T-cell tests are normally reported as the number of cells in a milliliter of blood. The T-cell value bounces around a lot. Time of day, fatigue, and stress can affect the test results. It's best to have blood drawn at the same time of day for each T-cell test, and to use the same laboratory. Infections can have a large impact on T-cell counts.
Vaccinations can cause the same effects.
Don't check your T-cells until a couple of weeks after you recover from an infection, or after you got a vaccination. These percentages refer to total lymphocytes. This percentage is more stable than the number of T cells. The lower the count, the greater damage HIV has done. See Fact Sheet 413 for more information on the viral load test.