The most common tests examine a blood sample for evidence that a person's body is fighting HIV. These tests detect HIV antibodies, which are substances the body creates in response to infection. Some tests look for both antibodies and an antigen, a substance the virus makes before antibodies develop.
However, no HIV test can detect HIV immediately after infection. This is because it takes some time for the immune system to produce enough antibodies for the antibody test to detect. The time between when a person gets HIV and when a test can accurately detect it is called the window period. The window period varies from person to person and also depends on the type of HIV test. Ask your healthcare provider about the window period for the test you had and whether you will need a follow-up test to confirm the results.1
The Centers for Disease Control and Prevention (CDC) has more information about HIV testing.
Diagnosis in Children and Youth
There are special challenges in diagnosing HIV in infants and youth.
Because HIV antibodies pass from mothers with HIV to their infants, finding HIV antibodies in an infant does not indicate that the infant has HIV. Maternal HIV antibody in an HIV-negative infant can persist as long as 12 to 18 months before it disappears. Therefore, an HIV antibody test cannot be used to diagnose HIV in infants younger than age 18 months.
Scientists have developed highly accurate blood tests for diagnosing HIV 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.
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. As a result, CDC recommends routine HIV screening in healthcare settings starting at age 13 years. The U.S. Preventive Services Task Force recommends routine screening for HIV beginning at age 15 years or earlier for adolescents at increased risk.