Two years ago the UK Health Protection Agency (HPA) expanded access to avidity testing (which indicates the likelihood that HIV infection occurred within the previous 6 months) to all newly diagnosed patients.
Additionally, the agency decided to return results to patients, with the understanding that this is more sensitive on a population than individual level.
Also called the Recent Infection Testing Algorithm (RITA) and previously referred to as STARHS (Serological Testing Algorithm for Recent HIV Seroconversion), the UK AxSYM avidity index (AI) is a guanidine-based test where a cut off AI < 0.8 is interpreted as recent infection (95%CI 5-8 months), with a 4.5% false positive rate (based on 1287 tested >1 year after diagnosis).
Of the 6284 results available, probable risk category was 50% gay men (n=2848) and 50% heterosexual (n=1146 men and 1706 women). Only 93 diagnoses related to IV drug use, but 491 samples were unclassified.
While 14% (95%CI 13.7-15.5%) of the overall cohort (917/6284) indicated recent infection, this varied significantly by risk group with more than 22% (95%CI 20.7-23.8%) of recent infections in gay men, 8% (95%CI 7.1-9.1%) in heterosexual transmissions (similar for men and women) and 4% (95%CI 1.2-10.6%) in IDUs.
There was also a striking inverse correlation between older age and risk of recent infection among gay men. Recent infection accounted for approximately 30% of diagnoses in men aged 15-24 (n=118), 25% of those aged 25-34 (n=268), 18% aged 35-50 (n=209) and 13% aged > 50 years (n=37).
For heterosexual transmissions, younger age was less dramatic, but accounted for approximately 18% of women and 10% of men aged 15-24 and 8% women and 13% men in the 25-34 age group.
These results included a caution on the need for estimates of population incidence rates in order to interpret these findings. A survey of the acceptability of this service for healthcare staff has already been published in HIV Medicine, with an ongoing patient survey still ongoing. This is essential due to the potential for results to be used together with sexual history in identifying possible source partners.
These results are an important new aspect of the important work carried out by the HPA.
The UK is the only country currently using avidity testing for all new diagnoses and that returns results to patients.
Aghaizu A et al. Recently acquired HIV infections: an overview of surveillance in the UK. 18th BHIVA Conference, 18-20 April 2012, Birmingham. Oral abstract O19.
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