HIV tests are used to detect the presence of the human immunodeficiency virus (HIV), the virus that can lead to AIDS, in serum, saliva, or urine. Such tests may detect antibodies, antigens, or RNA.

AIDS diagnosis

AIDS stands for acquired immunodeficiency syndrome. It is a clinical condition characterized by severe immunosuppression that can occur when HIV is untreated. A person with HIV is diagnosed with AIDS when their CD4+ T cell count falls below 200 cells per μL or they develop an AIDS-defining clinical condition.

Terminology

The eclipse period is a variable period starting from HIV exposure in which no existing test can detect HIV. The median duration of the eclipse period in one study was 11.5 days. The window period is the time between HIV exposure and when an antibody or antigen test can detect HIV. The median window period for antibody/antigen testing is 18 days. Nucleic acid testing (NAT) further reduces this period to 11.5 days.

Performance of medical tests is often described in terms of: Sensitivity: The percentage of the results that will be positive when HIV is present Specificity: The percentage of the results that will be negative when HIV is not present.

All diagnostic tests have limitations, and sometimes their use may produce erroneous or questionable results. False positive: The test incorrectly indicates that HIV is present in a non-infected person. False negative: The test incorrectly indicates that HIV is absent in an infected person.

Nonspecific reactions, hypergammaglobulinemia, or the presence of antibodies directed to other infectious agents that may be antigenically similar to HIV can produce false positive results. Autoimmune diseases, such as systemic lupus erythematosus, have also rarely caused false positive results. Most false negative results are due to the window period.

Principles

Screening donor blood and cellular products

Tests selected to screen donor blood and tissue must provide a high degree of confidence that HIV will be detected if present (that is, a high sensitivity is required). A combination of antibody, antigen and nucleic acid tests are used by blood banks in Western countries. The World Health Organization estimated that, , inadequate blood screening had resulted in 1 million new HIV infections worldwide.

In the US, the Food and Drug Administration requires that all donated blood be screened for several infectious diseases, including HIV-1 and HIV-2, using a combination of antibody testing (EIA) and more expeditious nucleic acid testing (NAT). These diagnostic tests are combined with careful donor selection. , the risk of transfusion-acquired HIV in the US was approximately one in 2.5 million for each transfusion.Adverse reactions associated with blood transfusion . From the Puget Sound Blood Center. Accessed 5 October 2006.

Diagnosis of HIV infection Tests used for the diagnosis of HIV infection in a particular person require a high degree of both sensitivity and specificity. In the United States, this is achieved using an algorithm combining two tests for HIV antibodies. If antibodies are detected by an initial test based on the ELISA method, then a second test using the western blot procedure determines the size of the antigens in the test kit binding to the antibodies. The combination of these two methods is highly accurate

Human rights The UNAIDS/WHO policy statement on HIV Testing states that conditions under which people undergo HIV testing must be anchored in a human rights approach that pays due respect to ethical principles.UNAIDS/WHO policy statement on HIV Testing (PDF), accessed 5 October 2006. According to these principles, the conduct of HIV testing of individuals must be Confidential; Accompanied by counseling (for those who test positive); Conducted with the informed consent of the person being tested.

Confidentiality

Considerable controversy exists over the ethical obligations of health care providers to inform the sexual partners of individuals infected with HIV that they are at risk of contracting the virus. Some legal jurisdictions permit such disclosure, while others do not. More state funded testing sites are now using confidential forms of testing. This allows for monitoring of infected individuals easily, compared to anonymous testing that has a number attached to the positive test results. Controversy exists over privacy issues.

In developing countries, home-based HIV testing and counseling (HBHTC) is an emerging approach for addressing confidentiality issues. HBHTC allows individuals, couples, and families to learn their HIV status in the convenience and privacy of their home environment. Rapid HIV tests are most often used, so results are available for the client between 15 and 30 minutes. Furthermore, when an HIV-positive result is communicated, the HTC provider can offer appropriate linkages for prevention, care, and treatment.

Anonymous testing Anonymous testing may be available for people who wish to take an HIV test without giving their name. Instead of providing a name, the person undergoing the test receives a number and then presents that number to obtain the results of the test, which do not contain the person's name.

Routine testing recommendation In the United States, one emerging standard of care is to screen all patients for HIV in all health care settings. In 2006, the Centers for Disease Control (CDC) announced an initiative for voluntary, routine testing of all Americans aged 13–64 during health care encounters. An estimated 25% of infected individuals were unaware of their status; if successful, this effort was expected to reduce new infections by 30% per year. The CDC recommends elimination of requirements for written consent or extensive pre-test counseling as barriers to widespread routine testing. In 2006, the National Association of Community Health Centers implemented a model for offering free, rapid HIV testing to all patients between the ages of 13 and 64 during routine primary medical and dental care visits. The program increased testing rates, with 66% of the 17,237 patients involved in the study agreeing to testing (56% were tested for the first time). In September 2010, New York became the first state to require that hospitals and primary care providers offer an HIV test to all patients between the ages of 13 and 64 years. An evaluation of the law's impact found that it increased testing significantly throughout the state.

