Two major civic laboratories had discovered that more than half the symptomatic patients tested negative for COVID-19 in an antigen test came positive later with the real-time PCR method. This highlights the high rate of false negatives seen in rapid antigen testing started this month under BMC’s ‘Mission Universal Testing’ strategy.

For example, 60% of 538 symptomatic patients of COVID-19 who had tested negative in antigen tests at Nair Hospital, came out positive later as retested under RTPCR method. At Kasturba hospital, the finding was reversed for 65% of the 43 patients.

Starting of this month, the BMC hadperformed around 8,872 antigen tests cumulatively and had found 1,152 of them being tested positive. Out of these, two BMC hospitals have reported 60-65% positivity in RT-PCR tests on the antigen negative samples. Head of Molecular Diagnostic Laboratory, Kasturba Hospital, Dr Jayanthi Shastri, said that the reason for high percentage of false negatives in symptomatic patients is due to the sensitivity of antigen test as each kit literature is 50% but its specificity is 100%. Test sensitivity has the capability to properly identify those with the infectionwhile test specificity has the capability to identify those without the disease.

Doctors said that the samples tested by the two hospitals were of walk-in patients who were visiting out-patient departments with coronavirus-like symptoms or patients from their hospital wards. When coronavirus appeared first in March, the only test available was the real-time polymerase chain reaction (RT-PCR) which tests swab samples from nose and throat providing a diagnosis in 6 to 8 hours. Although RT-PCR is still considered the most effective test, newer methods such as antigen tests are identified to provide a diagnosis within 15 and 30 minutes. They are also considered as useful tools for surveillance. But, there are concerns regarding the high rate of false negatives in these antigen tests.

An official said that at present antigen tests are being performed at fever camps whenever they find any symptomatic patients. In the camps, the body temperature is tested using a thermal gun and oxygen levels by a pulse oximeter. If the oxygen level is below 95, then the antigen test is performed. If a patient with COVID-19 symptoms still test negative under the rapid antigen test, they then suggest to get themselves retested under RT-PCR.

As per the ICMR protocol, any sample test which comes negative in antigen testing should be re-tested with RTPCR if the person shows coronavirus symptoms. Patients who are negative with no symptoms should be monitored for a few days and in case they develop symptoms, then they should undergo the RT-PCR tests.

Leena Patel Deherkar, Kandivlicorporator said that recently at three fever camps in her ward, antigen tests were conducted on patients showing symptoms of coronavirus.

Source: ET Healthworld