Experts flag risks in India's use of rapid tests for virus

NEW DELHI, India (AP) — In June, India began using cheaper, faster but less accurate tests to scale up testing for the coronavirus — a strategy that the United States is now considering.

These rapid tests boosted India’s testing levels nearly five-fold within two months. But government numbers suggest some parts of the country might have become over reliant on the faster tests, which can miss infections. Experts warn that safely using them requires frequent retesting, something that isn’t always happening.

Cases surged faster than labs could scale up testing once India’s harsh lockdown was relaxed. So far authorities have rationed the use of the more precise molecular tests that detect the genetic code of the virus. But on June 14, India decided to bolster these with faster tests that screen for antigens, or viral proteins.

Albeit less accurate, these tests are cheap and yield results in minutes. Most don’t require a lab for processing or any specialized equipment or trained personnel. The plan was to rapidly increase testing to identify infected people and prevent them from spreading the virus. Samples tested using both tests increased from 5.6 million in mid-June to 26 million two months later, and nearly a third of all tests conducted daily are now antigen tests, health officials say.

But India’s experience also highlights the inherent pitfalls of relying too heavily on antigen tests, at the expense of more accurate tests. The danger is that the tests may falsely clear many who are infected with COVID-19, contributing to new spread of the virus in hard-hit areas.

Rapid test results can be backstopped with more accurate laboratory tests, but these are slower and expensive. Experts also warn that since the two types of tests vary in accuracy, they need to be interpreted separately to properly assess the spread of infection -- something India isn’t doing.

The U.S. faces a similar need to strike a balance between speed and precision, with overburdened labs struggling to keep pace with the outbreak. Researchers at Harvard and elsewhere are proposing developing a $1 saliva-based antigen test for all Americans to test themselves daily, something that has not yet been approved by the Food and Drug Administration.  ...