Covid-19: Detection and diagnostic tests
Dr. Aarti Chitkara, a senior resident doctor at AIIMS and has been with their covid care facility for the past 2 months, explains the different types of tests for Covid-19 detection:-
• RT-PCR test: This test collects swabs from two places: the nasopharynx that is the back of the nose wall and the oropharynx that is the back of the throat. These swabs are transferred into a VTM (Viral Transport Medium) and sent to the laboratory for processing. For this test, the DNA material of the virus is amplified by 1000 times and then detected in calibrated machines.
• TrueNat or CBNAAT: The process for this test is similar to RT-PCR but the results are detected in a shorter time. The characteristic of the CBNAAT test is such that it can even detect a dead virus inside you thereby showing you as Covid-19 positive for longer.
• Rapid antigen test: This test declares whether you are Covid-19 positive or negative within a few minutes. It is generally taken when the patient is already displaying symptoms and so if the rapid antigen is positive then the patient is considered Covid-19 positive.
The reason for multiple tests is because every test has its sensitivity and specificity. Sensitivity refers to how accurately a test can detect the illness and specificity refers to the ability of a test to correctly identify people without the disease. At present, the RT-PCR is considered the gold standard of Covid-19 testing. However, no test can be 100% accurate. So, if the person is not sick and has yet detected positive then the result is considered false positive and if the results are negative when the person is actually ill then it is a false negative.
There can be several reasons why the results are negative when the patient is actually Covid-19 positive. This might be because the sample collected is inadequate, or that the temperature for the samples was not maintained during transport, or the virus load at the time of sample collection in the patient is low, etc. So, if you have come in contact with a high-risk patient and are displaying symptoms, then regardless of the test results, consider yourself as Covid-19 positive and take treatment accordingly.