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Current tests in use in British Columbia
When this study was conducted, in British Columbia the first HIV test applied to a blood specimen was an antibody test, a 3rd generation enzyme immunoassay (EIA). This test has high sensitivity and specificity and is used as a screening test. If the test is non-reactive, the individual is considered to be HIV uninfected. Any degree of reactivity on EIA testing led to a series of further tests to confirm or rule out HIV infection. The window period (the period between infection and the ability of the test to detect antibodies) of the EIA is about 20 to 22 days, which means that if HIV infection has occurred in the last 3 weeks, the test will likely be negative, although the individual is acutely infected with HIV. If acute HIV infection is suspected in an individual whose EIA test is non-reactive, the health care provider may consult with BCCDC to request that further testing be done.
If the EIA is reactive, a confirmatory Western Blot test is done. If the Western Blot is reactive, the individual is considered to be HIV-infected. If the Western Blot is non-reactive or indeterminate, an individual RNA nucleic acid amplification test (NAAT) is performed. If viral RNA is detected, the individual is considered to have a result suggestive of acute HIV infection. If viral RNA is not detected, it is unlikely that the individual is infected with HIV. Results of all confirmatory testing are interpreted by a medical microbiologist and occasionally, follow-up testing will be required to confirm or rule out HIV infection.
Detecting Acute HIV infection
During the course of this study, two new HIV testing technologies were introduced in British Columbia which helped reduce the window period of HIV tests and helped identify individuals with acute HIV infection.
4th Generation EIA test: This test detects antibodies to HIV similar to the 3rd generation EIA, but also detects the presence of p24 antigen, an HIV protein which can be detected in blood prior to the appearance of antibodies. The test identifies specimens that are positive for p24 antigen or HIV antibody, but does not distinguish between them. This combination test has a window period of approximately 16 to 18 days.
Pooled RNA NAAT test: Pooled RNA NAAT testing (currently used by Canadian Blood Services to screen donated blood) pools specimens from individuals with non-reactive 3rd generation EIA tests. Pools are tested using RNA NAAT; if a pool is positive, i.e. HIV RNA is detected, the specimens from the pool are tested individually to identify the RNA positive specimen. This protocol identifies people who have acute HIV infection, who are antibody negative and RNA positive. The window period of pooled RNA testing is approximately 10 to 12 days. In a number of STI clinics in North America, where pooled RNA NAAT testing has been added to standard HIV testing, the total number of individuals newly diagnosed with HIV has increased by between 3 and 11%.
RNA NAAT and 4th generation EIA testing were at the time regarded as investigational tools to assist with early identification of acute HIV infections, and the Western Blot test remained the gold standard for confirmation of HIV infection. Thus, individuals who are identified by the study protocol as having acute HIV infection may require follow-up blood testing for final confirmation with a reactive Western Blot. Most individuals will develop a reactive Western Blot within about 4 weeks following acute HIV infection.
What are the tests being used in the context of the CIHR study?
In the context of our study, we used both 3rd and 4th generation EIA for screening. If either test was reactive, Western Blot was used as the first confirmatory test and if reactive, the person was considered to have established HIV infection (meaning the person is HIV infected but past the acute HIV phase). If only the 4th generation EIA was reactive, an individual RNA NAAT was performed in addition to the Western Blot. If viral RNA is detected, the individual is considered to have a result suggestive of acute HIV infection.
In addition, all the non-reactive tests from the dual screening protocol were tested by pooled RNA NAAT (as described above). Samples which were non-reactive by 3rd and 4th generation EIA testing, but reactive in the RNA NAAT test will be considered to have a result suggestive of acute HIV infection.