HIV is the etiological agent of AIDS and is transmitted through bodily secretions, especially by blood or sexual contact. The virus preferentially binds to the T4 helper lymphocytes and replicates within the cells using viral reverse transcriptase, integrase and protease enzymes. Current assays detect antibodies to one or more of several viral proteins. Public health guidelines recommend CD4 counts and viral load testing upon initiation of care for HIV; 3 to 4 mo before commencement of ART; every 3 to 4 mo, but no later than 6 mo, thereafter; and if treatment failure is suspected or otherwise when clinically indicated. Additionally, viral load testing should be requested 2 to 4 wk, but no later than 8 wk, after initiation of ART to verify success of therapy. In clinically stable patients, CD4 testing may be recommended every 6 to 12 mo rather than every 3 to 6 mo. Guidelines also state that treatment of asymptomatic patients should begin when CD4 count is less than 350/mm³; treatment is recommended when the patient is symptomatic regardless of test results or when the patient is asymptomatic and CD4 count is between 350 and 500/mm³. Failure to respond to therapy is defined as a viral load greater than 200 copies/mL. Increased viral load may be indicative of viral mutations, drug resistance, or noncompliance to the therapeutic regimen. Testing for drug resistance is recommended if viral load is greater than 1,000 copies/mL. Initial screening is generally performed using a third-generation immunoassay for antibodies to HIV1/HIV2. The antibody screening tests most commonly used do not distinguish between HIV1 and HIV2. A reactive screen result is followed by repeat testing in duplicate. Positive or indeterminate results should be confirmed by Western blot assay where positive is defined by the Centers for Disease Control and Prevention (CDC) as presence of two of the three viral proteins: gp41, gp120 (from the viral membrane), and p24 (from the viral core). The newest HIV testing algorithm was developed jointly by the Association of Public Health Laboratories and the CDC. This new algorithm provides for earlier detection of acute infection as well as identification of established infection. Recommendations for initial screening call for the use of a fourth-generation immunoassay capable of the simultaneous detection of HIV antigen and antibody. The fourth-generation assays demonstrate the ability to detect infection 7 days earlier than third-generation assays. Positive initial screens should be followed by a rapid immunoassay that differentiates between HIV1 and HIV2 antibody. A negative or indeterminate supplemental antibody result should be followed by a nucleic acid amplification test (NAAT), during the period after infection has occurred but before the development of antibodies to the virus, to determine if HIV viral RNA is present. The HIV screening test is routinely recommended as part of a prenatal work-up and is required for evaluating donated blood units before release for transfusion. The CDC has structured its recommendations to increase identification of HIV-infected patients as early as possible; early identification increases treatment options, increases frequency of successful treatment, and can decrease further spread of disease. The CDC recommends the following:

• Include HIV testing in routine medical care; screening of all patients between the ages of 13 and 64 years of age as part of routine medical care, unless the patient requests to opt out.
• Implement new models to diagnose HIV infections outside medical settings; promote availability of rapid waived testing kits like OraQuick.
• Prevent new infections by working with persons diagnosed with HIV and their partners; adapt a voluntary opt-out approach that includes elimination of pretest counseling and written consent requirements.
• Further decrease prenatal transmission of HIV by incorporating HIV testing as a routine part of prenatal medical care and also perform third-trimester testing in areas with high rates of HIV infection among pregnant women.

HIV genotyping by polymerase chain reaction (PCR) methods may also be required to guide selection of medications for therapeutic regimens, assess potential for drug resistance, and monitor for transmission of drug resistant HIV. Genotyping is also useful to determine eligibility for new medications once resistance to conventional drugs has been identified.