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Subject: Human Immunodeficiency Virus 1/2 Antibody Screen
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HIV is a highly variable virus that mutates readily, and numerous virus strains may be classified into types, groups, and subtypes. There are two types of HIV: HIV-1 and HIV-2. Both types are transmitted by sexual contact, through blood, and from mother to child, and they appear to cause clinically indistinguishable AIDS. However, it seems that HIV-2 is less easily transmitted, and the period between initial infection and illness is longer in the case of HIV-2.
The diagnosis of HIV infection is established by one of the following methods: detecting antibodies to the virus; detecting the viral p24 antigen; detecting viral nucleic acid (NAT); or culturing HIV. The most widely used test is the detection of IgG antibody against HIV-1 and HIV-2 antigens in serum. HIV-1 antigens include p24 (a nucleocapsid protein) and gp 120 and gp 41 (envelope proteins). Antibodies to gp41 and p24 antigens are the first detectable serologic markers following HIV infection. IgG antibodies appear 6–12 weeks following HIV infection in the majority of patients and by 6 months in 95% of patients. IgG antibodies to HIV generally persist for life. Positive tests should be confirmed with repeat antibody testing, testing to differentiate HIV-1 and HIV-2, and confirmatory testing (e.g., type-specific HIV RNA, western blot assays). Assays for IgM antibodies are not used because they are relatively insensitive.
Fourth-generation HIV tests can detect both antibody and p24 antigen. The main advantage of these assays is the ability to detect HIV infection during the “window period,” where antibody may not be detected.
HIV has evolved into several groups: M, N, O, and P. Group M (“main”) is considered the pandemic strain and comprise most strains of HIV. Group O (“outlier”) represents far fewer strains from Cameroon, Gabon, and Equatorial Guinea. Group N (“non-M/non-O”) and group P are represented by very few isolates and have only been documented in Cameroon. Viruses from group M are subsequently divided into 10 distinct subtypes (A–J). HIV testing was originally developed to detect HIV subtype B, the most common subtype in the United States and Europe. The estimated frequency of non-B subtypes in the United States is approximately 2%. The CDC does not recommend routine testing for HIV-2 in settings other than blood centers.
Normal range: Negative.
Screening of HIV-1 and/or HIV-2 infection, organ transplant donors, testing individuals who have documented and significant exposure to other infected individuals, and testing exposed high-risk individuals for detection of antibody (e.g., persons with multiple sex partners, persons with a history of other STDs, IV drug users, infants born to infected mothers, allied health care workers, and public service employees who have contact with blood and blood products) (Figure 17-1)
(From Morbidity & Mortality Weekly Report. June 21, 2013, 62:490–494.)
Positive in HIV infection; a positive screen test is considered “preliminary” and requires confirmation by definitive, specific testing, like HIV-1 RNA or Western blot assay. The patient with a positive test should be told of the result and advised on avoiding risk of transmitting HIV.
A negative screen test is regarded as a true negative, and requires no confirmation; the patient may be informed that the test is negative.
Common causes of false-negative results can occur due to acute infection and failure to detect certain HIV subtypes.
Rare causes of false-negative results include immune dysfunction due to defective humoral response or agammaglobulinemia, immunosuppression due to malignancy or medications, delay in seroconversion following early initiation of antiretroviral therapy, and fulminant HIV infection.
False-positive test results for HIV infection have been documented after participation in an HIV vaccine trial.
Indeterminate results may occur due to partial seroconversion during acute HIV infection, advanced HIV infection with decreased titers of p24 antibodies, or infection with HIV-2.
Other causes for an indeterminate test result in persons who are not infected with HIV include
Cross-reacting alloantibodies from pregnancy
Autoantibodies (collagen–vascular diseases, autoimmune diseases, and malignancy)
Receipt of an experimental HIV-1 vaccine