Thread by @Rumindahg

#PCR test, how to understand the results

First a reminder that the nasal swab for #COVID19 can be negative in a patient infected with covid19. This range is between 5-40%

The #PCR test results are not necessarily a positive or negative scenario. It depends on the Ct value👇

https://twitter.com/apoorva_nyc/status/1343929897341902851

#Ct or cycle threshold is the number of cycles the PCR has to run to get a positive result

Lower the Ct value means higher the viral load & apparently greater the infectivity, severe illness & death

But Ct values vary between machines & even in the same machine due to the user

#Ct values change over the timing of the illness. From high values at onset of symptoms & tailing off after around 5 days. These values might stay on for a long time depending on the patient, even after a live virus is gone. A low value probably means patient is not infectious.

What are these #Ct values?

Difficult to answer as mentioned above it can vary.
There is no general consensus what Ct value equates +ve test but ≤37 is a number thrown around

In Sri Lanka AFAIK we discharge patients if the #PCR is >32

Remember higher the value lower the virus

Another general consensus is that if #Ct value is >30 patients is not infectious

In summary
-PCR can be negative in an infected person
-the Ct value determines the viral load. Lower the Ct value higher the load
-Ct value is a good predictor of infectivity, severity & death

*high value means not infectious