The most common tests examine a blood sample for evidence that a person's body is fighting an HIV infection. These tests detect HIV antibodies, which are substances the body creates in response to being infected with HIV.
However, during the first several weeks of infection, these tests may not reveal the infection. This is because it takes some time for the immune system to produce enough antibodies for the antibody test to detect. This is a time when it is very easy for a person with the virus to pass it on to someone else. Ninety-seven percent of people will develop detectable antibodies in the first 3 months after infection, but for a small percentage of people it can take longer). In these cases, different tests can directly look for pieces of the virus's genetic material in the blood.
The National Institute of Allergy and Infectious Diseases has more information about HIV testing.
There are special challenges in diagnosing HIV in infants and youth.
Because the HIV antibody from HIV-infected mothers passes to their infants, finding the HIV antibody in an infant does not indicate that the infant has become HIV-infected. Maternal HIV antibody in an uninfected infant can persist as long as 12 to 18 months before it disappears. Therefore, an HIV antibody test cannot be used to diagnose HIV infection in infants younger than age 18 months.
Scientists have developed highly accurate blood tests for diagnosing HIV infection in infants. One laboratory test, called polymerase chain reaction, can detect extremely small quantities of HIV's genetic material in an infant's blood and allow a diagnosis to be made in the first few months of life.1
The challenge in youth is different: Many young people think they're not at risk for HIV. This makes them less likely to seek testing. One in 5 of all Americans with HIV don't know they're infected, but almost 3 in 5 HIV-positive youth ages 13 to 24 don't know their HIV status, according to the Centers for Disease Control and Prevention (CDC). As a result, the CDC recommends routine HIV screening in health care settings starting at age 13. The U.S. Preventive Services Task Force recommends routine screening for HIV infection beginning at age 15 years or earlier for adolescents at increased risk.
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