Kolkata patients could possibly be a case of false positive diagnosis 

One of the two positive cases tests negative on retest yesterday 

Younten Tshedup

The news of all 55 primary contacts testing negative on the RT-PCR test on the night of November 29 came as a relief for many Bhutanese, who were worried of another lockdown.

All those who came in contact with the two individuals, a patient and an escort, who tested positive for Covid-19 in Kolkata, tested negative in the country.

Last evening, the 55-year-old man who had initially tested positive, tested negative during the retest. The results of the 47-year-old patient, who also tested positive, is pending as of last night.

On November 27, a group of Bhutanese referral patients and escorts left for Kolkata. All the 45 passengers on board were tested for Covid-19 prior to their departure. An individual is allowed to travel outside only after he or she tests negative. RT-PCR tests are conducted 72 hours before the departure.

Two individuals tested positive for Covid-19 after arriving in Kolkata. The incident has once again reignited the debate on reliability of testing procedures.

A similar incident was also reported last month.

Two Bhutanese referral patients in Kolkata after testing positive initially, tested negative during the retest, a few days after.

One of the two cases, a 47-year-old cancer patient was admitted at the Royal Bhutan Army (RBA) hospital in Lungtenphug, Thimphu since October, meaning his movements outside were limited.

The 55-year-old man from Phuentsholing, who was escorting his sick wife, arrived in Thimphu as a medical emergency case on November 25. Medical emergency cases are allowed to move from a high-risk area to a lower risk following an antigen test. If the results are negative, individuals are not required to stay in quarantine for seven days.

The 55-year-old man and his wife also tested negative on the RT-PCR test on November 26 before their departure. Everyone who boarded the aircraft was cleared for travel based on their Covid-19 test results.

The two individuals tested positive in an RT-PCR test, which was conducted as a requirement by the hospital in Kolkata prior to admission into the facility. As of yesterday, no one from the group including the escort of the 47-year-old cancer patient and the partner of the 55-year-old man, tested positive for the virus.

Health experts said contracting the disease upon arrival or at the airport was highly unlikely, as it would be too early for them to turn positive within 24 hours.

They explained that if they were infected while in the country, at least some of their immediate contacts should have tested positive in the country.

Amidst all these speculations, cross-contamination of samples is also being considered. Cross-contamination happens when a negative sample comes in contact with a positive sample during a test.

Health officials say contamination of laboratory reagents could also lead to cross-contamination.

Health officials are speculating the case to be another case of false positive diagnosis, similar to one reported earlier in October from Kolkata.

The reverse transcription- polymerase chain reaction (RT-PCR) test for Covid-19 is considered the gold standard test for diagnosis.

Health officials said that it was not very common to have false negatives or positives on the RT-PCR test.

RT-PCR test involves a chemical reaction that repeatedly duplicates certain targeted segments of the virus’s RNA until there’s enough of it to be detected, meaning that with a well-designed test, it is highly certain that any reaction and duplication occurring is only with genetic material from the SARS-CoV-2 virus, ruling out any false positive or negative results.

Officials explained that a false negative result on the PCR test could happen during early or late infection with a low viral load. Sampling technique, improper sampling or problems during the sample processing could also result in a false-negative result.

The rare false-positive in a PCR test could happen from cross-contamination of samples. A cross-reaction could also lead to a false positive on the PCR.

A false positive result from a cross-reaction is more common with an antibody test than a PCR test, according to experts. Very rare, but a cross-reaction with a virus having similar properties like that of a Covid-19 could lead to cross-reaction.

The probability of a false negative result on a PCR ranges between 2 to 30 percent. A UK based study found that a false positive result on a PCR ranges between 0.8 percent to 4 percent.

Meanwhile, this is the second time where Bhutanese referral patients in India have tested positive for the virus.

In the first incident in October, no close contacts of the patient tested positive.

“Based on the above considerations, we believe that this too would be a case of false positive diagnosis,” an official said.

However, besides the public panic, contact tracing and testing of the contacts have cost implications for the government.

Lyonpo Dechen Wangmo said that the ministry has asked for a retest of the two individuals in Kolkata. “They have their own protocol for retesting the patients.” Any decision on future referrals would be taken based on the results of the retest.

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