Is there any reference article for this?
Cts < 29 are strong positive reactions indicative of abundant target nucleic acid in the sample
Cts of 30-37 are positive reactions indicative of moderate amounts of target nucleic acid
Cts of 38-40 are weak reactions indicative of minimal amounts of target nucleic acid which could represent an infection state or environmental contamination.
There are many papers that show calibration curves from standards with known concentration (in terms of "copy numbers" of the amplicon sequence). For instance:
http://aac.asm.org/content/55/9/4038/F1.expansion.html
http://femsec.oxfordjournals.org/content/87/3/576
What we lern from such curves is that, in sufficiently well optimized qPCR assays, a single molecule will give a ct value of around 40 (give and take some), and that 106 copies will give a ct value around 20.
Now you can calculate at least the order of magnitude of quantified molecules in a sample that refer to a measured ct value. If you like to consider this as "abundant", "moderate" or "minimal" depends on the kind of samples and on how many cell-equivalents per sample you are analyzing.
There are many papers that show calibration curves from standards with known concentration (in terms of "copy numbers" of the amplicon sequence). For instance:
http://aac.asm.org/content/55/9/4038/F1.expansion.html
http://femsec.oxfordjournals.org/content/87/3/576
What we lern from such curves is that, in sufficiently well optimized qPCR assays, a single molecule will give a ct value of around 40 (give and take some), and that 106 copies will give a ct value around 20.
Now you can calculate at least the order of magnitude of quantified molecules in a sample that refer to a measured ct value. If you like to consider this as "abundant", "moderate" or "minimal" depends on the kind of samples and on how many cell-equivalents per sample you are analyzing.
Those are excellent references for this Jochen~
Also helpful is to use the calculation for initial copies per
reaction, Xo, (provided one already knows the y-intercept of the standard curve, the measured sample Cq value, and the qPCR exponential amplification efficiency EAMP):
Xo = EAMP(b-Cq)
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