The antigen(s) detected by rapid antigen tests are expressed only when the virus is actively replicating; therefore, such tests are best used to identify acute or early infection.
Preferred Sample: Nasopharyngeal Swab
Turn around time : 30 minutes to 2 hours
How antigen testing works
Antigen tests seek out specific protein i.e. the ‘spike glycoprotein’ that studs the surface of the coronavirus.
A swab from the nose is collected for this test. The swab is then dipped in a solution that inactivates the virus, and then transferred onto a test strip. The test strip houses antibodies that bind to coronavirus spike protein antigens.
If the sample is positive for coronavirus, coloured lines will show up on the paper strip in 15-20 minutes.
Antigen testing : Advantages
- It reduces the burden on RT-PCRtests, which needs a more technical infrastructure/equipment in a laboratory.
- It is highly specific i.e., the results that are positive are true positive. So, patients who test positive can get into isolation faster.
- Antigen testing costs less and takes less time.
Antigen testing : Limitations
- An antigen test can only reveal an active infection.
- It is a less sensitive test as compared to RT-PCR. Since antigen testing doesn’t involve any processes of amplifying the virus or its genetic material, a swab sample may have too little antigen to be detected. This could produce a false negativeresult.
So as a precaution, a negative test should be followed up by the more accurate RT-PCR test, to confirm a true negative for COVID-19.