Diagnosis

HIV/AIDS is diagnosed via laboratory testing and then staged based on the presence of certain signs or symptoms. HIV testing is recommended for all those at high risk, which includes anyone diagnosed with a sexually transmitted illness. In many areas of the world a third of HIV carriers only discover they are infected at an advanced stage of the disease when AIDS or severe immunodeficiency has become apparent.

HIV testing

Most people infected with HIV develop specific antibodies (i.e. seroconvert) within three to twelve weeks of the initial infection. Diagnosis of primary HIV before seroconversion is done by measuring HIV-RNA or p24 antigen. Positive results obtained by antibody or PCR testing are confirmed either by a different antibody or by PCR.

Antibody tests in children younger than 18 months are typically inaccurate due to the continued presence of maternal antibodies. Thus HIV infection can only be diagnosed by PCR testing for HIV RNA or DNA, or via testing for the p24 antigen. Much of the world lacks access to reliable PCR testing and many places simply wait until either symptoms develop or the child is old enough for accurate antibody testing. In sub-Saharan Africa as of 2007–2009 between 30–70% of the population was aware of their HIV status. In 2009 between four and 42% of the population was tested.These figures represent substantial increases from ten years previous.

Classifications of HIV infection

Two main clinical staging systems are used to classify HIV and HIV-related disease for surveillance purposes: the WHO disease staging system for HIV infection and disease, and the CDC classification system for HIV infection. The CDC's classification system is more frequently adopted in developed countries. Since the WHO's staging system does not require laboratory tests, it is suited to the resource-restricted conditions encountered in developing countries, where it can also be used to help guide clinical management. Despite their differences, the two systems allow comparison for statistical purposes.

The World Health Organization first proposed a definition for AIDS in 1986. Since then, the WHO classification has been updated and expanded several times, with the most recent version being published in 2007. The WHO system uses the following categories:

• Primary HIV infection: May be either asymptomatic or associated with acute retroviral syndrome.
• Stage I: HIV infection is asymptomatic with a CD4⁺ T cell count (also known as CD4 count) greater than 500/uL. May include generalized lymph node enlargement.
• Stage II: Mild symptoms which may include minor mucocutaneous manifestations and recurrent upper respiratory tract infections. A CD4 count of less than 500/uL.
• Stage III: Advanced symptoms which may include unexplained chronic diarrhea for longer than a month, severe bacterial infections including tuberculosis of the lung as well as a CD4 count of less than 350/uL.
• Stage IV or AIDS: severe symptoms which includes toxoplasmosis of the brain, candidiasis of the esophagus, trachea, bronchi or lungs and Kaposi's sarcoma. A CD4 count of less than 200/uL.

The United States Center for Disease Control and Prevention also created a classification system for HIV, and updated it in 2008. In this system HIV infections are classified based on CD4 count and clinical symptoms, and describes the infection in three stages:

• Stage 1: CD4 count ≥ 500 cells/uL and no AIDS defining conditions
• Stage 2: CD4 count 200 to 500 cells/uL and no AIDS defining conditions
• Stage 3: CD4 count ≤ 200 cells/uL or AIDS defining conditions
• Unknown: if insufficient information is available to make any of the above classifications

For surveillance purposes, the AIDS diagnosis still stands even if, after treatment, the CD4⁺ T cell count rises to above 200 per µL of blood or other AIDS-defining illnesses are cured.

A generalized graph of the relationship between HIV copies
 (viral load) and CD4⁺ T cell counts over the average course
of untreated HIV infection.

           CD4⁺ T Lymphocyte count (cells/mm³)

           HIV RNA copies per mL of plasma

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