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Most of us have heard a horror story about COVID-19 testing, where it feels like the swab goes right through to the person's brain.

Obviously, swabs do not go anywhere near the point of jabbing someone's brain.

In fact, Australian guidelines say swabs should only go a few centimetres up someone's nostril.

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For most people over the age of 12, a swab will go between 2 and 3 centimetres into the nostril.

But that will vary from person to person.

The national guidelines for testing, which come from the Public Health Laboratory Network (PHLN), say the swab should go up to 3 centimetres deep or until there is resistance in the cavity.

For children between six and 12, the depth is 2cm.

From the ages of two to six, the swab will go about 1.5cm into the nostril.

And for those under the age of two, the swab will only go 1cm deep.

The swab needs to be rotated against the nasal wall to absorb the secretions required for testing.

This cavity is quite small and lined with membranes containing a lot of tiny blood vessels, making it quite a sensitive area.

And while it helps to trap dust and other particles in the air from going into your lungs, it's not all that pleasant having a foreign object — like a swab — in there.

So the swab being rotated in that tight, sensitive area could be behind that "brain tickle" sensation you might have heard about.

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These 2-3cm tests are done using mid-turbinate swabs, which are also called deep nasal swabs.

Mid-turbinate swabs refer to structures on the inside the nose called turbinates, which are made of bone and soft tissue.

They're located a bit more than a centimetre into the nasal cavity.

But there are other swabs out there too.

Those horror stories may just have come from the use of nasopharyngeal swabs, which go further back into the nasal cavity.

A diagram from the US Department of Health and Human Services (DHHS) shows these swabs going deeper until they reach the nasopharynx, just above the roof of the mouth.

Well, that depends on the person's head.

The US DHHS diagrams, which were adapted for use by the PHLN, says the swab should reach a depth equal to the distance from the nostrils to the outer opening of the ear.

But an Ottawa Public Health explainer says swabs should only go about half that distance, somewhere between 4-6cm.

Australia's national guidelines recommend the use of the 2-3cm swab.

"However, this advice does not preclude the use of a nasopharyngeal swab," a Federal Department of Health spokesperson said.

According to the update log for the national guidelines, references to nasopharyngeal swabs were updated with the current deep nasal swabbing advice on April 1.

So, the longer ones haven't been ruled out, but the shorter ones are favoured.

And while these are just recommended guidelines, the states take them on board.

In NSW, for example, these swabs have been widely used across the state since June.

"NSW Health's preferred method for testing is the mid-turbinate swab, which is less invasive than the nasopharyngeal swab," a NSW Health spokesperson said.

But this depends on the type of test you're going for.

If you're being tested for other respiratory illnesses at the same time, which is something your doctor might request to work out what could be behind your symptoms, then the nasopharyngeal swab might be used.

Currently, the guidelines say the same swab that goes up your nose can also be used to take a sample from the back of the throat.

But it's worth noting that the PHLN guidelines suggest the mouth sample should be taken before the nasal sample.

It's important both the back of the throat and the nasal cavity are swabbed "because the virus may be found in one location and not the other early in an infection", a NSW Health spokesperson explained.

Obviously, the shallower swabs are more comfortable than the deeper swabs.

And because the deeper nasopharyngeal swabs are more likely to procure a reactive cough or sneeze from a potentially infected person, they're riskier for clinicians to administer.

The PHLN says the shallower swabbing method is recommended "to optimise the chances of virus detection while minimising discomfort for the individual being tested".

A US study of about 500 participants found mid-turbinate swabs were 96 per cent effective at detecting COVID-19, and that was when administered by the patients themselves, not trained professionals.

Another study from the US, with 40 participants, found that both types of swabs identified COVID-19 patients in the early course of their disease but the nasopharyngeal swabs were more reliable later in the disease course.

But if the test is being combined with other tests, the longer swab might be the better option.

Here's what the ACT guidelines say:

"If also testing for other respiratory viruses, an NPS [nasopharyngeal swab] remains the specimen of choice for Mycoplasma pneumoniae and Bordetella pertussis.

"For other respiratory viruses, a deep nasal swab is adequate."

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