The most commonly used hematology tests to monitor HIV infection are the complete blood count (CBC) and lymphocyte subsets. As the name CBC implies, this test measures the number of red blood cells, white blood cells and platelets. The most commonly followed lymphocyte subsets include CD4 and CD8 cell counts and percentages.
Many of the blood cell counts discussed below -- including the red blood cells, white blood cells, platelets, differentials, the CD4 cell count, and the CD8 cell count -- are listed on lab reports as the number of cells per cubic millimeter (mm3 or cu.mm) of blood. Sometimes, lab reports list the number of cells per microliter (µL) of blood. A microliter is a thousandth of a milliliter and is the same as a cubic millimeter.
The Complete Blood Count (CBC)
Red Blood Cell (RBC) Count: Red blood cells, also called erythrocytes, are responsible for delivering oxygen throughout the body. There are between 3.6 to 6.1 million red blood cells in a single cubic millimeter of blood. Anemia, a condition generally defined as a decreased number of red blood cells, can be caused by certain anti-HIV drugs or be a sign of an underlying illness. Women of child-bearing age may also experience anemia as a result of blood loss from their menstrual periods. One of the most common physical symptoms of anemia is fatigue.
Hematocrit and Hemoglobin: Hematocrit measures the percentage of blood volume that is occupied by RBCs. Generally speaking, red blood cells should make up 40 to 52 percent of the total blood volume in men and 35 to 46 percent in women. Hemoglobin is a protein normally found within the RBCs that carries oxygen throughout the body. Normal hemoglobin levels range from 12 to 16 grams per deciliter of blood (g/dL). Healthcare providers usually keep track of the hematocrit and hemoglobin rather than the RBC count itself.
Red Blood Cell Indices: This category includes the mean corpuscular hemoglobin (MCH), the mean corpuscular hemoglobin concentration (MCHC), the mean corpuscular volume (MCV), and the red cell distribution width (RDW). These indices are used to help define anemias.
Platelets: Platelets, also called thrombocytes, are elements in the blood that are necessary for blood to clot. A normal platelet count is between 150,000 to 440,000 per cubic millimeter of blood. Some people with HIV have low platelet counts -- called thrombocytopenia -- which can be caused by some drugs, as well as by HIV itself.
White Blood Cell (WBC) Count: White blood cells, also called leukocytes, defend the body against infection. They form in the bone marrow and consist of several different types and sub-types. On average, a healthy adult has between 4,000 and 11,000 white cells per cubic millimeter or microliter of blood. A high WBC count often means that an infection is present in the body, while a low number can mean that a specific disease or drug has impaired the bone marrow's ability to produce new cells. Most people with HIV have WBC counts at the low normal end of the range.
Differential White Blood Cell Count: The differential is a count of the number or percentage of WBCs made up by each major type of WBC. Neutrophils (also called polymorphonuclear leukocytes, PMNs, or poly's for short) are WBCs that fight most bacterial infections. The neutrophil count may be lowered by certain medications used by people with HIV, such as Retrovir (AZT) and Cytovene (ganciclovir). If the neutrophil count becomes too low, there is an increased risk of bacterial infections. Lymphocytes are the key WBCs involved in immune responses (see CD4 count below) and are often lowered by HIV infection. Monocytes play important roles in fighting certain types of infections by maturing into macrophages that can ingest bacteria and cellular debris. Eosinophils are involved in fighting certain parasitic infections and are sometimes elevated due to allergic reactions. The function of basophils is not well understood.
CD4 Count (Helper T-cell Count): Counting the number of helper T-cells -- technically called CD4+ lymphocytes -- is perhaps the most important tool used to assess the overall health of the immune system in people with HIV. Helper T-cells, as the name implies, are responsible for signaling other immune system cells to fight an infection in the body. The normal CD4 count is somewhere between 500 and 1500 cells per cubic millimeter. Without anti-HIV treatment, the average HIV-infected person undergoes a decrease in helper T-cell count of about 50 to 100 cells per cubic millimeter each year. Opportunistic infections such as Pneumocystis pneumonia (PCP) typically occur once the helper T-cell count falls below 200. Other infections typically occur when the count is less than 50 to 100. For this reason, medications to prophylax (prevent) certain infections are started once the helper T-cell count falls to certain levels, such as below 200 for PCP prophylaxis. The helper T-cell count also plays a major role in deciding when to start anti-HIV treatment. Currently, the U.S. Department of Health and Human Services (DHHS) recommends anti-HIV drug treatment for all HIV-positive people with helper T-cell counts below 200 and encourages treatment for all HIV-positive people with helper T-cell counts below 350.
CD4 Percentage: In a healthy adult, helper T-cells account for between 32% and 68% of the total number of lymphocytes (which includes B-cells and other types of T-cells). The CD4 percentage is sometimes a more reliable measurement than the CD4 count because it tends to vary less between measurements. For example, one person's CD4 count may vary between 160 and 240 over a period of several months while their CD4 percentage remains constant at, say, 15%. The reason for this is that the CD4 count isn't actually a direct count of CD4 cells, but rather a calculation based on the results of three other tests (the CD4 percentage, the lymphocyte percentage, and the WBC count), each of which can vary slightly each time it's measured. Occasionally the CD4 count may be relatively high while the CD4 percentage is low (less than 21%). In this situation, many healthcare providers would consider the immune system to be significantly impaired based on the CD4 percentage.
CD8 Count, CD8 Percentage, T-cell Ratio: CD8 cells, also called suppressor T-cells, play a role in fighting viral infections such as HIV. A healthy adult usually has between 150 and 1,000 CD8 cells per cubic millimeter. In contrast to CD4 cells, people with HIV often have elevated numbers of CD8 cells, the significance of which is not well understood. Lab reports may also list the T-cell ratio, which is the number of CD4 cells divided by the number of CD8 cells. Since the CD4 count is usually lower and the CD8 count higher than normal, the ratio is usually low in people with HIV. A normal T-cell ratio is usually between 0.9 and 6.0. The expected response to effective combination anti-HIV treatment is an increase in CD4 count, a decrease in CD8 count, and an increase in the T-cell ratio.