More testing may mean more diagnoses

It is difficult, however, to find further examples at present where increasing the proportion of people who are diagnosed and on treatment has resulted in reducing the number of new infections.

This is because HIV-testing rates have to be very high over a sustained period before the effect of controlling viral load in the HIV-positive population starts to outweigh the increase in prevalence due to more new people testing positive than are dying. More linkage to care and treatment, especially in settings where treatment rates have been low, will reduce the death rate and therefore not only prevalence, but even incidence, may increase because people stay alive to infect others.

Increasing the proportion of people on treatment may lead to decreased incidence in settings where treatment access is already high. Incidence, however, is extremely difficult to estimate due to the inaccuracies involved in establishing time of infection and the difficulty of estimating a denominator in sub-populations of uncertain size such as gay men.

For example, in Washington DC¹ a nearly fivefold increase in the annual number of HIV tests performed between 2004 and 2009 has led to a 17% increase in the annual number of diagnoses – although it also led to earlier diagnosis (average CD4 count at diagnosis rose from 216 to 343 cells/mm³) and a decline in people diagnosed with HIV-related conditions, from 47 to 28%.