Delhi: 11% patients who tested Covid-19 negative in antigen tests found infected through RT-PCR
The Centre had in September asked all states and UTs to mandatorily retest all symptomatic cases who tested negative in rapid antigen tests through RT-PCR.
Nearly 11% of 32,903 coronavirus patients in Delhi, who tested negative through rapid antigen tests, were found to be infected with Covid-19 when they took a real-time reverse transcription polymerase chain reaction, or an RT-PCR test, between September 1 and November 7, PTI reported on Sunday.
In order to effectively curb the spread of coronavirus infection, the Union Health Ministry had in September asked all states and Union Territories to mandatorily retest all symptomatic cases, who were found to be Covid-19 negative in rapid antigen tests, through RT-PCR, so that no positive case is missed. Antigen tests have a high rate of false negative results.
In Delhi, out of the 56,862 symptomatic patients whose test results came negative through a rapid antigen test, 32,903 were tested again through RT-PCR. Out of these, 3,524 were found to be Covid-19 positive, a Right to Information query filed by PTI has revealed.
The case positivity rate in RT-PCR tests, which is considered the “gold standard” for Covid-19 screening, was 20.97% in September, while it was only 4.77% for rapid antigen test, the RTI reply stated. Positivity rate is the percentage of people who test positive for coronavirus as against those who were tested.
In October, positivity rates of RT-PCR and rapid antigen tests were 16.76% and 4.58%, respectively. Between November 1 and November 7, the rapid antigen positivity rate was 8.16%, while for RT-PCR it was 27.2%.
The RTI response further stated that out of 27,533 symptomatic negative cases in rapid antigen test in September, only 4,597 were retested through RT-PCR and 623 were found positive. The next month, 24,737 out of 26,316 symptomatic negative cases were tested again through RT-PCR and 2,300 were found to have contracted the virus.
Between November 1 and November 7, health authorities retested 3,569 of 4,013 symptomatic rapid antigen test negative cases and 601 were found to be infected, PTI reported.
In July, the Delhi High Court had pulled up the Aam Aadmi Party government for choosing rapid antigen testing over the RT-PCR testing method. The High Court cited the serological survey carried out by the National Centre for Disease Control to question the government’s decision to go with the rapid tests. The survey revealed that 23.48% of Delhi residents had antibodies – that is, they may have at some point been exposed to the coronavirus. The survey was conducted between June 27 and July 10, and a total of 21,387 samples were collected from 11 districts to test for antibodies.
In such a scenario how can the Delhi government go with RAT as its frontline test, when its rate of false negatives was very high, the court had then asked.
What is rapid antigen testing?
Like the RT-PCR test, and unlike antibody tests, rapid antigen testing looks for the presence of the SARS-CoV-2 virus in the body. In an antigen test, a swab is taken from a person’s nasal cavity and it is tested to detect fragments of proteins which are found on or within the SARS-CoV-2 virus. The RT-PCR test also looks for the presence of the virus, but here the samples are tested for genetic material of the virus. The antibody test is different – in this, blood samples are tested for the presence of antibodies that develop while or after a person has developed some level of immune response to the virus.
The RT-PCR tests are “incredibly accurate”, according to the United States Food and Drug Administration. Antigen tests, on the other hand, are not as sensitive, and the US drug regulator says this aspect of antigen testing means that “positive results from antigen tests are highly accurate, but there is a higher chance of false negatives, so negative results do not rule out infection”, and need a confirmation via the RT-PCR test.
In its defense, the antigen test produces results much faster. While the RT-PCR test may take two to five hours at the least, the antigen test can give a result in 30 minutes.