With millions of Kiwis now checking their Covid virus status each day, it is important for the public to be aware of aspects of rapid antigen tests (RAT) including their limitations and the differences when compared with polymerised chain reaction (PCR) tests. Put simply, RAT tests are less sensitive but simpler and faster to conduct than PCR tests.
RAT tests employ artificial antibodies to detect Covid viruses. This type of test can be easily conducted by a person without any special training or expertise and do not require specialised expensive laboratory equipment. PCR tests, on the other hand, are much more sensitive than RAT tests but require analysis by professionals with technical training using specialised expensive laboratory equipment usually in large testing facilities. Because of these complications, PCR tests are also more time consuming and more expensive. PCR tests involve amplification (that is, reproduction) of the genetic (DNA) material of the Covid virus via a molecular process known as polymerised chain reaction. Such genetic amplification, which gives PCR tests their sensitivity, is not possible with RAT tests.
Because RAT tests are less sensitive, they are prone in marginal cases to give more false negatives than PCR tests. To illustrate using a hypothetical situation, applying a RAT test with 80% sensitivity to 100 Covid-infected people would lead to a positive result in 80 people with the remaining 20 receiving a false negative result. On the other hand, a clearly positive RAT test result is almost certainly a genuine result. RAT tests are better at detecting Covid-19 when people have developed symptoms. Therefore, they are recommended for people who feel unwell with Covid-19 symptoms or for those who live in the same household as someone who has Covid-19 (NZ Ministry of Health advice). RAT results are less reliable for people who don’t fit these categories.
In conducting a RAT test for yourself, the process of swabbing to ensure a good contact sample is taken from the lower part (2.5cm or one inch) of your nasal passages is probably the single most important step to achieve an accurate result. Other steps are also important, therefore, follow the kit instructions closely, time your test, delay the test until two days after Covid exposure and then repeat regularly during the following seven days. When a person is symptomatic and has a high population of virus in their system, RAT tests can be very accurate. When a person is in the early stages of infection, and there are low levels of virus in the nose and throat, RAT tests may fail to detect the virus.
Finally, if your test result is a surprise based on your prior expectation of a false positive or negative result, then take appropriate precautions and repeat the test on to two days later or have a PCR test done.