Background
In 2013, the BCCDC published results of a pilot study [1] which confirmed the value of pooled nucleic acid testing (PNAT) to identify acute HIV infections in a high risk population of men who have sex with men. PNAT increased HIV diagnoses by 11.5% compared to standard 3rd generation HIV antibody screening. This increase is a result of the shorter window period [2] for PNAT, which is about 10-12 days from infection to detection, compared to about 22-23 days for current 3rd generation screening.
The high cost of PNAT precludes its use for routine screening of low risk populations, which includes most individuals testing in BC, where few acute HIV infections are expected. An alternative to universal PNAT is 4th generation screening, which detects both HIV antibodies and p24 antigen. P24 antigen is a viral core protein which appears in the blood earlier than HIV antibodies (window period about 17-18 days) [3], but slightly later than HIV RNA.
The BC Public Health Microbiology and Reference Laboratory recently completed a second study [4] comparing PNAT and 4th generation tests to 3rd generation screening in the same population. In that study, 4th generation screening increased overall HIV detection rates by about 2% – 4% compared to 3rd generation, whereas PNAT resulted in a 6% increase.
Implementing routine 4th generation screening is therefore expected to increase the number of acute HIV infections identified during routine HIV screening in BC.
Change to practice
Effective May 27, 2015, the first-line screening test for all HIV testing in BC will be a 4th generation test.
Laboratory ordering and reporting to clinicians will not change, except that the initial test will be reported as HIV-1 & 2 Ab/Ag EIA.
The HIV confirmatory algorithm will remain unchanged. 4th generation reactive specimens which are Western blot negative or indeterminate will be tested by HIV-1 NAT to determine if an acute infection is likely.
As Western blot is still the gold standard for serological confirmation of HIV infection, a follow-up specimen will be requested to confirm infection in individuals presumptively identified with acute HIV infection.
Impacts on practice
Routine 4th generation HIV screening is likely to lead to earlier identification of a few cases of acute HIV infection annually in BC. Diagnosing individuals and informing them of their HIV status earlier in the course of infection provides an opportunity to reduce HIV transmission through behaviour changes at the time when high viral loads lead to the highest risk of transmission.
References
- Gilbert M et al. Targeting screening and social marketing to increase detection of acute HIV infection in MSM in Vancouver, British Columbia. AIDS 2013;27:2649-54.
- SmartSexResource STI Updates: Understanding the window periods of HIV tests.
- Taylor D et al. Probability of a false negative HIV antibody test result during the window period: a tool for pre- and post-test counselling. Int J STD AIDS 2014;26:215-24.
- Krajden M et al. Pooled nucleic acid testing increases the diagnostic yield of acute HIV infections in a high-risk population compared to 3rd and 4th generation HIV enzyme immunoassays. J Clin Virol 2014;61:132-7.