New Delhi: Amid the surge in COVID-19 cases, people queueing up for RT-PCR tests don’t know whether to rejoice on getting a negative report or not. It was due to such false-negative reports (at least 20 % reports suggest) of symptomatic patients that experts recommended COVID-19 treatment for everyone showing classic symptoms irrespective of RT-PCR results.
The minimum sensitivity (ability to detect positives) demanded by the Indian Council of Medical Research (ICMR) for validating an RT-PCR test is 95%. That means up to 5% false-negative results are expected.
Four broad factors determine the accuracy of an RT-PCR test, explained The Indian Express (TIE) in a report.
Viral Load
COVID-19 manifests in a person by the fifth day after exposure. A person going in for a test before that may test COVID negative. However, this may not be a major factor behind the recent false-negative trend as certain COVID-19 mutants apparently show early symptoms.
A number of symptomatic patients who tested false negative in RT-PCR assays have subsequently been confirmed COVID-positive in Bronchoalveolar Lavage (BAL) tests that collect samples from the lower respiratory tract through a bronchoscope. This has led to conjectures that certain mutants bypass the upper respiratory tract to target the lung.
Capacity Explosion
There were only 14 COVID test labs in February 2020. Now there are 2,400. This rapid expansion required approving hundreds of labs for RT-PCR testing and training thousands of technicians in a hurry.
As a safeguard, ICMR in July 2020 listed 30 Quality Control labs to check on all the COVID labs it approved, the report said.
Human Element
Tests can go wrong at various stages — from bad sample collection and storage to faulty extraction and amplification. All RT-PCR kits include an internal control (IC) to safeguard against a scenario when no RNA is extracted/amplified, leading to a false negative. The IC can be exogenous or endogenous. It is exogenous when an artificial RNA template molecule is added to each sample before RNA extraction. The test is considered void when the synthetic RNA is not detected post-extraction and a re-test is prescribed. And, an endogenous control uses a human ‘house-keeping’ gene present in the sample; its non-detection after the RNA extraction procedure invalidates the test, TIE reported.
Viral Mutation
An RTR-PCR test targets specific area/s of one or more viral genes to detect its presence. A test may return false-negative results if a mutation occurs in the segment of the genome assessed. Tests that use multiple genetic targets, like the ones in use in India, are less likely to be fooled by mutations.
Price crash
In one year, the price of the RT-PCR kit fell from over Rs 1,100 to under Rs 40. Since import duty exemption was withdrawn in October 2020, most foreign brands left the Indian market due to the additional 15% cost burden. A number of Indian brands, too, have been refusing to sell below Rs 100. Many have found dedicated corporate clientele who do not mind paying a little extra to ensure quality. Some are even offering multiple PCR kits at different price levels.
Questions regarding the quality of the cheaper competition notwithstanding, every test kit available in the market apparently meets ICMR’s validation benchmarks.
