Both rapid antigen test (RAT) and RT PCR test are equally effective in detecting SARS-CoV-2 infection when done at a regular frequency, at least every three days. A National Institutes of Health (NIH) study, published June 30 in The Journal of Infectious Diseases, has shown that while RT PCR test is a more sensitive test vis-à-vis RAT, both tests have a sensitivity of 98% when taken regularly as part of a screening program.

The study involved 43 adults newly infected with SARS-CoV-2; the participants had tested negative in the week before their enrolment in the study. A saliva sample and two nasal swabs were collected daily for 14 consecutive days for Quantitative reverse transcription PCR (RT-qPCR), Quidel SARS Sofia antigen FIA and live virus culture. The live virus culture was done to obtain a rough measure of the period during which subjects could spread infection to others. None of the participants required hospitalization for their symptoms.

The researchers then compared the three COVID-19 viral testing modalities - PCR testing of saliva, PCR testing of nasal samples and rapid antigen testing of nasal samples. 

The results showed that the sensitivities of RTqPCR and antigen tests vary over the course of SARS-CoV-2 infection. Both RTqPCR and the Quidel SARS Sofia antigen FIA show maximum sensitivity during the period in which live virus is detected in nasal swabs, but the sensitivity of RTqPCR tests - both from saliva and nasal samples - increases more rapidly prior to the infectious period or early during the infectious period.

• All tests showed >98% sensitivity for detecting infection when done at least every three days. 
• When the test frequency was reduced to once per week, the sensitivity of RT PCR was still around 98%, but, the sensitivity of antigen tests decreased to 80%.
• RT PCR detected the virus even beyond the infectivity period when the virus is most likely no longer transmissible. On the other hand, the sensitivity of RAT declined rapidly every day making it a less effective modality to identify persons during later stages of infection.

The study concluded that with timely reporting of results, RTqPCR tests can decrease further spread of the virus. This study has also shown for the first time that RAT was comparable to RT PCR in terms of performance when done at least twice in a week and can increase the chances of detecting Covid-19 infection. And, daily screening using antigen tests can also achieve ~90% sensitivity for identifying infected individuals while they are viral culture positive, the period during which transmission of infection could occur.

At present, a negative RAT has to be confirmed by RT PCR. This study has demonstrated that frequent testing at least twice in a week, whether with RAT or RT PCR, can efficiently identify the highly infectious persons.

The second wave in the country is receding, but with unlocking and reopening of economy, there is the risk of exposure to the infection, especially from asymptomatic individuals. RAT is faster and gives immediate results; it is easier on the pocket than RT PCR, which is a laboratory test and above all offers the convenience of testing at home, especially in settings where the facilities for serial RTqPCR testing with rapid reporting of results are unavailable. This can be a very useful and effective strategy in our rural and tribal areas.

(Source: NIH News Release & The Journal of Infectious Diseases, jiab337, https://doi.org/10.1093/infdis/jiab337, June 30, 2021)