BREAKING DOWN COVID-19 TESTS
Cheap and quick, antigen test kits help screen for the presence of the virus but results are not as accurate as an RT-PCR
An entire school in Mukdahan province was in shock earlier this month following results of antigen test kits (ATKs) which showed many students, teachers and officers were positive for Covid-19 after the school’s reopening. More than 1,000 students were left with no choice but to self-quarantine.
A few days later, things turned upside down — a relief to be more precise — when the RT-PCR tests returned negative. It was reported that the ATKs the school had bought were not certified by the Food and Drug Administration (FDA) and could render false positives.
The availability of ATKs has been rapidly rising in Thailand, following the reopening of businesses earlier this month and after FDA-approved Covid-19 self-test kits were allowed to be sold at general stores or online. Since late September, various types and brands of ATKs have made their way to the market.
On the bright side, the widespread availability of ATKs can be seen as an improvement in access to a self-screening tool among the public now that infection risk is everywhere. However, on the flip side, the variety of ATKs has also led to confusion. False negatives and false positives occur more frequently because many factors come into play when it comes to the accuracy of their results.
Internal medicine specialist Dr Pathumpoom Kooanupong from Paolo Samutprakan Hospital said although antigen test kits serve as an additional weapon against the spread of Covid19, it is important to understand that they are just a preliminary screening method that has errors.
“When it comes to laboratory tests to confirm disease, there are two types of test — screening and diagnostic,” Dr Pathumpoom explained. “Think of blood sugar tests. An at-home blood sugar test in which people prick their fingertips and check the blood with a test strip is regarded as a screening test. But when you visit a hospital and have a proper blood sugar test, that’s a diagnostic test.”
The same principle applies to Covid19 as the only function of ATKs is to be a screening tool. However, reverse transcription-polymerase chain reactions, or RT-PCR, is a common diagnostic tool.
The advantage of ATKs, nonetheless, is that they are cheaper compared to an RT-PCR. Providing results within 15 minutes, ATKs are both fast and affordable.
The ATKs are, however, not without limitations, Dr Pathumpoom warned.
“The sensitivity and specificity of ATKs are lower compared to an RT-PCR,” he said. “The test kits are likely to render false results as seen in the news. In case of false positives, those who get tested will become anxious and worried. However, false negatives are much worse because when you receive a negative result, you are likely to continue your life as normal, resulting in the spread of the virus.”
Many factors can affect the sensitivity and specificity of ATK test results. The first is the period when the body is exposed to the virus until the time of the test. For example, doing a Covid-19 self-test one or two days after contracting the virus can lead to a false negative. Ideally, an asymptomatic person should get an ATK test five to seven days following exposure to someone who is suspected or confirmed to have Covid19. People at infection risk or those with symptoms can get the test right away.
The way the specimen is collected also plays a significant role in ATK accuracy, along with the use of uncertified or poor-quality test kits.
Currently, there are various types of home test kits available on the Thai market — ATKs for professional use, ATKs for home use, and ATKs using saliva. According to Dr Pathumpoom, people are advised to opt for homeuse ATKs rather than professional-use counterparts, mostly used by medical personnel.
“I wouldn’t recommend people buy professional-use ATKs and use them at home given that the test method is quite complicated. People require a certain level of Covid-19 education and knowledge to be able to use such a test properly,” he said.
ATKs that test saliva samples are increasingly being used given their convenience. However, they are not suitable for everyone, the specialist added.
“When it comes to saliva-based ATKs, there are many factors that can affect the accuracy of the results. The tested saliva, for example, must come from deep inside the throat. Also, before getting the test done, the person must refrain from food, beverages, smoking and chewing gum for at least half-anhour to one hour. Oral bacteria also plays a part in the test’s accuracy.
“The saliva-based ATKs are suitable for the elderly or those who are unco-operative for the nasal swab,” Dr Pathumpoom advised.
Thailand only allows the sale of FDA-certified antigen test kits and currently, there are 129 brands authorised.
Unfortunately, there are many brands on the market that do not meet medical standards. Before making any purchase, Dr Pathumpoom recommends that people take a look at the Medical Device Control Division’s website (bit. ly/3CpCoLf) to see if products are FDA-approved.
In this ongoing battle against the novel coronavirus pandemic, an ATK is a firearm against the spread of the life-threatening virus. However, from his experience attending a maximum of 200 Covid-19 tests per day at the peak of the outbreak, Dr Pathumpoom believes vaccines are the best and most effective weapon against the disease as of now.
“After vaccine rollout, the number of new cases has gotten lower. The number of ICU patients, those with lung inflammation and those on oxygen is also noticeably lower. While ATKs are designed to help stop the spread, vaccines can apparently reduce the severity of the disease.”
In the future, the specialist strongly believes that better ATKs — an equivalent to RT-PCR in terms of efficiency, perhaps — will be developed and become widely available. But in the meantime, using standard ATKs properly and appropriately is the way to go.
“For general people with zero risk of Covid-19 infection, an ATK might be unnecessary. But for working people who go out and meet others, an ATK once a week or once every two weeks will do no harm.
“Meanwhile, people who are at risk of infection or exposed to someone confirmed positive for Covid-19 should get tested with an RT-PCR if they have access to it.”
THERE ARE VARIOUS TYPES OF HOME TEST KITS AVAILABLE ON THE THAI MARKET — ATKS FOR PROFESSIONAL USE, ATKS FOR HOME USE, AND ATKS USING SALIVA