Hello Ferrzobel Danganan Bicaldo

I agree with you. The swab test for covid-19 is an accurate and reliable test for diagnosing covid-19. A positive test means one is likely to have covid-19. A negative test means one probably does not have covid-19 at the time of the test. Nevertheless, antigen tests are still fairly accurate, particularly when someone is experiencing symptoms and their viral load is very high. However, they can be less accurate when someone has a lower viral load, like in someone without symptoms. This could lead to false negative test results.

In situation where test turnaround time is critical, there is value in providing immediate results with antigen tests, even though they may have lower sensitivity. This will help quickly identify people with COVID-19, thereby adopting infection prevention and control measures and thus preventing transmission.

Best.

Hello Ferrzobel Danganan Bicaldo

The antigen tests are really good and they can even detect SARS-CoV-2 (N antigen) even under low viral titers. Definitely, there is no doubt that an RT-PCR overshadows it because of high specificity and sensitivity, plus it is also directed towards other genes (including Spike). Nevertheless, if one tests negative for SARS-CoV-2 antigen, it can be dubious only during the early stages of infection. Interestingly, I would want to tell you that since people experience different symptoms, also remain asymptomatic, I have come across cases where samples from different sites (oropharyngeal and nasal) have shown opposite results for the same individual. A rapid antigen test from the oropharynx showed positive results, while the RT-PCR from the nasal cavity showed negative results. So at times, the results of the tests are also dependent on the site from which the sample was collected.

Nevertheless, if one tests negative for antigen at a particular instant, they should be identified as negative only; provided the individual experiences symptoms at a later stage or during the same duration. In that case, an RT-PCR can precisely identify the status of the individual. As correctly mentioned by Malcolm Nobre, RAT has proved to be more convenient in tracing out positive individuals because of its rapidity and reliability. Hope it answers your query.

Sensitivity is detecting your target (antigen) when a person is truly infected

Specificity is not detecting your target (antigen) when a person is truly not infected. If I want more positives I lower the cutoff and the sensitivity rises and the specificity falls. If I want more specificity I raise the cutoff. Hopefully the test makers have carefully calibrated the cutoff where they achieve the best a given assay can achieve.

A person can test negative but still actually be infected is frequently a sampling problem. As Jadin Chada said he has seen different results depending on where the sample came from. This is not surprising, especially at the very beginning of infection or at the very end. During the peak of infection even a bad test is usually right. Sampling becomes a problem at either end of the infection.