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Lab using a new assay
Mar 16, 2008

I'm not on any meds, have been infected for about 30 months. My CD4s are in the 600 to 800 range, %age mid-30s, my VL barely above detectability threshold (I'm touching wood as I type); though my ratio has declined slightly in the last nine months, from 1.4 to 1.2.

My last VL result (late-January) came back as 50 cpm. Which I thought was a bit odd, since I thought 50 was the definition of "undetectable" with the ultrasensitive assay. Then I saw this note:

"New viral load method implemented 12-14-2007. The results are comparable to the old method with an average log difference of 0.15 lower for the new assay."

Could you explain please? (Sort of sounds like the "re-centering" of scores that takes place occasionally with tests like the SAT and GRE.) Is this a new method being used or is the assay now more sensitive (down to 20 cpm or whatever)?

Thanks.

Response from Dr. Holodniy

Likely a new method, although hard to be certain. This definitely happens when labs go to a new viral load assay. Depending on the technology used, some recalibration will be required, either up or down when comparing results to the old assay. In most cases, it takes a 2-3 results with the new test to "rebaseline" where someone is. Given your CD4 count/percent, and the very low viral load, I would not be concerned. Remember in most cases, undetectable does not mean zero. There may be very low levels or intermittent small bursts of viral load, that will be under the radar of the assay cutoff of 50. On the other hand, the assays are not perfect and there will be some wobble or blipping for those who are mostly consistently below 50, that sometimes the result will be > 50. That is most likely related to assay variability.



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