Twice tested negative on RT-PCR, the confirmatory test for Covid-19

Younten Tshedup

A woman in Phuentsholing who recently tested positive on the rapid diagnostic test (RDT) could possible be ‘false positive,’ experts said, after she tested negative twice on the reverse transcription-polymerase chain reaction (RT-PCR), the confirmatory test for Covid-19.

The woman had tested positive for IgG antibodies during the health ministry’s ongoing serosurveillance in high-risk areas. A serosurveillance is a process of monitoring the presence or absence of a specific substance in the blood serum of a population.

Clinical microbiologist with the national referral hospital, Dr Tshokey said that the ministry is using RDT kits to detect possible infection through the detection of antibodies among the high-risk population during the serosurveillance.

There are two brands of RDT kits currently used in Bhutan to detect antibodies besides the rapid antigen testing and the RT-PCR.

Dr Tshokey said that the Phuentsholing woman had tested positive for IgG antibodies using both the brands of antibody test kits. “This means that the person had been exposed to the virus some three weeks ago or even before.”

However, the negative results on the RT-PCR confirmed that the woman had no active infection (infectious) at the time of testing.    

It was learnt that the woman had no recent travel history and had not come in contact with any positive patients.

Dr Tshokey said that the most logical thing to do in such cases was to test her family members and close contacts. “If this person was infected without her knowledge, at least we expect some of the family members who are in close contact to test positive.”

All her primary and close contacts including family members tested negative for Covid-19. “The only possibility then was that the RDT results could have been false positive,” said the microbiologist.

The woman has been quarantined for now.

Testing is a major challenge

Health experts say that having a good and reliable test kit for detecting Covid-19 is still a challenge. This is mainly because Covid-19 is a new disease and the behaviour of the virus is still evolving.

Dr Tshokey said that no test is 100 percent in terms of sensitivity and specificity. “There are limitations to every test kit and specifically with the Covid-19 test kits because it’s a fairly new disease.”

He said that unlike test kits for other diseases that have to undergo robust scrutiny and testing by international bodies before the approval for use, Covid-19 test kits are allowed based on an emergency use authorisation (EUA) following a fair evaluation.

He added that some of the practical test kits are given the EUA, including the ones the country is currently using. The test kits are imported from South Korea.

Surveillance to feel the pulse of the pandemic

The health ministry on July 30 started conducting serosurveillance to check for possible local transmission in communities in the light of increasing positive cases across the border.

High-risk populations including health workers, officials in the frontline and mobile population such as taxi drivers, among others are tested using RDT kits to detect antibodies in the body.

Antibodies are produced as a body’s response when an antigen (foreign substance) enters the body.

Ideally, for any infection, the body first produces the IgM antibody during the initial phase of the infection. The body starts producing a large amount of antibodies (IgM) in response to the infection. Subsequently, the number of viruses starts declining, as the antibody fights the virus. Usually, by the second week, the number of IgM antibodies drops and the body then starts producing IgG antibodies.

However, Dr Tshokey said that some studies have shown that for coronavirus, the IgM and IgG antibodies could also be produced simultaneously.

He said the RDT is a good test for serosurveillance, which detects antibodies in people even if the individual has been infected several months ago. The test detects any recent or past infections through the presence of antibodies produced by the body.

With about 20 percent of the Covid-19 patients being asymptomatic, the microbiologist said that people might never know if they are infected. With increasing cases across the border, the test was initiated to see if there were infected people without symptoms in the community, he added.

Meanwhile, as of August 5, the ministry has tested over 16,000 people using RDT along the border communities in the country – Samtse, Chukha, Dagana, Sarpang, Zhemgang, Pemagatshel and Samdrupjongkhar.

The ministry towards the evening yesterday posted that a second round of serosurveillance was found necessary to ascertain the evidence of community transmission along the bordering areas as they reported spikes in the Covid-19 cases.