White blood cells, sometimes referred to as leukocytes in medical literature, are produced by the immune system to help defend the body against infection. They are formed in the bone marrow and either enter the blood or migrate to key organs, such as the spleen, lymph nodes, or gut. There are several different types of white blood cells (see "differential" below). A high white blood cell count likely indicates that an infection is present somewhere in the body, whereas a low number might indicate that an infection or disease – such as HIV or cancer – has slowed the ability of the bone marrow to produce new WBCs.

Red blood cells, sometimes referred to as erythrocytes, are responsible for delivering oxygen throughout the body. There are lots of them in a single milliliter of blood – between 3.6 to 6.1 million. A low red blood cell count can indicate anemia, which can lead to fatigue. Some anti-HIV drugs can cause anemia, as can some AIDS-related infections and cancers.

Both Hemoglobin & Hematocrit can help you and your doctor learn a bit more about your red blood cells. Hemoglobin is a protein used by red blood cells to distribute oxygen to other tissues and cells in the body. Hematocrit refers to the amount of your blood that is occupied by red blood cells. It's like chocolate milk: how much of it is chocolate and how much of it is milk? For men, the hematocrit should be between 40% and 52%; for women, it should be between 35% and 46%. A low hemoglobin number or hematocrit percentage are good indicators of anemia.

Both Hemoglobin & Hematocrit can help you and your doctor learn a bit more about your red blood cells. Hemoglobin is a protein used by red blood cells to distribute oxygen to other tissues and cells in the body. Hematocrit refers to the amount of your blood that is occupied by red blood cells. It's like chocolate milk: how much of it is chocolate and how much of it is milk? For men, the hematocrit should be between 40% and 52%; for women, it should be between 35% and 46%. A low hemoglobin number or hematocrit percentage are good indicators of anemia.

This test measures the amount of hemoglobin in red blood cells. Both hemoglobin and hematocrit are used to calculate this number. Low levels indicate anemia.

This test measures the amount of hemoglobin in red blood cells. Both hemoglobin and hematocrit are used to calculate this number. Low levels indicate anemia.

Red blood cells can come in different sizes. RDW looks at the range of these sizes in a blood sample. If anemia is suspected, based on other blood counts, RDW test results are often used together with MCV results to figure out what the cause of the anemia might be.

Platelets are tiny cells produced by the bone marrow to help your blood clot in the event of a cut or scrape. A high number might be seen in people with cancer, a blood disease, or rheumatoid arthritis. A decreased platelet count is called thrombocytopenia. There are a number of possible causes of thrombocytopenia, including a disorder of the immune system that causes antibodies produced by the spleen to kill platelets (idiopathic thrombocytopenia purpura, or ITP). This can be problematic and often warrants immediate medical care. HIV treatment can help reverse thrombocytopenia, but can sometimes cause platelet levels to become too high (thrombocytosis), increasing the risk of blot clots.