New Delhi: Delhi Deputy Chief Minister Manish Sisodia Saturday defended his government’s focus on Rapid Antigen Testing over Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) tests in the national capital, saying the rate of false negatives in the two processes is almost the same, and that due to their immediacy, antigen tests have helped curb the spread of the disease.
In accordance with Indian Council of Medical Research (ICMR) guidelines, Delhi is only conducting retests on symptomatic rapid antigen negatives.
Delhi’s sero survey data, however, had revealed that a large number — nearly 23 per cent — of those exposed to the virus were asymptomatic. A study by ICMR in April also revealed that 28 per cent of surveyed positive cases were asymptomatic.
Experts that ThePrint spoke to said while the RT-PCR test is more effective in detecting cases, the Delhi government’s focus on retesting only symptomatic rapid antigen negatives implies resource optimisation. They also said while the risk of transmission from asymptomatic patients is low, a large prevalence of asymptomatic patients implies community transmission.
However, a study quoted by the World Health Organization (WHO) has also showed that 44 per cent Covid-positive persons catch the infection from asymptomatic cases.
Delhi’s focus on antigen tests
The ICMR allowed Delhi to start rapid antigen testing on 18 June for containment zones and hospital settings. In its guidelines, the ICMR said symptomatic individuals who test negative will have to mandatorily be get an RT-PCR test done to rule out a false negative, as the specificity of the antigen test ranges from 99.3 to 100 per cent, while its sensitivity ranges from 50.6 to 84 per cent. The RT-PCR test has been referred to by the WHO as the gold standard for testing.
Data analysed by ThePrint reveals that during the period between 30 June (when data on both kinds of tests was first made available) and 8 August, Delhi has conducted almost twice the number of rapid antigen tests as RT-PCR tests and CBNAAT and TrueNAT tests (the other two tests approved by the ICMR).
Data from the daily health bulletin reveals that Delhi has conducted 4,98,214 rapid antigen tests against 2,64,717 RT-PCR tests in this period. Between 1 and 8 August, it conducted almost 2.5 times the rapid antigen tests (96,350) as RT-PCR tests (39,190).
ThePrint had reported earlier that despite the spurt in testing, focus on rapid antigen tests made false negatives a huge concern, and how a Delhi government report in the high court revealed that nearly 18 per cent of rapid antigen tests sent for follow-up RT-PCR tests came back positive.
On 27 July, the Delhi High Court rapped the government for focusing on rapid antigen testing and conducting less than 50 per cent of its RT-PCR testing capacity. On 4 August, the high court noted the government’s resistance to the court monitoring its testing strategy, and asked the government to file a status report on RT-PCR tests across the capital.
Defending the Delhi government Saturday, Sisodia said: “I don’t think there is a problem with antigen tests. The false reading rate for antigen is almost same as that of RT-PCR, which is around 30 per cent.”
He also said the immediacy of the antigen test has helped curb the spread of the disease.
“Antigen test has its merits. PCR test results come in two days and by then, many people are already infected by an individual. However, antigen test has immediate results, which is effective in controlling the further spread,” he said.
Delhi’s strategy is attuned to resources available
While the NCDC’s sero survey and the ICMR’s April study raised concerns about those with mild or no symptoms spreading the infection, experts ThePrint spoke to said Delhi’s testing strategy is attuned to the resources available.
“RT-PCR tests require skilled personnel, specific labs, and is a time-consuming process, as machines can only take 200 samples at a time and the results come in 4-6 hours. For rapid antigen tests you don’t need skilled personnel or labs and results come in 30 minutes,” said Dr Sanjay Rai, professor of community medicine at the All India Institute of Medical Sciences.
“Delhi has already ramped up testing and one-fourth of the population has already been infected, so the focus should move from number of cases to comorbidities and hospital admissions,” Rai said.
Experts also said while the rapid antigen test can detect only if there is high viral load, and RT-PCR test can detect the virus even if two viral particles are present, the focus on symptomatic patients is also because the viral load is higher in them.
“The risk of transmission is more from symptomatic patients as the viral load is much higher in them. In contrast, the viral load is much lower in asymptomatic patients. So you have to use your resources carefully,” said Dr D. Prabhakaran of the Public Health Foundation of India.
“Testing everyone is not possible, so follow-up RT-PCR tests for the high risk groups and symptomatic patients are the best way to optimise resources now.”
With studies indicating a large number of asymptomatic carriers, the experts also said this is an indication of community transmission, and an admission by the government will help curb spread via asymptomatic carriers.
“The government’s own sero survey and ICMR study shows large numbers of asymptomatic carriers. This proves community transmission. Instead of contradicting itself by saying no community transmission, if the government admits to this then people will be more careful and wear masks and practise social distancing, which will help curb the spread of the virus,” said Dr T. Jacob John, former director general of the ICMR.
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