What a test for recent infection might reveal about HIV incidence in England and Wales

Background: A laboratory method has been developed that detects recent HIV infection and allows incidence to be estimated by testing single stored ant ibody-positive specimens. A theoretical exploration of the method's surveil lance utility was carried out. Methods: Using various data sources, HIV incidence rates were postulated. T he confidence intervals (CI) for these postulated incidences were calculate d using the expected number of recent infections for each postulated incide nce, the actual number tested for HIV, and the known number of HIV-1 positi ves. A test for trend was used to determine when an important change in inc idence could be recognized. Results: If the incidence was 5% per annum (p.a.) in homosexual/bisexual me n attending sexually transmitted diseases (STD) clinics in London, 64 recen t infections would be expected in the 392 HIV-seropositive specimens and, i f observed, would result in a 95% CI of 3.1-7.9% p.a. for the incidence rat e. An incidence of 1% p.a. in pregnant women would be most unlikely as this would require detection of 193 recent infections, 26 more than the total 1 67 HIV-seropositive specimens found in 1997. In African women attending STD clinics in London, 30% of prevalent infections would be-classified as rece nt if the incidence was 5% p.a. Further, if the incidence in homosexual/bis exual men were to fall by 50% over 3 years, a decrease of this magnitude wo uld be recognized as significant within 2 years. Conclusions: The detuned assay will increase the information from HIV seros urveys even where prevalence and incidence are relatively low. Existing sur veillance systems should be redesigned to take full advantage of the method . (C) 2000 Lippincott Williams & Wilkins.