CD4 / CD8
What is it?
CD4 and CD8 tests measure the
amount of CD4 and CD8 cells (commonly called the “CD4/CD8 count”) present in
your body, both of which are relevant to the progression of HIV infection.
The CD4 count reveals how strong the patient’s immune system is, numerically
shows how far the HIV disease has advanced, and helps predict future
infections and complications.
CD4 T-cells are a type of
lymphocyte (which is one type of white blood cell). They initiate the immune
system’s response to viruses such as HIV. CD4 cells can be infected and
killed by HIV, so the CD4 count generally declines as HIV is taking its toll
on the body. CD8 T-cells, on the other hand, are the immune system’s killers
of abnormal or infected body cells. As HIV infection worsens, the number of
CD8 cells present in the body tends to increase.
Tests may measure the
percentage of lymphocytes that are CD4 T-cells: in a healthy, HIV-negative
person this number is about 40%, while a percentage below 20% usually
reflects a risk that opportunistic infections may develop. Tests may also
measure the CD4:CD8 ratio, which indicates how many CD4 cells are present in
the body per CD8 cell. A normal result is greater than 1, but if the number
drops below 1 (that is, if there are fewer CD4 than CD8 cells in the
sample), the HIV infection is thought to have significantly worsened.
In order to show the
progression of HIV, the CD4/CD8 count must be considered over time. The
tests are used in combination with the viral load test, to determine the
overall status of the disease in the body.
What should I do to
prepare?
Talk with your doctor, who
will be the one to order the CD4/CD8 count test, along with the viral load
test, after HIV-positive status has been diagnosed. If and when HIV therapy
is begun as a response to the first viral load and CD4/CD8 tests, your
doctor should order another viral load test and a CD4 cell count about 2-8
weeks after you start treatment, to evaluate the effect of the therapy. On a
long-term basis, you should be prepared to re-take viral load tests and CD4
cell counts about every 3-6 months as you are being treated for HIV.
What happens?
The same blood-drawing
procedure as for an ELISA test, a Western blot test, or a viral load test is
followed to obtain a sample. The test calculates the number of CD4 T-cells
per cubic millimetre of blood. It measures the amount of CD4 cells present
in the blood, which only accounts for about 2% of the total CD4 cell count
in the body (most CD4 cells are in the lymphocytes).
How does it work?
A standard reference range is
not available for this test; however, normal CD4 counts in adults usually
range from 500 to 1,500 cells per cubic millimetre of blood. Individual CD4
count measurements are quite sensitive and may vary greatly from test to
test; what should be considered is the general progression of the CD4 count
over time.
If, in the initial months
after your HIV-positive diagnosis, your CD4 count declines consistently,
your doctor may start you on anti-HIV treatment and/or begin preventive
treatment against opportunistic infections. If the anti-HIV therapy is
effective, the CD4 count should increase or stabilize. The standard
definition of a person with AIDS is a CD4 count below 200, regardless of
other factors in the patient’s health.
Risks & Errors
It is important to note that
the progression of HIV can only be determined by a series of tests,
not an individual CD4/CD8 count result. Each individual test result is
subject to many extenuating factors, such as time of day (CD4 levels are
usually lower in the morning than in the evening), smoking, stress levels,
illnesses such as pneumonia, influenza, or herpes simplex virus infection,
and cancer chemotherapy.
Another possible test error
can occur if some of the body’s other CD4 cells (the 98% not normally
present in the blood) move into the bloodstream, or if CD4 cells in the
blood move out into other parts of the body. Also, CD4/CD8 counts do not
always reflect general health: some patients can live well with a very low
CD4 count, while others develop infections and complications earlier,
despite a higher CD4 count.
Drug Resistance Tests
What is it?
A drug resistance test, often considered alongside a
patient’s viral load, is designed to monitor whether anti-HIV drugs
administered over time are having their desired effect, and specifically to
detect whether the body has developed resistance to any specific drugs.
A growing number of people are being infected with strains
of the disease that prove resistant to at least one form of anti-HIV drug
therapy. It is speculated that HIV drug resistance is on the rise, largely
due to many new infected people being infected by HIV-positive patients who
have been infected for a long time, have taken anti-HIV drugs, and developed
a resistance to the virus that they transmit to their partners along with
the virus itself.
What should
I do to prepare?
First of all, drug resistance can be avoided or slowed
through a number of measures. You should start treatment with a broad
spectrum of powerful anti-HIV drugs to suppress the virus as early as
possible. Educate yourself about the different anti-HIV drugs available, and
absolutely make sure to take your anti-HIV medication exactly as
prescribed: do not miss doses, miscalculate amounts, or fail to follow
instructions about, for instance, eating with medication or taking pills on
an empty stomach.
Communicate with your doctor regularly, making sure that
the effects of your anti-HIV drugs are monitored frequently throughout your
treatment. Drug-resistance tests can even be used before the patient starts
anti-HIV therapy; however, the tests will not always monitor accurately
before regular treatment has begun.
What
happens?
Pharmaceutical drugs are designed to target many types of
invasive pathogens, and either kill them or prevent them from reproducing
inside the body. If these pathogens continue to reproduce during treatment,
they can mutate (change their form) to avoid the intended effect of the
drugs. This is called drug resistance. When drug resistance occurs in
the HIV virus, anti-HIV therapy drugs lose their ability to halt the
reproduction of the pathogen. Over time, the treatment could stop having any
effect whatsoever.
In order for anti-HIV drugs to be effective, they must
first attach themselves to the necessary enzyme. Certain mutations can
prevent a drug from binding with the enzyme and, as a result, make the drug
less effective against the virus.
HIV drug-resistance mutations can occur both before and
during anti-HIV drug therapy. The same blood-drawing procedure as for the
above tests is used to obtain a sample.
How does it
work?
There are two types of tests that measure drug resistance,
called phenotypic and genotypic tests. Phenotypic testing
measures resistance to drugs directly and can pinpoint particular resistance
to particular anti-HIV drugs. Genotypic testing can help determine the cause
of drug resistance through discovering which mutations are causing the
failure of the drugs to take effect.
Though a viral load test can provide an initial window
into whether or not drug resistance is happening, separate drug-resistance
testing is important because it helps doctors and patients understand
exactly why failure has occurred and what adjustments to drug therapy need
to be made.
Risks &
Errors
There are a number of potential risks
that may cause resistance to anti-HIV drugs. First of all, the patient must
be extremely strict about adhering to the drug regimen, exactly as
prescribed (in the correct amounts and on schedule): in fact, an anti-HIV
drug regimen requires at least 95% perfect compliance in order for the drug
to work properly.
The specific dietary requirements for
the drug must also be strictly followed so the whole drug amount can be
properly absorbed into the bloodstream. This is also complicated by diarrhea
and vomiting (common symptoms among people with HIV), which may cause
anti-HIV drugs to be expelled from the body before they have had time to be
fully absorbed. Factors such as body weight, height, and age can also affect
the metabolic speed at which a drug is absorbed by the body.
Sources:
www.labtestsonline.org
www.aidsmap.com
www.aidsmeds.com
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