What it measures
CD4 is a marker found on helper T cells, the lymphocytes that coordinate the immune response. The test counts how many CD4 cells are in a microlitre of blood, using flow cytometry. Results are often given with the CD4 percentage and the CD4 to CD8 ratio.
Why people test it
- Monitored in people living with HIV, alongside viral load.
- Measured when the lymphocyte count is low or an immune deficiency is suspected.
- Monitored on some medicines, and after some transplants, that lower T cells.
Typical range
Ranges vary by lab and method, and counts are much higher in children. The count can also change with recent illness. Your report shows the lab's own range.
Finger-prick or vein?
A vein sample is needed. The cells are counted while fresh, so the sample has to reach the lab quickly, and a finger prick does not give enough blood.