I wrote earlier about a sudden jump in CD4 count from the steady 400's to 900 using a new lab. My provider had questions about the results and contacted that lab. I was then contacted by my provider that my actual CD4 count was 480 and that the new lab uses a different formula to calculate CD4 counts. That's quite a difference. Is this a common problem? It would mean that if the new lab had calculated my CD4 at 400 it would acutally be half 200? Needless to say I will resume using the original lab at my providers clinic even though it means a 90 minute trip each way, just to get lab work. But less confusion. Can using different formulas to calculate CD4 counts results in marked variation?

Yes, different calculations can be used, but that seems well out of the range of variation we would expect between assays.