It may be very difficult to tell if someone who has just started HIV pre-exposure prophylaxis (PrEP) has actually caught HIV just before or just after they started PrEP.

This is what seems to have happened in a case where a 31-year-old man from New York, who had just started PrEP, had multiple inconsistent HIV test results shortly after starting. Even though he was put on a third drug as a precaution after a very weak positive result 28 days after starting PrEP, his HIV test results were still inconsistent for some time after this.

It was eventually concluded that this patient had probably caught HIV shortly before starting PrEP – but that the two PrEP drugs he was taking, while not stopping the infection, suppressed his HIV so it was difficult to detect. In theory, people like this could be on PrEP for a considerable time before testing positive, with implications for the development of resistance.

Guidelines emphasise that people starting PrEP should always take an HIV test before starting to rule out the possibility they already have HIV. This is important because the two drugs currently comprising PrEP, while effective at preventing HIV, are not strong enough to treat established infection.

It had been known from the early days of PrEP research in animals, however, that even if PrEP fails it can cause a so-called ‘blunted’ HIV infection with a lower viral load and a weaker antibody response than usual, and the same could happen if infection occurs just before PrEP is started.

As the writer says, while guidelines recommend that people taking PrEP test for HIV every three months, they “provide no guidance on optimal screening for, and management of, acute/early infection specifically amongst individuals on PrEP,” and current tests may be inadequate to detect HIV infection in these circumstances. Read more AIDSmap