Header image courtesy of Rod Long (via Unsplash)
Whether it’s for self-assurance or protecting others, taking a Covid-19 test is always a responsible thought. However, venturing out during Hong Kong’s worst Covid outbreak is a much less stellar idea. Fortunately, at-home antigen tests are now at our disposal, easily available from pharmacies and stores. Unlike laboratory tests that require long hours of waiting and several days to obtain the results, rapid antigen tests provide results in as little as 15 minutes.
But how to choose at-home Covid-19 testing kits is a different matter. Allow us to go through frequently asked questions and top tips to help you navigate these rapid tests.
In layman’s terms, antigens are the little “spikes” on the surface of a virus—these spiky ends are essential to the process of infection. Once our body identifies an antigen, our immune system swiftly calls “weapons” and “soldiers” to fight against the intrusion, triggering an immune response. Some of the useful “weapons” being produced are antibodies, which latch onto antigens and help our cells better differentiate the invading virus, so that the body will not open the gate for similar viruses to enter easily.
Rapid antigen tests (RATs) are designed to detect active antigens (SARS-CoV-2 nucleocapsid protein antigen) in our airways during the acute phase of infection. By taking the test, you need to swab the nostrils or other respiratory pathways. If you test positive on a rapid test, this means that there are viral loads in your nostril secretion. Under the current criteria, it is sufficient to establish a positive infection status.
Usually, the accuracy of a test kit is measured through sensitivity (the percentage of infected patients that a test can identify) and specificity (the percentage of healthy people it can rule out). A test with 90 percent sensitivity and 99 percent specificity means that the test caught 90 percent of infected patients, but there’s still 10 percent who somehow got away.
For the healthy group, 99 percent of them passed the test, except that the remaining one percent were misclassified as “positive.” Remember to look at the sensitivity value along with specificity. A highly sensitive but poorly specific test is likely to give false positive results, meaning that you test positive, but you don’t actually have the virus.
Rapid antigen tests are most sensitive in people with symptoms during the first week of illness. At the earliest phase of infection, the viral loads are too low to spot, leading to lower sensitivity. Plus, rapid tests are not substitutes for nucleic acid or polymerase chain reaction (PCR) tests, which amplify genetic materials and detect tiny virus fragments.
No. Rapid antigen tests only detect an active, current Covid infection. BioNTech (Comirnaty) uses mRNA to code for spike proteins, and Sinovac (CoronaVac) uses the inactivated virus. Both vaccines are harmless and unlikely to alter the nucleic acid or antigen test results.
However, you may test positive in some antibody serology tests after getting jabbed. Typically, rapid antigen tests will not require blood samples. Before taking a test, get information on what kind of tests it is and what their results imply.
According to the World Health Organization, antigen tests are recommended to have at least 80 percent sensitivity and 97 percent specificity. The Consumer Council has set up a search engine to confirm whether a test kit lives up to the standards set in Hong Kong, mainland China, Europe, and the US. Use the site to help you navigate through your buying options.
Unlike nasopharyngeal tests, which poke deep inside the nose to reach the upper part of our throat (and often involve our eyes tearing up), antigen tests take samples only from the anterior nasal area. Without the proper procedure, going too deep inside the nose runs the risk of hurting it. Run a number of circles inside the nostrils while keeping the swab pressed against the nostril wall—that will do for a rapid test.
Yes, test kits can expire. When shopping for your options, make sure the test kit pack has an expiry date either printed or illustrated with an hourglass symbol. Do not buy products with no expiry date, or from unknown, untrustworthy sources.
Also, the performance of the test kit is extremely vulnerable to temperatures. Putting them in hot temperatures, even temporarily, will cause irreversible damage to the proteins inside and compromise the accuracy. Keep the test kits in a sheltered area under room temperature, so that they will not lose their testing abilities easily.
Do not read the test cassette if the C-band (the control band) is missing. Also, each test kit has its own recommended reading time. After squeezing a few drops into the cassette, wait and record the results within the designated reading time. The results will be considered invalid if the cassette is left out longer than it should.
Getting negative results on antigen tests does not means you are 100 percent Covid-free; there is still a slim chance of getting false negative results. To increase sensitivity, test yourself at least every three days. Crank up the frequency if you have experienced symptoms. It’s better to err on the side of caution than to get infected without knowing.
Most antigen tests are highly specific. Getting false positive results are very rare. Do the test again, and self-isolate immediately once your infection status is confirmed.
If you are a first-time Covid-patient, declare your positive test results first. Follow the Hospital Authority’s guidelines to evaluate your conditions and see if you are suitable to quarantine at home. Meanwhile, look at the discharge criteria and inform your family, friends and employers. Your vaccination status will determine your quarantine period.
In the event of a medical emergency, dial 999 for urgent help.
Click here to read for advice on conducting home quarantine.
Click here for a comprehensive handbook on home quarantine.