AIDS laboratory tests



Associate Editors-in-Chief: Ujjwal Rastogi, MBBS [mailto:urastogi@perfuse.org]

Overview
HIV tests are usually performed on venous blood. Many laboratories use fourth generation screening tests which detect anti-HIV  antibody (IgG and IgM) and the HIV p24 antigen. The detection of HIV antibody or antigen in a patient previously known to be negative is evidence of HIV infection. Individuals whose first specimen indicates evidence of HIV infection will have a repeat test on a second blood sample to confirm the results. The window period (the time between initial infection and the development of detectable antibodies against the infection) can vary since it can take 3–6 months to seroconvert and to test positive. Detection of the virus using polymerase chain reaction (PCR) during the window period is possible, and evidence suggests that an infection may often be detected earlier than when using a fourth generation EIA screening test. Positive results obtained by PCR are confirmed by antibody tests. Routinely used HIV tests for infection in neonates, born to HIV-positive mothers, have no value because of the presence of maternal antibody to HIV in the child's blood. HIV infection can only be diagnosed by PCR, testing for HIV pro-viral DNA in the children's lymphocytes.

Many people are unaware that they are infected with HIV. Less than 1% of the sexually active urban population in Africa has been tested, and this proportion is even lower in rural populations. Furthermore, only 0.5% of pregnant women attending urban health facilities are counseled, tested or receive their test results. Again, this proportion is even lower in rural health facilities. Therefore, donor blood and blood products used in medicine and medical research are screened for HIV.

Historical Perspective of HIV testing

 * 1981: First AIDS case reported
 * 1984: Human immunodeficiency Virus (HIV) identified
 * 1985: First test for HIV licensed (ELISA)
 * 1987: First Western Blot blood test kit
 * 1992: First rapid test
 * 1994: First oral fluid test
 * 1996: First home and urine tests
 * 2002: First rapid test using finger prick
 * 2003: Rapid finger prick test granted CLIA waiver
 * 2004: First rapid oral fluid test (also granted CLIA waiver)
 * 2006: CDC recommends routine HIV screening in U.S. health-care settings
 * 2007: CDC launches Expanded HIV Testing Initiative in U.S.
 * 2007: WHO/UNAIDS global guidelines recommend routine HIV screening in health-care settings
 * 2010: First HIV diagnostc test approved that detects both antigen and antibodies

Types
Laboratory tests for detecting HIV infection are of three types:
 * 1) Screening tests
 * 2) Supplemental tests
 * 3) Confirmatory tests

After confirmation of the diagnosis, severity of disease and rate of progression are estimated by measurement of:
 * 1) CD4 count.
 * 2) HIV viral load.

AIDS screening tests
HIV testing is mandatory in the U.S. in certain cases:


 * Blood and organ donors
 * Military applicants and active duty personnel
 * Federal and state prison inmates under certain circumstances
 * Newborns in some states
 * As of January 2010, HIV testing is no longer mandatory for those wishing to emigrate to the United States or for refugees

Immunodeficient state and side-effects of HIV medication can itself cause various complications which should be monitored during the course of treatment. Monitoring for the development of these complications includes several laboratory and serological tests.

Most HIV tests used to screen for HIV infection detect the presence of antibodies against Human Immunodeficiency Virus. Detectable antibodies usually develop within 2–8 weeks after infection, but may take longer; the period after initial infection with HIV before detectable antibodies develop is the “window period”. These are of three types.
 * 1) ELISA test (based on antigen-antibody and enzyme substrate reactions).
 * 2) Rapid Tests (Dot blot and Latex Agglutination Tests).
 * 3) Simple Tests (Particle agglutination tests).

Both Simple and Rapid Tests are ready available and cheaper as compared to ELISA.

Home test
Home Access HIV-1 Test System, the only home HIV test currently approved by the FDA, may be purchased from many drug stores and online. Procedure: Individual performs the test by pricking finger with a lancet, placing drops of blood on treated card, and mailing to lab for testing. Identification number on card is used when phoning for results; counseling and referral available by phone. Results: in as little as three days.

Urine test
Procedure: Urine sample collected by health care provider and is tested at lab. Calypte is the only FDA-approved HIV urine test. Results: a few days to two weeks.

AIDS supplemental tests
These are used to validate results obtained by the screening tests and are of two type:
 * 1) Western blot test
 * 2) Immunofluorescent tests.

AIDS confirmatory tests
These test aim at the following:
 * 1) Demonstration of Viral Antigen (P24).
 * 2) Isolation of HIV.
 * 3) Detection of viral nucleic acid.

The confirmatory tests can diagnose HIV infection even during the window period (initial two to three weeks of infection), in which both the screening and the supplemental tests fail to diagnose the infection. However these are done in the reference centers thus time consuming and costly.

HIV testing in Pregnancy

 * All pregnant women in the United States should be tested for HIV infection as early during pregnancy as possible. A second test during the third trimester, preferably at <36 weeks’ gestation, should be considered for all pregnant women.


 * A second test is recommended for women in the following conditions:
 * Known to be at high risk for acquiring HIV. Women at high risk are:
 * Women who use illicit drugs.
 * Women who have STDs during pregnancy.
 * Women who have have multiple sex partners during pregnancy.
 * Women who live in areas with high HIV prevalence.
 * Women who have HIV-infected partners
 * Women receiving health care in jurisdictions with elevated incidence of HIV or AIDS among women.
 * Women living in facilities in which prenatal screening identifies at least one HIV-infected pregnant women per 1,000 women screened.
 * Rapid HIV screening should be performed on any woman in labor who has an undocumented HIV status unless she declines.If a rapid HIV test result is positive in these women, antiretroviral prophylaxis should be administered without waiting for the results of the confirmatory test.

Importance of testing

 * Testing pregnant women is particularly important not only to maintain the health of the patient, but because interventions (i.e., antiretroviral and obstetrical) can reduce the risk for perinatal transmission of HIV.


 * Evidence indicates that, in the absence of antiretroviral and other interventions, 15%–25% of infants born to HIV-infected mothers will become infected with HIV; such evidence also indicates that an additional 12%–14% of infants born to infected mothers who breastfeed into the second year of life will become infected.

Screening protocol

 * The patient should first be informed that she will be tested for HIV as part of the panel of prenatal tests, unless she declines, or opts-out, of screening.


 * For women who decline, providers should continue to strongly encourage testing and address concerns that pose obstacles to testing.


 * Women who decline testing because they have had a previous negative HIV test should be informed about the importance of retesting during each pregnancy.

Laboratory test

 * An RNA test should be used in conjunction with an HIV antibody test for women who have signs or symptoms consistent with acute HIV infection.

HIV infection among Infants and Children
School-age children exposed to HIV before birth are at increased risk for language problems and could benefit from early diagnosis and classroom intervention, according to a new study.


 * Diagnosis of HIV infection in a pregnant woman indicates the need to consider whether the woman’s other children might be infected.


 * Infants and young children with HIV infection differ from adults and adolescents with respect to the diagnosis, clinical presentation, and management of HIV disease.


 * Antibody tests for HIV are expected to be positive in the sera of both infected and uninfected infants born to seropositive mothers.

Laboratory test
A definitive determination of HIV infection for an infant aged <18 months is usually based on HIV nucleic acid testing.