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Human Immunodeficiency Virus Antibody EIA

The first case of AIDS-associated infection occurred in 1981 when the CDC reported on an unusual cluster of Pneumocystis pneumonia cases. Without treatment, most persons develop acquired immunodeficiency syndrome (AIDS) within 10 years of HIV infection. Antiretroviral therapy delays this progression and increases the length of survival, but is most effective when initiated during the asymptomatic phase. It is estimated that on average, an HIV-positive person aged 25 years who receives high-quality care will survive an additional 39 years. Identifying persons early in the course of infection reduces morbidity and mortality, prevents new infections, and can reduce health-care expenditures.

CDC recommends routine HIV screening between the ages of 13 and 64 years in all health-care settings. Persons at high risk for HIV infection should be screened at least annually. The burden of HIV is greatest among gay, bisexual, and other men who have sex with men (MSM). The disproportionately high rates of diagnoses among African Americans and Latinos, suggest that adults from these subpopulations might also benefit from more frequent testing. Other groups identified as potentially high risk include teenagers and senior citizens. CDC estimates that people over 60 years of age account for 19% of all newly diagnosed AIDS cases.

HIV can be transmitted during pregnancy, labor & delivery, or breastfeeding. CDC has recommended that all pregnant women be counseled and encouraged to be tested for HIV infection. HIV testing should be included in routine prenatal testing. Substantial progress has been made in reducing perinatal HIV transmission rates from 25-30% in 1991 to <2% currently. This reduction is attributed to HIV screening of pregnant women, use of antiretroviral drugs, and elective cesarean deliveries.

Human immunodeficiency virus exists as two distinct viral species, designated HIV-1 and HIV-2. Each species is further subdivided into subgroups, including M, N, & O for HIV-1, and subgroups A-G for HIV-2. The vast majority (99.6%) of HIV infections worldwide are caused by HIV-1, group M. Less prevalent viral types are largely confined to West Africa.

Detection of antibody to the HIV-1 virus is the best method of screening for HIV infection. HIV-1 antibody appears at about 11 days and peaks between 70 and 189 days. Most individuals produce detectable levels of antibody within 3 months of infection. The interval between infection and detection of HIV antibody is called the window period. Improvements in HIV-1 antibody tests have steadily reduced the window period as seen in the following table.

ELISA Generation Window Period (days)
First 42
Second 36
Third 22
Fourth 16

HIV-1 antibody tests have been replaced by newer combination tests for HIV-1 and HIV-2 antibody, which did not distinguish between HIV-1 and HiV-2. Specimens that tested reactive to HIV ½ antibody were confirmed by Western blot. Any specimen that iwas reactive for HIV-1/2 antibody, but negative or indeterminate by HIV-1 Western blot was subsequently tested specifically for HIV-2.

The fourth generation HIV assay decreases the detection window to 16 days post-infection. In addition to HIV-1 group M & O and HIV-2 antibodies, this new generation test detects HIV p24 antigen. Results are reported as HIV antigen/antibody and either reactive or non-reactive. The test does not distinguish between HIV types or whether p24 antigen or antibody is detected.

The use of Western blot as a confirmatory test became problematic because it was less sensitive than 3rd generation EIA. Patients who were truly infected may have had positive EIA result and a negative or indeterminate Western blot result if testing was done early after onset of infection. Third generation HIV-antibody immunoassays detected seroconversion as early as 22 days after infection, while Western blot did not show reactivity until 4 weeks or more.

In 2013, CDC and Clinical Laboratory Standards Institute (CLSI) designed a new HIV testing algorithm that replaced Western blot with an HIV-1/HIV-2 differentiation immunoassay that is known as Multispot. Besides detecting seroconversion earlier than Western blot, Multispot also eliminates most of the indeterminate Western blot results that occurred due to nonspecific reactivity from alloantibodies.

Specimens that do not confirm positive by Multispot may require further analysis by HIV RNA PCR, which can detect circulating virus within 11 days after infection.

Positive HIV antibody results in children younger than 18 months does not establish a diagnosis of HIV infection because children of HIV infected mothers may have detectable levels of maternal antibody up to 18 months after birth. These children should be tested with an HIV DNA PCR at birth, 2 weeks, 4-6 weeks, 6-12 weeks and 4-6 months of age.

Reference value is nonreactive. Specimen requirement is one SST tube of blood.

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