Antibody tests HIV antibody tests are specifically designed for routine diagnostic testing of adults; these tests are inexpensive and extremely accurate.

Window period Antibody tests may give false negative (no antibodies were detected despite the presence of HIV) results during the window period, hence an interval of three weeks to six months between the time of HIV exposure and the production of measurable antibodies to HIV seroconversion is implemented. Most people develop detectable antibodies approximately 18 to 30 days after exposure, although some do seroconvert later. The vast majority of people (99%) have detectable antibodies by two months after HIV exposure.

During the window period, an infected person can transmit HIV to others although their HIV infection may not be detectable with an antibody test. Antiretroviral therapy during the window period can delay the formation of antibodies and extend the window period beyond 12 months. This was not the case with patients that underwent treatment with post-exposure prophylaxis (PEP). Those patients must take ELISA tests at various intervals after the usual 28-day course of treatment, sometimes extending outside of the conservative window period of 6 months.

ELISA The enzyme-linked immunosorbent assay (ELISA), or enzyme immunoassay (EIA), was the first screening test commonly employed for HIV. It has a high sensitivity.

In an ELISA test, a person's serum is diluted 400-fold and applied to a plate to which HIV antigens have been attached. If antibodies to HIV are present in the serum, they may bind to these HIV antigens. The plate is then washed to remove all other components of the serum. A specially prepared "secondary antibody" – an antibody that binds to human antibodies – is then applied to the plate, followed by another wash. This secondary antibody is chemically linked in advance to an enzyme. Thus the plate will contain enzyme in proportion to the amount of secondary antibody bound to the plate. A substrate for the enzyme is applied, and catalysis by the enzyme leads to a change in color or fluorescence. ELISA results are reported as a number; the most controversial aspect of this test is determining the "cut-off" point between a positive and negative result.

ELISA dongle Researchers from Columbia University have produced an ELISA test dongle capable of testing for HIV and syphilis. It is compatible to any smartphone or computer without additional support or battery power, and takes some fifteen minutes to analyse a drop of blood. The units cost approximately $34 each to manufacture.

Western blot

Like the ELISA procedure, the western blot is an antibody detection test. However, unlike the ELISA method, the viral proteins are separated first and immobilized. In subsequent steps, the binding of serum antibodies to specific HIV proteins is visualized.

Specifically, cells that may be HIV-infected are opened and the proteins within are placed into a slab of gel, to which an electric current is applied. Different proteins will move with different speeds in this field, depending on their size, while their electrical charge is leveled by a surfactant called sodium lauryl sulfate. Some commercially prepared Western blot test kits contain the HIV proteins already on a cellulose acetate strip. Once the proteins are well-separated, they are transferred to a membrane and the procedure continues similar to an ELISA: the person's diluted serum is applied to the membrane and antibodies in the serum may attach to some of the HIV proteins. Antibodies that do not attach are washed away, and enzyme-linked antibodies with the capability to attach to the person's antibodies determine to which HIV proteins the person has antibodies.

There are no universal criteria for interpreting the western blot test: The number of viral bands that must be present may vary. If no viral bands are detected, the result is negative. If at least one viral band for each of the GAG, POL, and ENV gene-product groups are present, the result is positive. The three-gene-product approach to western blot interpretation has not been adopted for public health or clinical practice. Tests in which less than the required number of viral bands are detected are reported as indeterminate: a person who has an indeterminate result should be retested, as later tests may be more conclusive. Almost all HIV-infected persons with indeterminate western blot results will develop a positive result when tested in one month; persistently indeterminate results over a period of six months suggests the results are not due to HIV infection. In a generally healthy low-risk population, indeterminate results on western blot occur on the order of 1 in 5,000 patients.Bartlett, JG. Serologic tests for the diagnosis of HIV infection, in UpToDate. Accessed 5 October 2006. However, for those individuals who have had high-risk exposures to individuals where HIV-2 is most prevalent, Western Africa, an inconclusive western blot test may prove infection with HIV-2.Scand J Infect Dis. 1992;24(4):419-21. Accessed 23 September 2008.

The HIV proteins used in western blotting can be produced by recombinant DNA in a technique called recombinant immunoblot assay (RIBA).

Rapid or point-of-care tests