CD4 count: immune status, staging and the AIDS threshold
In HIV, the CD4 count estimates how far the infection has damaged immunity by counting CD4 T cells, the white blood cells that coordinate the immune response.
Remember
In adults, a CD4 count below 200 cells/µL meets the count-based definition of AIDS (advanced HIV); stage numbers differ between systems, so learn the thresholds of 500, 350 and 200 rather than the stage numbers.
HIV infects and destroys CD4 T cells, white blood cells that coordinate the immune response. The CD4 count, the number of these cells per microlitre of blood, is therefore the main way to estimate how far the infection has damaged the immune system. The viral load (the amount of HIV genetic material in the plasma) shows something else: how actively the virus is multiplying and whether treatment is working.
A healthy adult has a CD4 count of roughly 500 to 1500 cells/µL. Cell-mediated immunity (the defence run by T cells) is only minimally affected while the count stays above about 350. Once the count falls below about 200, the body can no longer hold back opportunistic infections, which are infections by organisms that seldom harm people with healthy immunity.
In adults, a CD4 count below 200 cells/µL meets the count-based definition of AIDS (acquired immune deficiency syndrome, now often called advanced HIV). The count signals a high risk of opportunistic infection even before any illness has appeared. An AIDS-defining illness, such as Kaposi sarcoma (a cancer), can also establish the diagnosis at any CD4 count.
Systems divide the CD4 count differently, so stage numbers vary. In the three-stage adult grouping, stage 1 is 500 cells/µL or more, stage 2 is 200 to 499 and stage 3 is below 200. Some teaching splits the middle band (200 to 499 cells/µL) at 350. The same stage number can therefore mean different counts, so learn the thresholds of 500, 350 and 200 rather than the stage numbers.
Key facts
- Normal count
- roughly 500 to 1500 cells/µL in a healthy adult.
- Mild effect
- cell-mediated immunity is only minimally affected above about 350 cells/µL.
- AIDS by count
- in adults, a CD4 count below 200 cells/µL meets the definition (advanced HIV).
- Other route
- a serious opportunistic infection or cancer typical of advanced HIV can establish AIDS at any CD4 count.
- Three-stage classification for adults
- stage 1 is 500 cells/µL or more, stage 2 is 200 to 499 and stage 3 is below 200, the AIDS range.
- Viral load
- it shows how active the virus is, whereas the CD4 count shows how much damage the immune system has sustained.
Exam traps
- The viral load does not directly measure immune damage, because it shows how actively HIV is multiplying and is used with the CD4 count, especially to judge whether treatment is working.
- In adults, a cut-off of 100 cells/µL for AIDS would miss people with counts of 100 to 199, who already have advanced HIV and a high risk of opportunistic infection.
- A count below 200 cells/µL does not prove that an opportunistic infection or cancer has occurred: it shows high risk, but whether an illness is present has to be judged from the clinical picture.
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