Are PCR tests more accurate than saliva swabs? Everything you need to know

Follow the latest updates on the Covid-19 pandemic here

Regular testing for Covid-19, along with vaccination, has been hailed the fastest way out of the pandemic, and the UAE was one of the first countries to facilitate testing en masse.

The first big processing lab opened in Abu Dhabi in April 2020, after being set up and staffed in only 14 days.

Since then, nearly 80 million tests have been carried out in the UAE, with the country averaging around 330,000 a day.

Dubai International airport also has one of the world’s biggest in-house Covid-19 testing laboratories, processing 100,000 samples a day.

There are a variety of different tests for Covid-19 available, with the nasal PCR the most commonly used. But how deep does the swab need to go, and why might you keep producing positive results after recovering from the infection?

The National consulted several doctors to find out, including Dr Rania Hawayek, medical director of Circle Care Clinic in Dubai, and Dr Ruhil Badiani, a GP at the London Centre for Aesthetic Surgery.

How do PCR tests work?

PCR tests use special proteins and enzymes to make copies of any infectious particles in the sample, in order to identify them. This is a process that can be completed in a matter of hours.

An alternative type of test is a culture, where scientists wait for the natural growth of an infectious organism in a sample. These usually take two to three days. Hence, PCR tests are much more efficient and require a much smaller sample to yield a result.

Why does the swab have to go so far up the nose?

When testing for Covid-19, doctors need a nasopharyngeal sample. The nasopharynx is the area at the very back of the nose, forming the roof of the throat. This is where the swab is most likely to pick up the greatest numbers of viral particles.

A healthcare worker will carefully extend a long cotton swab into an individual’s nose or throat for a few seconds, allowing it to soak up mucus.

"These swabs are extremely unpleasant, but they are the most likely to catch the virus, since they access the areas that the virus is most likely to live," said Dr Badiani.

"They are also the most heavily validated, meaning they have the most data suggesting they work."

Why do PCR tests seem to differ from place to place? Some hurt, some don't.

Several factors can affect how painful or uncomfortable the PCR test is, including your anatomy and the skill of the technician, said Dr Badiani.

"Some technicians have more experience than others, or a lighter touch. Some have more care to conduct it thoroughly and collect a proper sample. Others are pressured by the patient to do it quickly and not insert it as deep," said Dr Hawayek.

"Although the technique should be standard, it is often affected by human factors, both on the sides of the technician and the patient."

The goal with the swab should be to get to the nasopharynx without hitting anything along the way. Unfortunately, the geometry of the nasal passages is different from person to person.

The mucous membrane that lines your nose has a lot of nerve endings. In general, the body’s pretty tolerant the first couple of centimetres - as far as you can stick your finger in. Beyond that, the mucous membrane reacts to being touched.

Can the swab injure me if it goes too far?

"Although it feels that the swab is going into your brain it actually can’t get near it," said Dr Badiani.

There are several layers of protection between the nose and your grey matter, including the mucosal lining, the olfactory epithelium (the area involved in smell) and the dura mater, which when translated from Latin means ‘tough mother’, which is very difficult to penetrate.

Also, although it might feel like the swabs are inserted upwards, they are actually pushed along the floor of the nose, in a backwards direction, towards the nasopharynx.

"Believe it or not, the correct distance is the same as from the ear to the nostril," said Dr Hawayek.

If accidentally inserted upwards, the swab could cause damage to the turbinates, but to do that, the swab would have to be inserted with a lot of force, by someone who is ignoring the obvious resistance.

This is extremely difficult to do, she said.

In a very small number of cases, a PCR test has led to a leak of cerebrospinal fluid, but the risk is very low and those who have had this probably have a birth defect that has gone unnoticed until now.

People who have had extensive sinus surgery should also discuss with the ENT surgeon whether a nasal PCR swab is safe.

Nasopharyngeal tests can cause nose bleeds as the mucosal membrane is thin and has a lot of blood vessels.

Some people may also faint, but this is harmless, does not require any treatment and recovery is usually within a few minutes.

Are two nostrils better than one?

Two nostrils are often used in order to saturate the tip of the swab with fluid from the back of the nose.

The Centres for Disease Control and Prevention in the US published guidelines earlier this year and reported samples can be obtained from one nostril unless there is a deviated septum or blockage creating difficulty in obtaining the specimen.

Therefore, inserting the swab into both nostrils is technically unnecessary.

Should swabs remain in the nose for a certain amount of time?

Yes, they need to be rotated, and remain there for five to 10 seconds, as this is the time that research has shown is needed to saturate the tip of the swab with a sufficient quantity of nasal secretions to yield a true result.

Would a snotty nose give a better result than a dry nose?

Yes it would, as the virus is found in the nasal secretions. That is one reason why doing a PCR test too early after exposure and before symptoms appear is not very useful. It is most likely going to be negative, and give a false sense of security.

If you discover that you were exposed to a positive case, it is best to isolate and test a few days after exposure, or as soon as symptoms appear, rather than test too early.

Are swab tests in the throat as effective as PCR nose tests?

"Yes they are, when done properly, said Dr Hawayek. "The tongue, teeth and inner cheeks must be avoided, and the sample collected from both sides of the throat."

Are saliva tests where you spit in a pot as effective as nose tests?

In January, the American College of Cardiologists compared nasopharyngeal swabs with saliva tests and found that sample collection through saliva or nasopharyngeal swabbing does not differ significantly in sensitivity.

However, the technique for proper sample collection is a little tricky and has to be carefully followed.

It is best to avoid food for one to two hours before the test, and between one and five millilitres of saliva has to be collected in the tube.

"It is a more difficult specimen to extract - hence the reason why this is not done more widely," said Dr Hawayek.

How is the swab stored before it goes to the lab?

The swabs are stored in special transport tubes filled with a fluid called viral transport medium, which aids in the viral particle extraction.

These are stored in a fridge before being transported to the testing laboratory. The temperature at which it is stored depends on the time it will take to reach the testing facility.

Can a test result be incorrect?

Yes, the rate of false negatives can be as high as 30 per cent with different PCR tests, based on the timing of the sample, compliance with collection technique, and other factors.

Therefore, PCR tests are more useful in confirming the presence of an infection, rather than ensuring one is not infected.

"When a Covid-19 infection is suspected, whether because of exposure or symptoms, and a false negative result obtained, it's best to repeat the test," Dr Hawayek said.

"PCR tests can also be falsely positive, but this is fairly rare.

"These tests are so sensitive that they can continue to be positive even weeks after infection, because they can also detect dead, deactivated viral particles that may still be present in the respiratory tract."