Laboratory tests for HIV are performed on a sample of blood. Results usually take about a week, although this can vary depending on the laboratory. If the result is reactive (provisionally positive), the test should be repeated on a separate sample before a positive result is confirmed.
Standard ‘4th generation’ laboratory tests detect antibodies to HIV-1 and HIV-2 as well as to the HIV P24 antigen, a viral protein. They have largely replaced ‘3rd generation’ tests which detect only antibodies and thus have a longer window period (see below.)
Photo credit: © Miroslav Vajdić
Point of care test (POCT) packet
Rapid point of care tests (POCTs) can be performed in the clinic or doctor’s office and give a result in 1-20 minutes. They use a sample of fingerprick blood or oral fluid, are easy to administer and very convenient for doctor and patient. However, they may have a lower specificity than laboratory tests, which means that, where HIV prevalence is low, false positive results are more likely. It is essential to confirm any ‘reactive’ results with a laboratory blood test.
POCTs detect HIV antibodies, so their window period (see below) is longer than that of 4th generation tests. There are currently no rapid test devices which accurately detect HIV antigen. For this reason, the use of a POCT is not recommended if seroconversion is suspected, for example if a patient presents with a maculopapular rash and other infectious mononucleosis-like symptoms.
Watch how simple it is to perform a point of care test in the three short videos here (and scroll down the page to see how to read test results):
> Point of Care Test 1: Taking Blood
HIV RNA quantitative assays, a form of nucleic acid amplification test (NAAT), detect the virus itself in blood. They are not usually appropriate as a diagnostic test, but HIV specialists use them if very early HIV infection is suspected and a 4th generation test is negative. In such a situation, referral to an HIV specialist may be considered.
Photo credit: Jim Gathany
Source: HIV i-Base (i-base.info)
The ‘window period’ is the period of time after a person becomes infected with HIV and before a test can detect the infection. During the window period, an HIV test will be negative. Different tests have different window periods, so it is important to know which test is being used by the laboratory.
An HIV antibody-only test (3rd generation laboratory test or POCT) may be done from about six weeks after exposure to HIV, but a negative result cannot be taken as definitive until 12 weeks (the window period). This is because HIV antibodies usually appear about four to six weeks after infection but they can take longer.
The HIV P24 antigen appears much earlier, sometimes as early as ten days after infection and usually within four weeks. Blood tests which detect the antigen can be done at four weeks after exposure and are useful for detecting early infection. However, as HIV antibodies develop, the antigen becomes undetectable, so a 4th generation test which detects both antigen and antibody is needed. During early seroconversion the result of a 4th generation test may be positive for antigen and negative for antibody. The window period for a 4th generation test is usually considered to be four weeks.
If a person continues to be at risk of exposure to HIV infection, they remain within the window period.
HIV test results may be negative, reactive, positive or equivocal.
Negative: If a test result is negative, this means the patient does not have HIV unless the test was performed within the window period, in which case it should be repeated after the window period if HIV is suspected.
Reactive: A reactive test result usually means the patient has HIV but, whatever type of test was first performed, a confirmatory laboratory blood test should always be done to confirm the result.
Positive: A positive result is usually given after a first reactive test followed by a confirmatory blood test which is also reactive. At this point, an HIV diagnosis is confirmed.
Equivocal (or indeterminate): If an initial test is reactive and the confirmatory test is negative, this usually means the first result was a false positive and the patient does not have HIV. However, it may occasionally occur in very early seroconversion . A repeat test in 2-4 weeks is usually advised in this situation.
Failed test: Occasionally, the laboratory is unable to report a result if the blood sample provided is insufficient or there was some other technical problem, such as a faulty control. In this case, a new sample should be taken.
Point of care test: negative result
Point of care test: reactive result