The state health department has decided to run second checks on samples collected from private laboratories conducting Covid tests following allegations their results were faulty.

The samples collected from private laboratories will be tested separately and the results compared with those of the laboratories, a health department official said.

The health department has been receiving complaints for the past month-and-a-half about results being negative in the first instance and then positive at times within three to four days. There have been several instances of patients returning home after being declared Covid negative by a private clinic and then turning up at a state Covid hospital within two-three days and testing positive.

There are six private laboratories in Calcutta. Barring one, representatives from the remaining five were present at a meeting with senior department officials on Thursday where the decision to conduct random tests was announced.

These private laboratories like the government ones have been conducting Covid tests on swab samples using the RT-PCR method.

The RT-PCR method involves extraction of RNA of the virus to determine the viral load in a sample.

“If the RNA is not extracted properly, results can be misleading. It has happened on several occasions,” a virologist at the School of Tropical Medicine said. “Laboratories where RT-PCR methods are being followed need stringent monitoring.”

Health department officials cited the example of the School of Tropical Medicine. “Several weeks ago all test results started turning out to be negative. After investigation, it turned out that the machines had been infected during training sessions of pathologists,” a department official said. “We had to close the laboratory and decontaminate it before resuming tests.”