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Do you know anyone who has been tested #COVID19 positive from one lab & negative from another? In Pakistan, one of the many reasons for such discrepancies is the confusion over the Ct values –metric based on which our COVID positivity status is determined. A 
on Ct values
on Ct values
#CtValue refers to the number of cycles needed to amplify viral RNA to reach a detectable level. Ct value is inversely proportional to the patient’s viral load. I made this video clip explaining Ct values.
There are different kits available for the COVID19 RT-PCR test. Most tests use a 35-cycle protocol. If the virus isn’t detected within 35-amplification cycles, the test result -ve. If it is detected, then the test is +ve.
The scientists are debating over the Ct values. A few have claimed that 35 is too high, and patients are not likely to be infectious at such higher Ct values.
But there are different scenarios where a high Ct value may feature. For, e.g., Pre-symptomatic infection that subsequently develops into symptomatic infection with high viral load and infectivity.
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/926410/Understanding_Cycle_Threshold__Ct__in_SARS-CoV-2_RT-PCR_.pdf
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/926410/Understanding_Cycle_Threshold__Ct__in_SARS-CoV-2_RT-PCR_.pdf
The problem is that several diagnostic labs give a negative result when the Ct value is higher, and the patient is asymptomatic. Especially to the ones that are getting it done for traveling.
One of my colleagues spoke to a few health department officials –responsible for keeping an eye on the diagnostic facilities– about Ct values. Much to our dismay, they were not even aware of what Ct values are.
According to WHO’s current guidelines, “Where test results do not correspond with the clinical presentation, a new specimen should be taken and retested using the same or different technology.” https://www.who.int/news/item/20-01-2021-who-information-notice-for-ivd-users-2020-05
And if we give negative results to potentially infectious patients, the virus will spread. I understand that due to the lack of standardization across tests, most labs shy away from reporting the Ct values. And the test results are only viewed in a binary way — positive/negative.
However, Ct values should be provided to the requesting healthcare professional. https://www.who.int/news/item/20-01-2021-who-information-notice-for-ivd-users-2020-05
We @CancerbioC @pu_lhr_official are working on CT values & their association with disease severity in Pakistani patients. We will showcase our work in the coming days. For possible collaborations, please DM or send an email to director.crc@pu.edu.pk. @WHOPakistan @NIH_Pakistan
