Human Immunodeficiency Virus

The diagnosis of HIV infection usually depends on the detection of antibodies to the virus. The recommended screening test for HIV infection is an initial enzyme immunoassay, the ELISA test, which can detect the presence of antibody to HIV 2 to 8 weeks after infection. HIV ELISA testing studies in blood donors have found specificities of 99.7% and 99.99% (Chou et al., 2005). An initially positive ELISA should be repeated, and repeatedly positive ELISAs need to be confirmed by a more specific test, most often the Western blot. A positive ELISA combined with a negative Western blot should be considered a false-positive HIV test and indicates that HIV infection is not present. A positive ELISA and an indeterminate Western blot result can be a marker for early HIV infection or advanced acquired immunodeficiency syndrome (AIDS), or it can be a false-positive test result. The predictive value of a positive HIV test depends on the prevalence in the population being tested.

Rapid HIV tests are currently available that check saliva, whole blood, and plasma. The whole-blood tests measure capillary blood with a finger stick and do not require centrifugation. These tests are interpreted visually, do not require instrumentation, and provide test results in minutes. Rapid tests may be preferred for testing in patients who are not likely to return for results of standard testing, for pregnant women at delivery with no HIV testing during their pregnancy, and for testing during occupational exposure. Rapid tests for HIV have similar sensitivity and specificity to the standard ELISA tests. Clusters of false-positive oral rapid tests have been reported, emphasizing the need for confirmatory testing of positive rapid tests.

Tests to measure HIV directly include quantitative HIV RNA testing by PCR, which measures viral load or actual viral replication. Quantitative HIV RNA measurements are useful in evaluating indeterminate Western blot results and acute HIV infection, when the patient presents before seroconversion. Because neonates born to HIV-infected mothers often have maternal antibodies for months, early testing with HIV-1 DNA PCR can identify infants with HIV infection. In 2006 the U.S. Centers for Disease Control and Prevention (CDC) recommended routine, voluntary HIV screening for all patients age 13 to 64 in any health care setting.

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