A person testing for Covid19 while wishing he is negative would also want to get an accurate result – whether negative or positive. Hence the accuracy of the lab tests are of utmost importance given their role in helping to halt the spread of the virus. Lay persons rely entirely on the medical/scientific community and trust them implicitly for the results. Asking questions is considered impertinent. But discussions with the medical and scientific community can bring enlightenment in an otherwise grey area. There are two main types of tests for COVID-19 – the Nucleic Acid Amplification Testing (NAAT), commonly called the RT-PCR test and the Anti-body Test. NAAT detects viral RNA using molecular methods such as polymerase chain reaction (PCR). These tests are highly specific because they are based on the unique genetic sequence of Covid19. If a test comes back positive, it is positive. However, the sensitivity of the tests depends on the timing and how the sample is collected.
ICMR recommends use of nasopharyngeal (NP) swabs for NAAT because in most patients, the naso-pharynx, or the space above the soft palate at the back of the nose appears to have the highest concentration of virus. However, NP swab samples are technically challenging to obtain, and a sub-optimal collection may reduce test sensitivity and increase the likelihood of obtaining a false-negative result in a patient with the virus. As with all laboratory tests, a number of factors determine the accuracy of a COVID-19 test result. These include the instrument and chemical reagents used to perform the test and also the timing and quality of specimen collection and the biology of the individual patient. Testing a swab from the oro-pharynx or nose is likely to reduce sensitivity. Other sample types such as saliva or blood are likely to result in even lower sensitivity. The timing of the sample collection is also important because the amount of virus present in the naso-pharynx varies over the course of infection. Ideally, samples should be collected near the time of symptom onset to achieve the highest test sensitivity. Patients who are infected but not yet symptomatic may have false-negative test results, as may those whose symptoms are waning.
Now while the NAAT directly detects the virus, the Antibody or serology testing is used to detect an immune response in the patient. Most patients have detectable antibodies by day 14 following symptom onset, and the likelihood of detection increases over time. Medical science tends to rely on the NAAT tests because they detect molecules that are specific to Covid19 and because the specificity of nucleic acid tests for COVID-19 is very high, meaning that a positive result can generally be trusted.
