Real time PCR and percentage loss - (Jun/06/2011 )

Hi all,

I'm using qPCR to assess the amount fo viral RNA in different samples. One sample originally had a Ct value of 22.83; after treatment of the sample it's Ct value decreased to 25.95. The efficiency of the PCR is 100%. I would say that there is 8.69 (i.e. 2^3.12) times more RNA in the sample before treament, but my supervisor describes the change as a 90% loss from original sample. How is this calculated?! Maths most definitely not being my strong point! Thanks :-)

after treatment - before treatment = 3.15 -> 2^3.15 = 8.69 fold increase of original to after treatment. But you need to reference it to the "before treatment" not the other way around, so you just change it to:
before treatment - after treatment = -3.15 -> 2^-3.15 = 0.11 fold increase of after treatment to original, that means 11% of original, that is around 90% decrease from the original.
Both values are right, you just need to relate it to the right reference.