COVID-19: Experts weigh in on effectiveness of saliva tests versus nasal swabs

The Government has defended not bringing in large scale saliva testing for COVID-19 over concerns it's not as accurate as nasal swabbing - but some scientists say there's no reason not to replace the invasive nasal testing for spitting in a tube.

Associate Professor Arindam Basu from the University of Canterbury says saliva testing and nasopharyngeal swabs have similar levels of accuracy when it comes to detecting COVID-19.

"The saliva tests overall, when pooled together were 83.2 percent accurate," he said on Tuesday.

"This indicates that on an average such a test will miss about 17 percent of  true positive cases."

Comparatively, nasopharyngeal swab tests are around 85 percent accurate - which suggests they will miss approximately 15 percent of true positive cases.

"They are not all that different, as their 95 percent confidence intervals or credible intervals overlap," said Basu.

"Overall, in terms of how accurate they are, there is little to choose from saliva and nasopharyngeal swab tests. The advantage of saliva tests is obviously their ease of obtaining samples."

However one of the issues with saliva tests, according to the University of Otago's Professor David Murdoch, is that it can take extra steps in the laboratory to process.

"This can affect laboratory workflows and efficiency."

He says newer methods are in development, with the hope they will require less technical expertise and allow samples to be tested outside of a lab.

He agrees with Basu that saliva testing has a similar accuracy rate to nasal swabs, and says the evidence in support is "clear".

"Saliva testing for COVID-19 may play a useful role in the frequent testing of individuals, as long as resourcing and laboratory workflow issues can be adequately addressed."

New Zealand currently has two saliva tests available - SalivaDirect, which is developed by the Yale School of Public Health and Shield - developed by the University of Illinois and currently in use at Auckland Airport. 

The Government started offering "less invasive" saliva testing in January at quarantine facilities as a "precautionary measure" in the wake of more contagious variants of the coronavirus popping up in managed isolation facilities. 

Despite this, the Ministry of Health has completed just 140 saliva tests in total nationwide.

COVID-19 Response Minister Chris Hipkins said on Tuesday more saliva testing could happen here, but he won't say when. 

"I am not going to rule out expanding saliva testing."