Today, almost half the coronavirus tests being conducted in India are antigen tests.
The very latest data on testing for coronavirus - which NDTV has accessed with difficulty - shows that India is suddenly using many more of the less-reliable Rapid Antigen Tests (RAT).
The "gold standard' for coronavirus testing is the RT-PCR test because it has a much better level of accuracy than Antigen Tests.
Two months ago, 98 per cent of the tests that India was conducting was PCR tests. Today, almost half the tests being conducted are Antigen Tests: only 56 per cent of coronavirus tests in India are PCR tests - while Antigen Tests have risen from 2 per cent to 44 per cent (see graph).
Since Antigen Tests have a high false negativity rate (they report positive cases as negative), a problem recognised even by the ICMR (Indian Council of Medical Research), the daily numbers being released for India's rate of Covid-positivity are misleading. The Covid-positivity rate is likely to be much higher, if only RT-PCR tests are used.
There is no doubt of a role for RAT or Antigen Test surveys: to identify persons who are infected. But if an Antigen Test result is negative, it should not be taken seriously. The 'negative' antigen tested person may still actually have the coronavirus infection.