The human immunodeficiency virus (HIV) is an infectious viral organism that attacks immune cells and gradually destroys the ability of your immune system to fight against infection and disease. The last stage of HIV infection is known as acquired immunodeficiency syndrome, or AIDS. People with AIDS will eventually die from infection, cancer, or another ailment because their failing immune system is unable to combat the disease. The following are the major blood tests used to diagnose HIV infections and monitor the progression of the condition.

Enzyme-linked immunosorbent assay (ELISA) tests for the presence of antibodies to HIV in the blood. If such antibodies are present, they will adhere to a plate coated with fragments of the HIV virus. The ELISA reagent will detect the bound antibodies and change color, indicating a positive result. ELISA, which is faster and less expensive than other HIV blood tests, is commonly used as the initial screening test for HIV infection. Positive results are confirmed by a repeat ELISA and a Western blot test.

Western blot also tests for the presence of antibodies to HIV in the blood. In this test, a blood sample is exposed to a special paper that is impregnated with selected fragments of the HIV virus. Any anti-HIV antibodies that are present in the blood will bind to the viral fragments, producing a characteristic pattern. The Western blot very rarely produces false-positive results and is used to confirm the presence of HIV infection in people who test positive with ELISA.

CD4 count, or T-cell assay, is a test to measure the level of the immune system’s CD4 cells, which are part of the body’s defense against infection. As HIV targets and destroys CD4 cells in the initial stages of infection, CD4 levels fall sharply. Later, as the body starts to cope with the infection, CD4 cell counts rise again. Eventually, however, the rate of CD4 destruction exceeds that of CD4 cell creation, and the CD4 count declines. Measuring the CD4 cell count is thus an effective way of assessing the condition of the immune system and monitoring the effects of treatment for HIV infection.

Viral load is a test that provides a direct measurement of the quantity of HIV virus in your blood. The genetic material of the virus (RNA) is usually measured by the polymerase chain reaction (PCR), which can amplify and detect small amounts of specific RNA sequences in a blood sample. This test is valuable for the management of HIV infection because the viral load can aid in determining your prognosis and evaluating your response to treatment. People with high levels of HIV RNA in their blood, for example, are more likely to progress to AIDS than those with low viral levels.

Purpose of the HIV and AIDS-related Blood Tests

ELISA and Western blot:

  • To screen for HIV infection in individuals at high risk for the disease
  • To screen donated blood for HIV

CD4 count and viral load:

  • To determine the stage of HIV infection
  • To determine the level of HIV in the body (called viral load)
  • To aid in deciding when to start anti-HIV medications
  • To monitor the response to therapy

Who Performs HIV and AIDS-related Blood Tests

  • A doctor, a nurse, or a technician draws a blood sample.

Special Concerns about HIV and AIDS-related Blood Tests

  • The ELISA and Western blot assays cannot detect HIV infection until antibodies develop, which may take from 6 to 24 weeks after exposure. Thus, false-negative results can occur in the early incubation stage of HIV infection.
  • False-positive results occur in about 2 out of every 1,000 people tested with ELISA. The Western blot assay produces very few false-positives, although it may rarely give ambiguous results. In such cases, the test is repeated.
  • A recent viral illness and certain medications can alter the CD4 count.
  • CD4 counts vary from hour to hour throughout the day and must be evaluated in the context of your symptoms.
  • Certain illnesses and some vaccinations may temporarily increase your viral load.
  • Because the viral load test does not measure the extent of existing damage to your immune system, it must be used in conjunction with the CD4 cell count.

Before the HIV and AIDS-related Blood Tests

  • You will receive counseling about the test and the implications of the results.

What You Experience during HIV and AIDS-related Blood Tests

  • A sample of your blood is drawn from a vein, usually in your arm, and sent to a laboratory for analysis.

Risks and Complications of HIV and AIDS-related Blood Tests

  • None

After the HIV and AIDS-related Blood Tests

  • Immediately after blood is drawn, pressure is applied (with cotton or gauze) to the puncture site.
  • You are free to resume your usual activities.
  • Blood may collect and clot under the skin (hematoma) at the puncture site; this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.

Results of HIV and AIDS-related Blood Tests

  • Your blood sample is sent to a laboratory for analysis.
  • If you test positive for the HIV antibody with the ELISA or Western blot test, you will undergo a thorough health evaluation—including CD4 count, viral load, a skin test for tuberculosis, and various other blood tests.
  • Depending on the stage of HIV infection—as determined by the CD4 count and viral load—your doctor may prescribe medications to reduce the amount of virus in your body and delay the progression of the condition.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 11 Jan 2012

Last Modified: 11 Jan 2012