New Delhi: Indian cricket coach Ravi Shastri is isolating in England after having tested Covid-19 positive in an lateral flow assay test. The test is one of the many rapid tests that are available, and is recommended for those who have a history of contact with a Covid positive person but do not have symptoms themselves.
Lateral flow assay refers to a paper-based technique where the presence of a certain compound is detected by the change in colour of a marker that is a part of the testing kit. This is the principle of a home pregnancy test, where the marker changes colour when it detects the presence of human chorionic gonadotropin, a hormone associated with pregnancy.
In case of Covid-19, the colour change happens when it encounters an antigen specific to the SARS-CoV-2 virus. That is why we refer to these tests more commonly as rapid antigen tests (RAT).
The National Health Service (NHS) in the UK recommends that people should do a rapid test twice a week even without symptoms to test their Covid status as even asymptomatic cases can transmit the virus and it is possible that the person who is infected can go on to develop serious complications.
No need for manpower
The test involves the usual collection of swabs but is designed in such a way that it can be performed at home without the necessity of any trained manpower or sophisticated equipment.
“The test for people without symptoms of Covid-19 is called a rapid lateral flow test. This usually involves rubbing a long cotton bud (swab) over your tonsils (or where they would have been) and inside your nose,” the UK’s National Health Service (NHS) says. “Tests give a quick result using a device similar to a pregnancy test. You can do a rapid test at home or at a rapid lateral flow test site. Research shows rapid tests are 99.9% accurate.”
Ordinarily, it is believed that rapid tests are more prone to false negatives, which is why in India the norm is to retest a symptomatic person using RTPCR, should they test negative in the rapid test. No such second shot is needed when a person tests positive.
According to GAVI, the global vaccine alliance, lateral flow assays change colour if viral proteins are present.
“Lateral flow tests can be designed to analyse various body fluids, but in the case of Covid-19, most tests analyse material collected from the back of someone’s nose and throat. The swab is then inserted into a tube of liquid, after which a sample of this liquid is deposited on a small absorbent pad contained within the disposable testing kit,” GAVI says.
“The liquid is drawn along the pad by capillary action, until it encounters a strip coated in antibodies that are specific to proteins, also known as antigens, from the SARS-CoV-2 virus. If viral proteins are present, this will show up as a coloured line — much like a positive pregnancy test.”
(Edited by Arun Prashanth)