Covid-19 rapid antigen tests high on accuracy, but protocols crucial
As per ICMR guidelines, symptomatic persons who test negative for Covid-19 in the rapid antigen tests should be followed up with an RT-PCR test. Rapid antigen detection tests are quicker (take up to 30 minutes compared to 2-5 hours in RT-PCR test).Updated: Jul 22, 2020, 12:33 IST
About 1,365 (0.54%) of the 262,075 people who tested negative for Covid-19 through antigen tests are believed to have showed symptoms for the disease, and, among these, 243 (17.8%) were found to have the virus through lab tests, according to data with the government that appears to address some concerns over the rapid kit’s accuracy, although experts said the protocols and the opinion of diagnosticians in each case may have a bearing on this.
Between June 18, when testing with these kits began in the national capital, and 10:15am on July 15, there were 281,555 rapid antigen tests (RAT) of which 19,480 people were found positive for the virus. From the 262,075 people who were negative, 1,365 people showed symptoms of Covid-19 and were referred to take a test through the RT-PCR method, which is regarded as the most accurate type of test yet for detecting the Sars-Cov-2.
Of these, 243 were eventually found to be positive, meaning they had a false negative on a rapid antigen test, which has been crucial to ramping up testing – and thus tracing and isolation of active cases – in the country.
“The follow up testing on RT-PCR is only for symptomatic negative persons on rapid tests as per the protocol laid down by the Indian Council of Medical Research (ICMR). RAT negative persons are administered the RT-PCR tests only after due clinical classification by the medical officer in charge at the concerned antigen centre,” said Manish Sisodia, Delhi’s deputy chief minister who is currently also heading the health department at present.
As per ICMR guidelines, symptomatic persons who test negative in the rapid antigen tests should be followed up with an RT-PCR test. Rapid antigen detection tests are quicker (take up to 30 minutes compared to 2-5 hours in RT-PCR test). The accuracy levels of these kits are such that if someone tests positive, they are considered a ‘true positive’ case but a negative result might be a false one, which is why those with symptoms are advised to go for an RT-PCR (reverse transcription polymerase chain reaction) test.
The government’s data, seen by HT, showed that of the 11 districts in Delhi, Southwest conducted the maximum number of rapid antigen tests and hence, had the highest positive cases through this technique.
It has carried 39,804 antigen tests of which 3,798 were positive, while 38 among those who tested negative were followed up with an RT-PCR test, of which 11 tested positive.
Shahdara district conducted the maximum follow-up RT-PCR tests on RAT negatives. In all, 648 symptomatic persons were sent for follow-ups, of which 123 tested positive, the highest among all districts.
Dr Lalit Kant, former head, division of Epidemiology and Communicable Diseases, ICMR said that the process also depends on the judgment of the medical officer screening the people at the testing centres.
“In this season, more and more people develop ILI and SARI symptoms, which are Covid-19 symptoms too. So, the number of symptomatic persons already must have increased in Delhi by now. Clear guidelines must be issued to all medical teams of every district that no symptomatic persons should be left without and RT-PCR test. Anyone with cough and fever should be tested,” said Kant.
Dr T Jacob John, virologist and former professor at Christian Medical College, Vellore, Tamil Nadu said it is unlikely that across Delhi only 1,365 of the roughly 262,000 persons who tested negative in the rapid tests could have been symptomatic.
“The antigen tests in Delhi started off from the containment and buffer zones which anyway should have increased the number of positive results. Widespread use of rapid tests is a good thing that Delhi is doing. But, this needs to be backed up with RT-PCR tests on all symptomatic RAT negatives because RT-PCR is the most accurate testing method so far. Otherwise, it would defeat the purpose of scaling up testing. The perception among the public that a negative result in the antigen test is the final word in a person’s Covid-19 diagnosis needs to be changed by the authorities,” he said.
John further added that simple statistical extrapolation of the government data showed that if 17.8% people tested positive in follow-up RT-PCR tests out of the RAT negatives, then over 46,000 persons could, potentially, have had a false negative. “However, the sample size of 1,365 persons is too small. This statistical extrapolation can only give an indication. It is also indicative of the fact that in Delhi, post peak transmission of Covid-19 continues,” he said.