Covid-19: How accurate is the new Saliva test and how does it work?

Team Udayavani, Jul 6, 2020, 10:41 AM IST

A new saliva testing method could eliminate the need for nasal passage swabs, reduce costs and increase mobility of testing used to detect the coronavirus

Nasal and throat swab is the preferred specimen for detecting the virus, but PCR testing on saliva has recently been suggested as an alternative method.

Collection of nasal swab involves the patient tipping their head back and receiving the nearly six-inch-long collection lance up a nostril

Once up into the nostril, the collector tilts the swab forward and pushes it about three inches toward brain stem, stopping when the tip hits the pharynx, the part of the airway that connects the nose to the throat.The test swab is then packaged and sent to a lab for analysis.

A diagnostic test can show if you have an active coronavirus infection. Two type of diagnostic test include,  molecular (RT-PCR) tests that detect the virus’s genetic material, and antigen tests that detect specific proteins on the surface of the virus.

An antibody test looks for antibodies that are made by the immune system in response to a threat.

Currently, molecular test is widely used which include taking samples from the patient’s nose or throat. Then they put the swab in a sterile container and seal it for transport to a lab.

During the shipping process, the swab must be kept within a certain temperature range to keep the virus alive so that the test will be accurate.

Later the lab technician mixes chemicals with the swab to extract the genetic material of any virus that may be on the swab and uses special chemicals, called primers and probes, and a high-tech machine to conduct several controlled heating and cooling cycles to convert the virus’s RNA into DNA, and then make millions of copies of the DNA.

When DNA binds to specific probes, a special type of light is produced that can be seen by the machine and the test shows a ‘positive’ result for infection

Saliva Test

It is easier and less uncomfortable to take saliva than a swab. One must collect saliva in their mouth for a minute or two and then spit it into a jar. After which the jar is sent to a lab where the saliva is tested.

It uses the existing PCR test, but examined saliva instead of nasal samples. Polymerase Chain Reaction (PCR) test can pick up the genetic information of the virus (the RNA) when someone is actively infected.

Accuracy of the saliva test

Study shows that detection from saliva is less sensitive than a nasal swab, with a lower concentration of virus in saliva compared to swabs.

Many experts prefer Nasal and Throat swabs over saliva test because saliva is a complex mix of fluids containing water, electrolytes, proteins and enzymes and all of these dilute the viral genetic material of Covid-19.

But, on the other hand, saliva test is convenient in places with limited staff to collect swabs or where high numbers of tests are required. Also, saliva test is best for children and other people for whom a nasal swab is difficult.

Although, the saliva test is less sensitive but in some instances, a test with moderately reduced sensitivity is better than no test at all.