HIV (human immunodeficiency virus) is a virus that can cause AIDS (acquired immunodeficiency syndrome). Because AIDS causes a weakened immune system, blood tests that assess the disease's progression and the health of the immune system typically focus on the white blood cells, which are killed by HIV.
HIV and White Blood Cells
White blood cells are of particular importance in evaluating patients with HIV/AIDS because they are the cells that the virus predominantly infects, AIDSInfoNet explains. The type of white blood cell that HIV infects is called a CD4-positive T cell, which is involved in the body's ability to respond to infections. Consequently, after a patient has tested positive for HIV, the CD4 cells are analyzed to determine whether the patient has AIDS.
CD4 Testing
There are two different ways of evaluating a patient's CD4 cells to monitor the progression of HIV, LabTestsOnline states: the absolute CD4 count and the CD4%. Both methods involve taking a sample of the patient's blood and counting the immune cells present in the blood. An absolute CD4 counts measure the number of CD4-positive T cells per milliliter of blood. Because a patient's absolute CD4 numbers can vary without a change in the patient's health, sometimes doctors look at the percentage of white blood cells that are CD4 positive, which is also called the CD4%.
CD4 Cells and HIV Progression
After a patient has been infected with HIV, there is often a sudden decrease in CD4-positive T cells during what is called the acute phase of infection, Johns Hopkins University notes. After this acute HIV syndrome, which typically lasts a few weeks, the CD4 count increases again and can return to normal levels. As the viral disease progresses, however, CD4 counts gradually decrease, leading to a weakening of the immune system.
Interpretation
Normally, patients have an absolute CD4 count of between 500 and 1,600 cells per milliliter of blood, The Body website states. A CD4 count of less than 200 indicates that the patient has AIDS. The normal range for CD4% is between 30 percent and 60 percent; a CD4% of less than 14 percent is another diagnostic criterion for AIDS.
Effects on Treatment
A patient's blood tests can affect treatment decisions for patients with HIV/AIDS, The Body website states. Patients typically begin receiving medications that slow the progression of HIV when their CD4 count decreases below 350 or when their CD4% decreases below 15 percent. Patients with a CD4 count of below 200 are at risk of developing a type of pneumonia called pneumocystis pneumonia (PCP). Additionally, CD4 counts below 100 make patients vulnerable to toxoplasmosis and cryptococcal infections. Counts below 50 can cause a patient to develop a Mycoplasma infection. Consequently, patients falling into those ranges may receive prophylactic treatment for these dangerous infections.
