also not be put on any surfaces, for example a table.
After conducting the swab test, put it into the tube as quickly as possible – don’t leave it lying around where it may get spoiled.
5. Not blowing your nose
It’s important to blow your nose before you take the test.
Dr Edwards said “too much snot or mucus can sometimes block the extraction or test reaction”.
6. Swabbing a pierced nose
The testing leaflet says you should not swab the nostril if it is pierced. If your nose is pierced on both sides, remove the piercing on one side before swabbing.
You should also wait 24 hours if you have had a nosebleed, or swab the nostril that was not affected.
7. Too much force
The Government says to avoid using excessive force, pressure or bending when collecting swab samples.
This may break the swab, and then you cannot use it.
8. Not collecting enough sample
The swab needs to be rubbed over each tonsils with good contact four times, and rotated in the nose for ten full circles.
This is to make sure the swab picks up enough mucus.
If you don’t take the time to do this, there may not be a sufficient quantity of mucus for viral particles to be collected.
9. Testing too early or late
In some instances, the PCR test may not pick up the virus because it’s too early in the infection.
It could take several days, or even more than a week, before the virus levels in the cells of your nose and throat are high enough before the test can detect it.
This is unlikely if you have symptoms, however.
If the test is performed too late, for example when someone has had symptoms for more than a week, the virus may not be present in detectable quantities.
It is unlikely they are contagious, however.
On the other hand, it is possible that fragments of the virus could still be present in the throat for several weeks after the infection, in which case the person would not be infectious.
10. Using an old test kit
UKHSA has previously told Public Technology that as the pandemic has progressed, there has been an uptick in the number of tests taken using expired kits.
These may have been kept in people’s homes for some time, perhaps for a “just in case” scenario.
In these cases, a void result is recorded before the processing of the sample has even begun, the agency said.
11. Sending a home kit too late
You should always send a home testing kit the day you take it.
And you need to post the test one hour before the last collection at a priority postbox – the details on how to find one are given in the kit.
12. Not registering the kit
All home testing kits need to be registered online before they are sent to the lab in the mail.
WIthout doing this, the lab won’t know who the test belongs to and how to contact you.
13. Packaging the kit incorrectly
Your testing kit, whether at home or at a testing centre, will have a number of components for packaging.
Follow the instructions carefully to avoid making any mistakes.
If it’s not done correctly, the lab may consider the sample at risk of contamination.
There are also reasons why a Covid PCR test might fail that has nothing to do with you.
Laboratory errors include clerical error, testing the wrong sample, cross-contamination from someone else’s sample, or problems with the chemicals used.