Difference between VL Not Detected & VL <20cp/ml

Question

Hi Dr. First Happy New Year to you, your entire loved one and all your readers. As we know most of the Labs are using a new Cobas Ampliprep/Cobas Taqman HIV-1 test V2 with linear reportable range 40-10000000 copies/m, which is very sensitive. My VL result six months & three months ago was reported as Not Detected cp/ml, January 2010 test can up with a result <20cp/ml (all test were done in the same lab and using the same method). Is there any difference? How do you explain the difference in reporting? My understanding was that not detected means ZERO (in the blood), is that correct? If that the case then <20cp/ml means that there is some virus. By the way my absolute CD4 count increased by 200 reaching 940/ul while the CD4% gained 2% reaching now 50.6%. Thanks for your permanent help & assistance that is highly appreciated.

Answer

Not detected does not mean zero. Whether the threshold is <40 or <20, there is likely some detectable virus in most folks if one had the ability to accurately measure down to zero. That said, your numbers are fabulous and I would not be concerned about the possibility.