Understanding the window period of HIV tests – time from infection to a positive result – is important in order to provide appropriate information to clients, including when to test after a possible exposure to HIV.
Progress in HIV testing has resulted in tests with shorter window periods, which can reduce client anxiety by shortening the interval for testing following a risk event. Earlier diagnosis of HIV status also benefits both individuals and populations by earlier connection to HIV primary care and treatment, and behavior change resulting from knowledge of HIV status. Tests with shorter window periods are also more likely to detect clients in the acute stage of HIV infection (first few months after infection), when HIV viral loads are high and there is a greater chance of transmission of HIV to partners.
Current tests in use in BC
The most common HIV tests used are based on the detection of three biological markers of HIV infection (see Figure1):
- Viral RNA (genetic material), earliest to appear: detected by individual or pooled nucleic acid amplification testing (NAAT)
- p24 antigen (a viral core protein), next to appear: detected by 4th generation enzyme immunoassay testing (EIA)
- HIV antibody, last to appear: detected by 3rd or 4th generation EIA, point-of-care (PIC) or rapid HIV testing, and Western Blot
When a blood specimen is received by a laboratory, the standard protocol is to start with a sensitive screening antibody test (3rd or 4th generation EIA). Any degree of reactivity on EIA leads to further testing including Western Blot or individual NAAT to confirm infection.
If results are not clear, a medical microbiologist provides a clinical interpretation which is included with the result. The BC Public Health Microbiology Reference Laboratory currently also screens HIV antibody negative specimens using pooled NAAT at a small number of clinics in BC where the HIV prevalence is high.
What do I tell my clients?
The overall window period of HIV testing relates to the first test used, typically either 3rd generation EIA or 4th generation EIA tests. Window period estimates are typically based on averages or statistical projections from studies of a small number of sero-converting individuals (typically among repeat blood donors). Window period estimates should not be interpreted as absolute cut-offs, as time to a positive result can vary for different people. We estimate that greater than 95% of individuals will show detectable antibodies to HIV by 4 to 6 weeks, with greater than 99% having sero-converted by 3 months (as detected by Western Blot).
For early reassurance, a client can be tested at 6 weeks following a risk event or exposure, with testing repeated at 3 months. If you think your client may have a higher likelihood of HIV infection (e.g., a high-risk exposure, or sero-conversion symptoms), don’t delay testing as many people will have detectable antibodies by as early as 2-3 weeks. In this scenario, writing “4th generation HIV EIA” on the requisition form will ensure a test is performed that has the best capacity to detect acute HIV infection.
For clinics where pooled NAAT is available, we estimate that in these settings most individuals will have detectable RNA by 10-12 days after infection particularly if symptoms of sero-conversion are present. By 6 weeks, greater than 99% of individuals will have a reactive result.
For more information
- If you have any questions regarding the interpretation of a client’s HIV test results, or to determine if additional tests are indicated given your client’s history of recent exposures or risk events, please contact the PHSA Laboratory at 1-877-747-2522 and ask to speak to a medical microbiologist.
- HIV Laboratory Testing: a Resource for Health Professionals. BC Centre for Disease Control, June 2010.
Dr. Mel Krajden, Associate Medical Director, BC Public Health Microbiology and Reference Laboratory
Darrel Cook, Clinical Prevention Services, BCCDC
Darlene Taylor, Clinical Prevention Services, BCCDC