Why we get false negative tests:The diagnosis of COVID19 using RT PCR method is made based on the threshold cycle (Ct) value. Ct is defined as the cycle number when the sample fluorescence exceeds a standard threshold above the calculated fluorescence.1/

So lower the Ct value of a specific gene, the more of the gene exists in the swab .Unfortunately a Ct-based diagnosis has no constant cut-off value, & Ct cut-off values are different for each diagnostic reagent even for the same gene. 2/

A test is positive based on Ct value of 35.Ct value in a RT PCR test is quite accurate,but an error of 1/2 cycles is common.
So when there is a eqivocacy in the Ct value,such as 34-36, the result is interpreted as false negative or false positive depending on the Ct cut-off. 3/

A test can also be false negative in early COVID infection because the Ct value is inversely proportional to the amount of the target gene,if the viral load is less. 4/

Therefore it is necessary to standardize the Ct value, which ideally the ICMR should do.
The best way to get accurate results is to take 2 swabs from 2 diff sites (nasopharyngeal and throat swab) and do successive tests to reduce false negative results.5/