Hello good people,
This is not a normal question for me, but a friend in Public Health wanted to know, so I thought I would ask. The test for COVID is a PCR, if I am not mistaken. The question is, how much DNA would you need to detect an infection? I can imagine that this is not simple. Just wondering if anyone knows what the limits of detection are. picograms? nanograms? I suppose you could always re-PCR the first reaction to detect really low amounts. How many rounds would need to be done? After the PCR, what is done with the sample? I assume something more sophisticated than a simple agarose gel, but I don't know.
Thank you!
Ellen
I am responding, not because I am the foremost expert in this field, but because no one else has answered this interesting question, and I too (also being in Public Health) would like to know the correct answer. I have had some close relatives who attended your institution, so I also have empathy, in the fact, that there are no answers yet. So, I will take a stab at the answer, and then maybe more competent DNA biochemists can correctly respond.
So my cursory answer: The RT-PCR is done on DNA, starting with a single strand. The SARS-CoV-2 is a single stranded RNA virus, so the first step is copying this strand into a DNA strand, that is the RT. This DNA strand or segment is multiplied billions of times to produce enough to give a reading. It can be read by a radioactive marker, but most of the current tests use a fluorescent marker that is easier and quicker to measure. So, theoretically, the multiplication of a single segment of the original viral RNA by billions of times, produces a colored glow that would be qualitatively positive. More sophisticated calibration can potentially allow quantitative measurement of more than one initial RNA segment.
So, I may be wrong in my number, and I wish that someone with greater expertise will correct me, so that I may know the correct answer too. Good luck, stay safe, and thank you for allowing me to expound. Gary Ordog, MD
Thank you, Gary! I was sort of guessing the same thing. I am a biomedical scientist (PhD), so I have done PCR, but the methods needed for a rapid turn-around are a mystery to me! I think our PCR reactions used to take a minimum of 4 hours and we probably work with much more DNA.
Best wishes,
Ellen
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