What is Covid RT-PCR test's Ct (Cycle Threshold) limit in India?

[Nikhil VJ]

Hi All,

I wanted to know if the Indian government or related agencies have set any standard limit in a crucial setting in the Covid tests called "Cycle Threshold".

Are the folks collecting stats on covid cases - having any data on cycle values for those cases?

Have you or someone you know been diagnosed as a Covid-19 case? What was the cycle value in your test result? Does the lab that did the test share this data?

Sharing an article and some excerpts from it:

https://www.msn.com/en-us/health/medical/experts-us-covid-19-positivity-rate-high-due-to-too-sensitive-tests/ar-BB18wE8B

Experts: US COVID-19 positivity rate high due to 'too sensitive' tests

"With a cutoff of 35, about half of those tests would no longer qualify as positive. About 70 percent would no longer be judged positive if the cycles were limited to 30.

In Massachusetts, from 85 to 90 percent of people who tested positive in July with a cycle threshold of 40 would have been considered negative if the threshold were 30 cycles, Mina said. "

"The Food and Drug Administration said that it does not specify the cycle threshold ranges used to determine who is positive and 'commercial manufacturers and laboratories set their own.'"

"The CDC said its own calculations suggest its extremely hard to detect a live virus in a sample above a threshold of 33 cycles. "

This was one - there's many more if I search for "RT-PCR test cycle threshold value covid" on duckduckgo. (tip: google search is broken when it comes to anything controversial. Proverbial case of overprotective mother suffocating the child in the quest to protect it. Take the same query and run it in Bing, Duckduckgo etc also.)

Looking into this I'm seeing an analogy with vectorizing raster satellite imagery : Your software can easily fill the whole thing up with false positives, or can produce no result at all. Lot of fine tuning is required to get the "perfect setting" that minimizes the false positives and false negatives, and you often never reach a perfect setting that didn't have any mistakes. It's not a hard yes/no thing. You invariably need manual intervention (and even with AI interventions we're seeing problems), and it frustrates the hell out of people who assumed this technology thing is a silver bullet.

Inviting people with better knowledge on this topic to correct me: My understanding is that there is an exponential (maybe doubling, maybe some other factor) change from one Ct value to the other. To go from 33 to 40, would be.. well, non-trivial.

So one set of data needed is : What are the Ct limits being used in current testing? Is there a single value standardized by the government? If there is variation, then who decides?

Another set of data that can be just as useful : Have these Ct limits been changed since the pandemic began almost a year ago? How have they changed, and is there any co-relation between that and the Covid+ case counts? Is it possible to "explode" / "reign in" a pandemic by merely altering this setting without any ground level realities changing? If yes, then why is talk about it missing from the mainstream discourse?

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Cheers,
Nikhil VJ
https://nikhilvj.co.in

[Chandrasekhar S.]

CT value is not a reliable indicator. Earlier labs used to report CT value by default but now they do not - but one can ask for the CT value. 

https://www.icmr.gov.in/pdf/covid/techdoc/Advisory_on_correlation_of_COVID_severity_with_Ct_values.pdf

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[guneet...@sputznik.com]

This debate was quite heated in the US back in July / Aug. Most virologists and health experts agree that cycle threshold is not at all about false positives or false negatives. It only has significance when it comes to viral load. And from the ICMR advisory linked above, as well as WHO I think, the correlation between viral load and disease severity is not clear or reliable.

Most test reports, at least here in India or in the US do not report the number of cycles used to determine the result.

- Guneet