HIV diagnosis in adults

To detect, confirm and type HIV infection. HIV typing (differentiation between HIV-1 and HIV-2 infection) is required because proviral testing, viral load testing and drug resistance testing require HIV-1 or HIV-2 specific molecular HIV assays, and the HIV type will also impact therapy choice. When an HIV infection is confirmed, it is advised to refer the patient immediately to an HIV Reference Center (HRC) for treatment and follow-up of the infection in order to inhibit disease progression towards the acquired immune deficiency syndrome (AIDS) and minimize the risk for transmission of the virus.

HIV screening should be performed by 4th generation tests (combined antibody/antigen detection) or 5th generation tests (discriminate between results for antibody and p24 antigen) to allow an early diagnosis. Both 4th and 5th generation tests detect antibodies against HIV-1 of groups M and O, as well as HIV-2. Samples with a reactive or equivocal result need to be confirmed by specific supplementary tests and results are interpreted using well-defined criteria. The seroconversion might still be incomplete during primary infection, which can result into negative of indeterminate profiles of these confirmatory assays. For this reason, additional testing for the detection of p24 antigen and/or viral RNA is particularly useful in this stage of the infection.

HIV screening can be requested by all physicians and can be performed in any clinical laboratory. The clinical laboratory will send all samples with a reactive/positive or equivocal HIV screening result to an ARL of its choice for confirmation or exclusion of HIV infection, as well as for typing of infection (HIV-1, HIV-2, co-HIV-½). The clinical laboratory is advised to provide information on the screening assay that was used and on the respective screening result to the ARL, together with any other clinical information that might be relevant for choice of test and interpretation of results (timing of risk contact, clinical symptoms, prophylaxis, antiviral treatment, …), if provided by physician.

HIV screening is recommended for several indications, but can also be performed upon request of the patient, even in the absence of any of these indications or without the disclosure of a risk factor.

The sensitivity and specificity of the screening tests are excellent (99,2 to 99,8%). However, the overall positive predictive value is only 50% due to the low prevalence of HIV infection in Belgium. Therefore, samples with a reactive/positive or equivocal result need to be confirmed with additional tests in an ARL before communication of the initial screening results.
Every diagnostic test has a window period. The window period for an HIV test refers to the time between HIV exposure and the time a test can detect HIV in the blood. The window period depends on the type of HIV test used and can be longer in case of prophylaxis against HIV.
 

To perform a test for HIV diagnosis in adults:

1. Collect blood in a serum tube (normal or SST) by venipuncture (5 to 10 ml).
2. Request an HIV screening test. 
3. Store mother-tube at room temperature and send as soon as possible to any clinical laboratory. Store at 2-8°C if transport cannot be arranged within one day.

In case of a negative test result, the test did not detect markers of an HIV infection. However,

• The date of the potential risk contact should be taken into account when interpreting the result. HIV screening tests that use 4th or 5th generation tests can detect infection from 2 weeks after the risk contact. But as the degree of certitude is not maximal at that time, it is necessary to retest 45 to 50 days after the contact, in case of an initial negative result. If there was no additional risk contact in the meantime, an HIV infection can be excluded after 45 to 50 days.
• If a very recent HIV infection is suspected, it is advised to take a sample 1 to 2 weeks after the sampling date of the negative test or to contact your clinical laboratory or ARL for possible additional testing on the original sample.
• In the presence of pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP), a delayed antibody response is possible. It is recommended to test at least at the end of prophylaxis and 45 to 50 days after stop of prophylaxis.

In case of an inconclusive confirmation test result, the test did not fulfill the criteria for the confirmation of an HIV infection nor for the exclusion of an HIV infection:

• If a very recent HIV infection is suspected, it is advised to take a sample 1 to 2 weeks after the sampling date of the inconclusive test or to contact your clinical laboratory or ARL for possible additional testing on the original sample.
• Otherwise, send a new sample preferably taken after 2 weeks.
• When repetitive inconclusive test results are obtained, contact the ARL to discuss additional testing.

In case of a positive screening test result in the clinical laboratory that is not yet confirmed by ARL:

• Wait for the confirmation test results from ARL before taking any other action.

In case of a reactive/positive confirmation test result received from ARL, including acute HIV infection (sample taken in seroconversion period):

• Send a new sample as soon as possible to your clinical laboratory, to rule out sampling/labelling errors and contaminations.
• Complete and return the provided epidemiological form to the ARL. This way you contribute to a better HIV surveillance, which can improve public health prevention.
• Refer the patient to an HIV Reference Center (HRC) as soon as possible.