False positives in rapid antigen tests are possible despite 98% specificity of tests: Experts

Rapid Antigen Tests (RAT) are known to give false negative reports 50 per cent of the time but false positives in rapid antigen tests are also possible despite 98 per cent specificity of the tests, say experts. They say it is best to isolate asymptomatic and mildly symptomatic patients at home if the patient or the care provider has requested an RT-PCR test after a RAT positive test and if the test report is awaited as this prevents them from exposure to a Covid-19 ward in case it is a false positive.

Karnataka currently tests only RAT negatives with RT-PCR. Test sensitivity is the ability of a test to correctly identify those with the disease, whereas test specificity is the ability of the test to correctly identify those without the disease.

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Talking to DH, eminent epidemiologist, cardiologist and president of Public Health Foundation of India Dr Srinath Reddy cited US researchers saying, “There would be a staggering 6.5 million false-positive results each day” if a test with 98 per cent specificity is used every day for mass testing in each of the 325 million population of the US.

He said despite the small 2 per cent false-positive possibility in RAT, 'when used for mass testing, the number of falsely identified persons are very high.' For the Indian Council of Medical Research (ICMR) to validate a RAT kit, the minimum acceptance criteria of sensitivity and specificity, as a Point of Care Test without transport to a laboratory setup, 50% and above, and 95% and above, respectively.

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In a laboratory setup, with samples collected in Viral Transport Medium, the criteria are -- Sensitivity: 70% and above, and Specificity: 99% and above. Dr V Ravi, Senior Professor and Head, Neurovirology, NIMHANS, and member of State Covid-19 Technical Advisory Committee, told DH, "RAT positive is considered Covid-19 positive. There is no need for a confirmatory RT-PCR test. We cannot test everyone tested with RAT again with RT-PCR. Besides, it is neither the same sample nor the same day. A different sample is given for a confirmatory test on a different day."

Patients can now get tested on demand even if asymptomatic. Dr CN Manjunath, nodal officer for state Covid-19 testing said, "If a patient is RAT positive and only has mild symptoms, he/she should get home isolated if an RT-PCR test report is awaited. It is not necessary to get hospitalised. There are other ways of corroborating a false positive too like an inflammatory marker test, an X-Ray, or a CT Scan. No test, even a gold standard one can be 100% accurate."