Focus on CT value as questions rise about test accuracy

CHENNAI: Any swab sample that returns a cycle threshold (CT) value above 30 should be reevaluated to ensure that sampling errors and problems in sample collection do not lead to erroneous results, opined experts. The significance of CT value has come to the fore after instances of false positives being reported by some private laboratories recently.

The CT value determines the viral load. To ensure that the sensors detect the RNA of coronavirus while testing, the machine does what is called ‘cycles’. Each cycle doubles the RNA’s brightness to make it easier to detect. Which means lesser the viral load, more number of cycles are required. So, higher

CT values indicate lesser viral load. A wide range of RT-PCR kits being used for testing, and the instructions for results declaration and CT value is different for each brand.

According to microbiologists and public health experts, low viral load cases should be carefully examined before being declared positive to avoid causing panic among the test subjects and also to prevent erroneous increase in positive cases.

“Each kit manufacturer assigns their own CT value to define positivity and negativity. This is based on the lowest quantity of virus that can be detected by the kit,” explained Anubhav Anusha, global CEO and R&D head of GeneStore that manufactures the kit called GeneStore Detection Expert.

The accuracy levels can be affected by factors like improper collection and storage of nasal swabs, improper RNA extraction, cross contamination between samples, poor storage and handling of reagents and the extracted RNA samples. “Also, data analysis settings on the machines can also cause variations in data interpretation and false reporting," Anusha added.

Officials from the State Public Health Laboratory added that some samples are tested again because the gene curves analysis shows wide variations. “They should be in the same range to show accurate results which determines the positive or negative status of the sample. Any laboratory can easily interpret the data through computer before declaring a sample positive or negative," said a senior official.

Public health expert Dr K Kolandaisamy noted that most labs do no preserve samples for re-examination. Most among the 192 approved testing facilities in the State show similar range of positivity, he said, adding that a higher range calls for re-examination of samples.