CD4
Tests
CD4 (T-CELL) TESTS
WHAT ARE CD4 CELLS?
CD4 cells are a type of
lymphocyte (white blood cell). They are an important part of
the immune system. CD4 cells are sometimes called T-cells.
There are two main types of T-cells. T-4 cells, also called
CD4+, are "helper" cells. They lead the attack against
infections. T-8 cells, (CD8+), are "suppressor" cells that end
the immune response. CD8+ cells can also be “killer” cells
that kill cancer cells and cells infected with a virus.
Researchers can tell
these cells apart by specific proteins on the cell surface. A
T-4 cell is a T-cell with CD4 molecules on its surface. This
type of T-cell is also called “CD4 positive,” or CD4+.
WHY ARE CD4 CELLS IMPORTANT IN HIV?
When HIV infects humans,
the cells it infects most often are CD4 cells. The virus
becomes part of the cells, and when they multiply to fight an
infection, they also make more copies of
HIV.
When someone is infected
with HIV for a long time, the number of CD4 cells they have
(their CD4 cell count) goes down. This is a sign that the
immune system is being weakened. The lower the CD4 cell count,
the more likely the person will get sick.
There are millions of
different families of CD4 cells. Each family is designed to
fight a specific type of germ. When HIV reduces the number of
CD4 cells, some of these families can be totally wiped out.
You can lose the ability to fight off the particular germs
those families were designed for. If this happens, you might
develop an opportunistic infection.
WHAT FACTORS INFLUENCE A CD4 CELL COUNT?
The CD4 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 CD4 cell test, and to use the same
laboratory.
Infections can have a
large impact on CD4 cell counts. When your body fights an
infection, the number of white blood cells (lymphocytes) goes
up. CD4 and CD8 counts go up, too. Vaccinations can cause the
same effects. Don't check your CD4 cells until a couple of
weeks after you recover from an infection or get a
vaccination.
HOW ARE THE TEST RESULTS REPORTED?
CD4 cell tests are
normally reported as the number of cells in a cubic millimeter
of blood, or mm3. There is some disagreement about the normal
range for CD4 cell counts, but normal CD4 counts are between
500 and 1600, and CD8 counts are between 375 and 1100. CD4
counts drop dramatically in people with HIV, in some cases
down to zero.
The ratio of CD4 cells to
CD8 cells is often reported. This is calculated by dividing
the CD4 value by the CD8 value. In healthy people, this ratio
is between 0.9 and 1.9, meaning that there are about 1 to 2
CD4 cells for every CD8 cell. In people with HIV infection,
this ratio drops dramatically, meaning that there are many
times more CD8 cells than CD4 cells.
Because the CD4 counts
are so variable, some health care providers prefer to look at
the CD4 percentages. These percentages refer to total
lymphocytes. If your test reports CD4% = 34%, that means that
34% of your lymphocytes were CD4 cells. This percentage is
more stable than the number of CD4 cells. The normal range is
between 20% and 40%. A CD4 percentage below 14% indicates
serious immune damage. It is a sign of AIDS in people with HIV
infection. A recent study showed that the CD4% is a predictor
of HIV disease progression.
WHAT DO THE NUMBERS MEAN?
The meaning of CD8 cell
counts is not clear, but it is being
studied.
The CD4 cell count is a
key measure of the health of the immune system. The lower the
count, the greater damage HIV has done. Anyone who has less
than 200 CD4 cells, or a CD4 percentage less than 14%, is
considered to have AIDS according to the US Centers for
Disease Control.
CD4 counts are used
together with the viral load to estimate how long someone will
stay healthy. CD4 counts are also used to indicate when to
start certain types of drug therapy:
When to start antiretroviral
therapy (ART):
When the CD4 count goes
below 350, most health care providers begin ART. Also,
some health care providers use the CD4% going below 15% as a
sign to start aggressive ART, even if the CD4 count is high.
More conservative health care providers might wait until the
CD4 count drops to near 200 before starting treatment. A
recent study found that starting treatment with a CD4% below
5% was strongly linked to a poor
outcome.
When to start drugs to
prevent opportunistic infections:
Most health care
providers prescribe drugs to prevent opportunistic infections
at the following CD4 levels:
- Less than 200:
pneumocystis pneumonia (PCP)
- Less than 100:
toxoplasmosis and cryptococcosis
- Less than 75:
mycobacterium avium complex (MAC).
Because they
are such an important indicator of the strength of the immune
system, official treatment guideline in the US suggest that
CD4 counts be monitored every 3 to 4 months.
