Some border workers are now undergoing daily spit testing. Does this mean the end of nasal swabbing? We explain how the tests work and why the two tests co-exist.

Voluntary saliva testing has been rolled out at Auckland Airport and the Jet Park quarantine facility for border workers, sparking hope for some that the sometimes uncomfortable nasal swab test could be on the way out. We’re here to dash those hopes.

Yes, both tests use the same PCR processing – but that doesn’t mean one can replace the other.

Do saliva tests and nasal swabs test the same thing?

Yes, they’re both testing for Covid-19 using the PCR testing method. No, the cells they’re testing aren’t necessarily the same.

Nasal swabs take cells from your nasopharynx, which is the other side of the roof of your mouth and all the way at the back. You can understand why this might be uncomfortable. Border workers need one every two weeks.

Saliva tests, which are currently voluntary, involve spitting into a tube every day. They’re much less invasive, and because workers can collect the samples themselves, other healthcare workers are less likely to be exposed to the virus.

How does this make a difference to the results?

James Ussher, an associate professor at the University of Otago and director of the Webster Centre for Infectious Diseases, told The Spinoff the downside of saliva testing is a higher likelihood of a false negative.

“You are swabbing a slightly different space,” he said. “With saliva, you’re getting oral secretions, and depending on how well the swab is collected you’ve got the epithelial lining [e.g. cheek cells] of that area.” Basically, there’s a lot of spit in those tubes and not much of it will be virus. It’s a lot more raw material to test and can take longer to process, depending on resources.

“For most respiratory viruses, the cells at the back of the nasopharynx tend to be a more sensitive specimen,” he said. Anyone who’s experienced one will know the sample is much smaller, and less covered in secretions. It’s easier to process and is more likely to have a high viral load than oral samples. It’s the gold standard of respiratory specimens.

So saliva tests are less sensitive. But is it by much?

The two saliva testing brands currently in New Zealand, SalivaDirect and Shield, claim to have accuracy rates around 94% and 88.9% respectively. The Ministry of Health states nasal swab tests have at least 95% accuracy, and this table of nasal swab brand results appears to back that up.

Ussher said a small difference in sensitivity still matters “It’s worth pointing out that we’re in a different situation in New Zealand,” he said. “Our tolerance of risk is different to what it might be in the US, for example.” In places like the US, severe cases are a priority – not asymptomatic ones. The test doesn’t need to be as sensitive. “You don’t necessarily need to detect the low level and historic infections [there],” said Ussher. “You’re mainly looking for the people who have high viral loads.”

Hang on, does that mean the “gold standard” nasal swab test could miss 5% of Covid-19 cases?

That’s one of the reasons we test people in MIQ so often. A test might might show a false negative because the viral load is very low and can’t be detected by the test. You find out the truth by testing again, and again, and again. Four negatives shouldn’t make a positive (unless you’re doing year 10 multiplication).

Why are we using saliva tests at all?

It’s an extra layer of protection. Lesley Gray, a senior lecturer at Otago’s department of Primary Health Care and General Practice, said a combination of saliva testing and nasal swabs could help keep tabs on the more infectious variants of Covid-19, and provide a level of security for high-risk people. “Where workers, for example, are in high risk locations, the need for regular or frequent testing is in place,” she said. “I understand one of the aged healthcare providers [Ryman HealthCare and Hill Laboratories] has chosen to introduce saliva testing recently for staff.”

Ussher said using saliva tests at the border is “additional surveillance”. “You can accept a slightly lesser sensitivity in the test when you’re using it in that way,” he said.

Will we ever be rid of the nasal swab?

“I don’t see that happening in the short term,” said Ussher.

Gray said saliva testing could be introduced more widely, but still wouldn’t replace nasal swabbing. “I suspect public testing would require an easy to use, near person result like a colour change – like drug tests or pregnancy tests – to be useful for public use,” she said. However, she warned that if at-home saliva tests are introduced there could be longer delays on test results. “Saliva tests that still need to be sent off to the lab for result processing might overwhelm the labs that are having to process those of border workers or contacts of cases.”

So no, nasal swabs will likely stay the gold standard, even if we get at-home saliva tests.