Experts suspect flaws in antigen test behind decreasing Covid-19 figures in Delhi, Ahmedabad

Inherent flaws in the rapid antigen test now being used across India may have contributed to the dwindling Covid-19 figures reported by erstwhile hotspots Delhi and Ahmedabad, suspect public health specialists.

The national capital that reported 31 per cent positivity rate (out of 100 tests, 31 was Covid-19 positive) in the second week of June, now has a positivity rate of less than 10 per cent. The same has dipped in Ahmedabad too in the recent weeks though the Gujarat capital was considered one of the hotspots in June.

Experts suspect widespread use of the rapid antigen test may have contributed behind such happy numbers, notwithstanding the policy measures taken by the local governments to get a grip on the Covid-19 scenario.

RT-PCR is a nucleic acid test that looks for replication of the viral RNA in samples collected from nose or throat swabs. The antigen test detects viral proteins in similar samples. But there is a vast difference in efficacy between the two.

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“RT-PCR, the gold standard test to detect Covid-19 has a sensitivity of 60-65 per cent. If rapid antigen test is applied on the RT-PCR positive results, only about 70 per cent of them are picked up by the quick diagnostics, which has a sensitivity of 50-85 per cent. So only about half of the positive cases would be diagnosed with rapid antigen tests,” K Srinath Reddy, president of Public Health Foundation of India and a member of the ICMR task force on Covid-19 told DH.

The consequence would be a drop in the positivity rate. To avoid such erroneous conclusions the Indian Council of Medical Research guideline suggests that those who tested negative in rapid antigen test must again be checked with RT-PCR.

But no one knows whether such guidelines are being followed scrupulously as neither ICMR nor the state authorities have publicly released the disaggregated data.

“By ICMR protocol, people who have symptoms but test negative on the antigen test should be retested using the PCR, but there could be slips in the way this protocol has been followed. Bringing in the antigen tests may have partly contributed to this decline in the numbers and percentage of positive cases,” said Meenakshi Gautham, a research fellow with the London School of Hygiene and Tropical Medicine.

“But this is not to say that the containment and strengthened contact tracing have not had an effect. They must have but it's difficult to say how much of the decline is true and how much is superficial.”

In Ahmedabad, the rapid antigen test data is not regularly shared by the administration, said Dileep Mavalankar, director of Public Health Foundation of India, Gandhimagar. “Ideally ICMR should have come out with guidelines on identification of the suspect cases and anyone with influenza like illness or severe acute respiratory illness should be checked with RT-PCR.”

The public health specialists said clinical judgment of a doctor should always precede the tests and all the suspected cases should be recorded separately. “Doctors should act on clinical judgment and testing, which alone can not be the single window to initiate Covid-19 treatment” noted T Sundaraman, former executive director at the National Health Systems Resources Centre, Delhi.