Correlation between Percentage and Absolute Number of CD4s
Oct 21, 2009
I seem to recall having read somewhere (and believe that Dr. Young has mentioned in passing) that there is a "general rule of thumb" as to what a particular percentage "should" translate into as an absolute number in an average person. I am on Truvada and Isentress. In my May labs my CD4s went up to a staggering 664 (from 382 and 18.8% in March) with 19.6% and a vl of 296 to the following numbers in July: 433 with a 23.6% percentage and a vl of < 50 ( I had not reached this low vl in these 9 months ofbefore). The lab is always the same and the blood is always drawn in the morning. Given these crazy fluctuations in absolute numbers I was hoping to get a refresher on these general notions so that I could try to better assess the true state of my CD4 arsenal.
Response from Dr. McGowan
Thanks for your question. This is an area of confusion for many (including many doctors) and is a more sophisticated way of looking at your CD4 counts.
A good way of thinking about it is to view HIV as an imbalance of the immune system. The 2 major types of T cells are the CD4 (or helper) cells and the CD8 (or killer/suppresor) cells. Th normal balance is more CD4 cells than CD8 cells. HIV preferentially attacks the CD4 cells and lowers their numbers faster than the CD8 cells. This throws off the balance. So the proportion (or %) of T cells that are CD4 drops and the proportion of T cells that are CD8 goes up. Usually a CD4 count of around 200 equals a percentage of about 14-15. The % and the absolute CD4 count can fluctuate differently because we only measure the CD4 count in the blood and these T cells are constantly moving in and out of the blood and into the tissues to fight infections...so we only see the tip of the iceburg. The total number of T cells can be higher or lower throughout the day, but the % usually is more stable. Think of 2 pizza pies, one large one and one small one. If the pies are cut into quarters (each slice 25% of the pie), the 25% slice would be larger in the big pie and smaller in the little pie. This is the same for the CD4 number and %...HIV will control the size of slice but not the size of the pie. As long as your slice is getting bigger it doesn't matter if the pie gets larger or smaller, your cut is improving. (This is assuming you like pizza!).
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