To ramp up coronavirus testing, Delhi clinics put TB machines to use

In a bid to expand coronavirus disease (Covid-19) testing in the national capital, the Delhi government has started using two machines usually used to detect tuberculosis, to diagnose for the Sars-Cov-2 virus.

The CBNAAT (Cartridge-Based Nucleic Acid Amplification Test) and TrueNat machines have been approved by the Indian Council of Medical Research (ICMR) as confirmatory tests for Covid-19, which means an additional reverse-transcription polymerase chain reaction (RT-PCR) test is not needed to confirm the result, as required in the case of rapid antigen testing, for instance.

RT-PCR is the most reliable test for Covid-19.

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“Delhi has started using the CBNAAT and TrueNat machines over the last two to three days. These are just different mechanisms of doing the same molecular testing,” an official from the Delhi government said, asking not to be named.

The city scaled up its testing considerably in the third week of June by deploying the antigen-based method, first within containment zones and then in other areas as well. Antigen tests are highly sensitive, meaning those who test positive are considered to be “true positives”. However, as the specificity is low those who test negative have to undergo an RT-PCR test if they are symptomatic, or the doctor suspects the infection.

As of Wednesday evening, Delhi has tested a total of 736,436 samples, which translates to a testing per million rate of 37,167. The national capital has 116,993 cases of Covid-19.

While CBNAAT and TrueNat machines test fewer samples in a cycle, the tests are faster than a traditional RT-PCR test and can be used by hospitals’ emergency departments.

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“With different kits, the machines used for TB diagnosis can be used to test for Covid-19 too. The main advantage is that test results can be provided in an hour, in comparison to around six hours it takes for the RT-PCR test, so it can be used in cases of emergencies to check for Covid-19 before performing a surgery or procedure,” said Dr KK Chopra, director, New Delhi TB Centre, one of the centres using the method.

According to him, there are at least 32 such centres in Delhi with CB-NAAT machines and several more with TrueNat machines that were deployed this year. The machines are also available with most of the major hospitals in the city.

The Indian Council of Medical Research (ICMR) had approved these methods for preliminary Covid-19 testing in April, with many states already using it in remote areas that do not have RT-PCR machines. In May, with newer kits, the machines were approved for confirmatory testing. The machines are being used to diagnose Covid-19 in Andhra Pradesh, Bihar, Chhattisgarh, Jharkhand, Odisha, Telangana, and Uttar Pradesh, among others.

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“Earlier, the kits available for these machines could detect only the ‘E’ gene that is common to all coronaviruses. Hence, a confirmatory RT-PCR test had to be done for those that tested positive. Now, there is a two-step process in place which is as accurate as the RT-PCR test. The second step can detect the RdRp enzyme specific to Sars-Cov-2 that causes Covid-19, so there is no need for a confirmatory test anymore,” said Dr Chopra.

CBNAAT or GeneXpert has single-use cartridge that contains the reagents needed for RNA amplification. A biosafety level 2 (BSL2) lab is needed to conduct the tests. The TrueNat is a smaller, battery operated machine that needs minimal laboratory infrastructure.