Data | Less reliable rapid antigen tests ramped up in many States

Rapid Antigen Detection Tests (RADT) are not as reliable as RT-PCR tests

August 08, 2020 05:33 pm | Updated August 24, 2020 11:36 am IST

Committed to care:  Frontline workers inspect samples after rapid antigen tests in Kolkata on Friday.

Committed to care: Frontline workers inspect samples after rapid antigen tests in Kolkata on Friday.

With seroprevalence surveys - done in Delhi and Mumbai - suggesting a much higher rate of COVID-19 infections than what were recorded in those cities so far, States have tried to ramp up testing levels to detect more infections.

But most new tests have been through Rapid Antigen Detection (RADT) method, which is not as reliable as RT-PCR tests because of the 'false negatives' problem. Some States have also limited Reverse Transcription-Polymerase Chain Reaction (RT-PCR) tests despite having the capacity to conduct more.

Actual vs detected

The serosurvey held during June 27-July 10 in Delhi found that 22.86% of Delhi's population had been exposed to the virus. But by July 10, COVID-19 tests had recorded only 1,09,410 infections, equivalent to 0.59% of the population.

Similar surveys in three Mumbai wards (slums - S, non-slums - NS) also found that much fewer cases had been recorded compared to the numbers who had been exposed to the virus.

image/svg+xml

Chart appears incomplete? Click to remove AMP mode

Two tests

States have ramped up RADTs instead of RT-PCR tests in order to capture the positive cases more quickly. RT-PCR tests are more accurate, while RADTs register a high rate of false negatives. Graphs show total-tests (bars, left-axis) and % of RADTs (line, right axis) in select States for whom data were available.

image/svg+xmls
 

Accuracy vs speed

The RADTs can register as many as 50% 'false negatives' — i.e. samples of those who are actually infected by COVID-19 wrongly returning a negative result. RT-PCR is more accurate.

ICMR had directed that those tested as negative by RADT should be re-tested using RT-PCR. Data from Delhi show that very few have been re-tested.

image/svg+xml

Listen: Do rapid antigen tests provide reliable enough data to track the pandemic? | The Hindu In Focus podcast

Capacity vs actual

Instead of supplementing RT-PCR tests, States have replaced a chunk of them with RADTs. Delhi, for instance, has a RT-PCR capacity of 11,000 tests per day.

But on most days in July, fewer than half the capacity was used.

image/svg+xml
 

Source: State health ministries, ICMR, PIB, Delhi High Court

0 / 0
Sign in to unlock member-only benefits!
  • Access 10 free stories every month
  • Save stories to read later
  • Access to comment on every story
  • Sign-up/manage your newsletter subscriptions with a single click
  • Get notified by email for early access to discounts & offers on our products
Sign in

Comments

Comments have to be in English, and in full sentences. They cannot be abusive or personal. Please abide by our community guidelines for posting your comments.

We have migrated to a new commenting platform. If you are already a registered user of The Hindu and logged in, you may continue to engage with our articles. If you do not have an account please register and login to post comments. Users can access their older comments by logging into their accounts on Vuukle.