An Expert's Guide to COVID-19 Tests

Medscape

26 Jan 2022

The reverse-transcription polymerase chain reaction (RT-PCR) test has been the gold standard for the diagnosis of COVID-19 throughout the pandemic. This highly sensitive molecular test amplifies specific segments of the transcribed viral DNA collected in a sample. Specialised labs can usually provide results in 12 to 24 hours.

In contrast, rapid point-of-care ANTIGEN tests for COVID-19, which have been on the market since the middle of 2020 and can be performed anywhere, can return results in minutes by way of a test strip housed in a cassette of these lateral-flow immunochromatographic assays that detects viral proteins.
Rapid tests are about 1000 times less sensitive than molecular tests, but sensitivity is not the only consideration when deciding which test is most appropriate. Cost, availability, accessibility, and how quickly the results are needed should also be considered.
The cycle-threshold value is an RT-PCR parameter that corresponds to the number of amplification cycles necessary to detect a positive signal.The cycle-threshold value is inversely proportional to the viral load in the sample; that is, the lower the number of cycles needed to detect the signal, the higher the amount of virus likely in the sample.

Because molecular tests are extremely sensitive, in theory they can detect positive results 48 hours before a patient's viral load reaches the threshold of infectiousness, giving the person time to take appropriate precautions. However, results typically come back 24 to 48 hours after the sample is taken, which means the patient could already have infected other people. In contrast, rapid antigen tests will deliver a positive result only when the patient's viral load is high. And although this test is not as sensitive as a molecular test, it does deliver a result in a very short amount of time.

GeneXpert is a rapid test for COVID-19 with processing time of around 45 mins and less cost.