Covid-19: High use of RT-PCR tests in Tamil Nadu leads to stable positivity

TN has the second-highest number of infections after Maharashtra but it is commendable the state is testing as many samples

Tamil Nadu (TN) is one of the few states that continue to use the RT-PCR method to test for Covid-19 infections. In contrast, most others have been able to ramp up testing on the back of rapid antigen tests (RAT). This is probably the reason why TN’s positivity rates have stayed constant, dipping slightly now, whereas the positivity prints in Delhi and Bihar have seen wild swings.

For instance, TN’s positivity on August 18 was 8.7% with the state having carried out 67,532 tests and found 5,890 positive cases. A month earlier, the positivity rate was  only slightly higher at 9.3% while on June 18, it was only 8.5%. Now contrast this with Delhi, which started using RATs from June 18. While Delhi has a low positivity of 5.3% today, a month ago it was 7.1%; on June 18, when Delhi had just started RATs, the positivity was 29.8%.

RATs now account for 62% Delhi’s total tests whereas a month ago this ratio was just 52%. When Bihar tested only 10,273 samples a day on July 18, its positivity was 17.8%. On August 18, it tested 1,07,727 samples, and its positivity was just 2.3%.

TN has the second-highest number of infections after Maharashtra but it is commendable the state is testing as many samples as Maharashtra despite not using RATs. Uttar Pradesh (UP) and Bihar too are testing close to one lakh samples daily but the majority of these are RATs which are only half as accurate as the RT-PCR. Although Bihar does not provide a break-up between RATs and RT-PCR tests, positivity rates clearly show an increased reliance on RAT tests.