Your will find some information about this here: http://fixhepc.com/forum/experts-corner/364-viral-load-quantitative-vs-qualitative.html

Where I note that:

Hep C Viral Load can be quantitative (undetectable, <15, or 15-100,000,000) or qualitative (undetected/detected).

But what does that look like under the hood?

There is very little viral RNA in your body, so to find it and count it we do this:

• Make an RNA soup out of your blood
• Perform what is known as Polymerase Chain Reaction to amplify the quantity of RNA (rather like brewing beer)
• Have a look at the resulting soup

And here is to sort of thing we see:

So if you look at the bottom “negative serum” you see we have nothing. With 0073 and 5044 we have faint smudges. 0073 might be reported HCV RNA < 15 (quantitative) HCV RNA detected, and 5044 HCV RNA 42, HCV RNA detected. If you look at the positive serum, to the left you see 1:100 positive serum - this is where the "soup" was diluted 1 part soup to 100 parts water to dilute it - as you can see it still shows up, but is much fainter. Dilution lets us quantify differences between the large solid smudges. When we get to really low levels of RNA all we see really faint smudges. At some point the smudge is there "detected" but to faint to count. The <12, <15, <20 and <25 results we see relate to the minimum sensitivity of the test in use. The lowest we used to be able to get to was <25, but is now typically <15 and recently I have seen <12 in some results from Spain. Although it is obviously great to have nothing at all seen ie <15 and undetected this does not map as well to SVR as you would expect. No, I don't understand that either! http://fixhepc.com/forum/questions-and-answers/541-hcv-rna-undetected.html?start=18