When the scientific community discovered that human immunodeficiency virus (HIV) attacks and destroys T-helper cells (CD4), physicians began monitoring this cell population in affected patients. Flow cytometric analysis helps determine the absolute T-helper cell counts in a patient blood sample.
CD4 counts can be obtained using flow cytometry results in conjunction with absolute lymphocyte counts from a hematology cell counter or can be quantitated on a flow cytometer with the help of fluorescent beads added at a specific volume for comparison. Although laboratories have various choices of instrumentation and methods for quantifying CD4 counts using flow cytometry, the Center for Disease Control (CDC) provides guidelines for standardization and laboratories must ensure accuracy and precision of their methods for CD4 analysis.
A patient that is infected with HIV is considered HIV-positive until the absolute T-helper cell count decreases to a threshold of 200 cells/mm3. After the T-helper cell count falls below this threshold, the patient's physician may change the diagnosis to acquired immunodeficiency syndrome (AIDS). This important data can also guide physicians in the timing of treatments for HIV-positive individuals, especially for antiviral therapies and antibiotics to prevent infections.