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Variations in CD4 Counts
Nov 22, 1996

Can you explain, briefly, why there can be such huge variations in CD4 counts? I know that different labs/facilities will come up with different counts. For example, I was part of a study and my CD4 count from that lab was often 100+ less than the count from my doctor's lab. What gives?

Response from Dr. Cohen

A lot of that variation has to do not with the CD4 measurement itself, but with variation in the way the white blood cell count and differential is measured, and with the fact that the absolute CD4 count is subject to variation in three different tests.

The absolute CD4 count can be measured directly, but in most labs it is not. It is calculated using the following equation:

Absolute CD4 count = (white blood cell count) x (% lymphocytes) x (%CD4)

Inaccuracy or variability in each of those three components is multiplied together and will affect the CD4 count. For that reason, the CD4 percentage tends to be a less variable measurement.

For reasons that are mostly historical we have always tended to use absolute CD4 counts rather than CD4 percent. That's fine in most cases, but if you notice a lot of variation or if there's a serious discrepancy in a series of measurements, it's worth looking at the CD4 percents. You may find that they don't vary as much. Viral load measurements are also less variable.


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