TN man tests coronavirus positive at one lab, negative at another: Experts explain why

Recently, a diagnostics lab in Coimbatore was closed after a customer alleged that he was declared as COVID-19 positive ‘falsely’.
Swab collection for Corona RT-PCR test
Swab collection for Corona RT-PCR test
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As the number of new COVID-19 cases continue to rise in Coimbatore district, the police in Vadavalli had a strange complaint from N Manikandan on Wednesday. He alleged that Micro Labs, a private diagnostic centre in the city had declared him coronavirus positive and that was false. He went on to submit a result that declared him coronavirus negative from another private lab in the city the next day. This discrepancies in test results has brought the spotlight on the technicalities involved in RT-PCR tests which are mandatory to determine if a person has COVID-19. 

According to a report in Times of India, Manikandan had undergone a swab test for COVID-19 at Micro Labs on Tuesday to comply with the conditions of his workplace. By Tuesday evening, the labs informed him that he has tested positive for coronavirus. However, an asymptomatic Manikandan did not believe this and reportedly took another test in another private lab in the city on Wednesday. The results of this test returned negative for COVID-19. Armed with the results of the new test, Manikandan approached Micro Labs, and accused them of giving him false results.

When TNM contacted the local police to know more about the issue, a policeman said that they visited the concerned Micro Labs as the argument was happening. “Manikandan’s concern was largely around the stigma associated with the disease and how his family has been affected by it,” he added. 

Police also told TNM that they have forwarded Manikandan’s complaint to the district health department, which is authorised to look into these matters. A doctor from Micro Labs, meanwhile, confirmed that the district health department officers have been in touch with them.

When TNM contacted the Coimbatore City Corporation, a senior officer confirmed that there have been complaints about the lab results. “On Wednesday, around 50 persons went inside the lab and created an issue about the test results. Based on concerns around law and order, we have shut down the lab in Vadavalli. The district health department is carrying out an investigation on the authenticity of the tests and other technical matters. Based on that, further course of action will be taken,” he said.

This is not the first time discrepancies have been reported between the test results of different laboratories.

The doctor from Micro Labs told TNM that the test kits used by laboratories across the country prescribe different values of Cycle Threshold (CT) for declaring a sample as coronavirus positive or negative. She also said that CT value can be understood as ‘an indicator of the viral load’ in a sample and that different kit manufacturers use different CT values to declare a sample as COVID-19 positive or negative.

“We have a cut-off of 33 for CT value in our test kits. It means that samples that have viral load of up to 33 will be declared to have COVID-19. The World Health Organisation (WHO) guidelines for laboratories is to follow the protocol as per the test kits that we use. We use kits from Trivitron and as per the guidelines of that company, we should consider any sample with viral load up to CT of 33 as COVID-19 positive,” the doctor explained.

What is CT Value? 

According to a report in The Hindu, Cycle Threshold value (CT Value) is an indicator of the viral load in the infected person. However, the article added that there are no reliable ways to link the CT value in a sample and the severity or infectiousness of the disease in a person. 

Dr Jacob John, retired professor of Virology at Christian Medical College, Vellore, told TNM that when a particular sample has a CT value, that itself is enough proof that there are traces of the virus in the sample. 

“In my opinion it is not a reliable metric to judge the severity of the infection,” he added, pointing out that the viral load reflected in the test result is based on where the swab sample is taken from (throat or nose), which may not be uniform. Testing for the presence of the virus in the blood is more reliable, he adds, as swab samples are localised to a particular spot in the body unlike samples of blood, which is uniformly spread across the body.

Virus behaviour varies from person to person

There are many factors that contribute towards a sample testing positive or negative for coronavirus. A Velumani, the Chairman of Thyrocare Technologies Limited that owns and operates over 1000 diagnostic labs across the country told TNM that it is imperative for people to understand why a test returns positive or negative for the novel coronavirus.

“If a swab sample did not have an adequate amount of virus, the test result can be negative. Technicians and how they collect swab samples play a huge role,” he said. He added that the progression of COVID-19 is such that those who test positive today could have tested negative the previous day. This is because the incubation period of the virus can vary from immune system to immune system.

“It also depends on the incubation period of the virus inside the body. A person can be infected today, but still test negative but tomorrow when they take a new test, it might return positive. That could also be a reason behind the different test results from different labs,” he pointed out. 

Adding that he is aware of many officers who see such differences in lab results as crime, Velumani said that it is mostly not the failure on the part of the labs that conduct the tests.

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