One of the most widely-accepted ways to identify the presence of coronavirus in a person is through throat and nasal swabs. Both the popular tests currently under wide use - rapid antigen and RT-PCR - see healthcare professionals inserting swab and gently pressing the inside of a person's nostrils to get as much nasal discharge as possible for testing.
But why do some healthcare workers dig deeper?
Our nasal cavity is much larger than our nose, extending into the skull and draining into the back of our throat.
Inhaled coronavirus particles can attach to different soft tissues in the nasal cavity or throat. The goldmine is nasopharynx, in the upper part of the throat behind the nose, which is tapped by the healthcare professionals to know if a person has indeed contracted the virus.
The nasopharynx is located in the upper respiratory tract and Centers for Disease Control (CDC) has prescribed guidelines for sampling the mucosa from there. It says that only synthetic fiber swabs with thin plastic or wire shafts should be used for collecting the specimen, instead of calcium alginate swabs or swabs with wooden shafts.
But getting to nasopharynx can be uncomfortable as the testing swab has to move through four inches of soft, sensitive tissues. And as per prescribed guidelines, has to stay there for 15 seconds for sample collection.
A sample collected from the middle or shallow parts of nose is less likely to detect the virus. It is helpful only if a person has a high viral load in the nose.
Types of Covid-19 tests
Dr Hanan Balkhy, World Health Organization's (WHO's) Assistant Director-General for Antimicrobial Resistance, says there are three categories of testing.
“The first one is to identify whether the actual Covid virus genetic material exists, and that's called a NAAT test. It's the PCR testing where you would have a nasal pharyngeal swab or pharyngeal swab taken. Then they look for the genetic material of the virus itself,” she said.
“The second type of testing is when they try to identify one of the outer protesting of the viral shell or envelope. That's called an antigen testing. The third type is to detect within the human body, whether they have developed antibodies,” she added.