HIV tests may be giving incorrect negative results for several months in people taking pills that cut the risk of catching the virus 鈥� which could let people pass the infection on to others because they believe they are HIV-free.
Pre-exposure prophylaxis (PrEP) is a combination of two drugs that reduce the risk of getting HIV from unprotected sex to almost zero, if taken consistently. This approach has contributed to tumbling rates of new HIV infections in gay men in several western cities such as London and San Francisco.
Advertisement
But there are still risks 鈥� some people may miss doses, making PrEP less effective. And there have also been a few cases of people saying they caught HIV even though they took every dose. For these reasons, PrEP users are advised to take HIV tests every three months, to check they haven鈥檛 contracted it.
Time lag
However, Ivana Parker of the Centers for Disease Control and Prevention in Atlanta, Georgia, and her team have found that some HIV tests may not detect an infection until around five months later, if a person is using PrEP.
They discovered this by analysing stored blood and saliva samples from a trial of PrEP in injecting drug users in Thailand. The trial cut the rate of HIV acquisition, but a few people still got infected, especially if they missed doses.
When a person becomes infected with HIV, their immune system makes antibodies that recognise the virus, and this is what HIV tests detect. But Parker and colleagues found it took seven months after infection before antibodies could be detected in saliva tests, compared with two months in those who caught HIV while not taking PrEP. There was up to a three month delay when using blood tests for certain antibodies.聽The team鈥檚 results were presented at a meeting of the American Society for Microbiology in Atlanta, Georgia, this month.
The lag could be because the drugs in PrEP stop the virus from multiplying as much, causing antibodies to rise in number more slowly. This means it takes longer to produce enough antibodies for a test to be able to detect them.
Better testing
As a result, people using PrEP may be unwittingly passing on HIV for several months before getting a positive test result. To keep HIV under control, it鈥檚 best for people to start on a regime of three anti-retroviral drugs as soon as possible, so people with false negatives are also missing out on months of important treatment.
During this time, their virus may become resistant to the PrEP drugs which could be a problem 鈥� these drugs are commonly used, in combination with a third anti-retroviral therapy, to keep HIV under control.
Parker suggests that PrEP users get tested monthly rather than the currently recommended every three months. The CDC is trying to develop more sensitive tests that would give a positive result with lower antibody levels.
鈥淧erhaps we need to design a new testing strategy but I don鈥檛 think that鈥檚 a reason not to give PrEP,鈥� says of Cambridge University. 鈥淧eople are going to have high risk behaviours 鈥� anything you can do to reduce their HIV rate is a good thing.鈥�
Parker says: 鈥淭here are many ways that you can prevent HIV and you have to work out what works best for you.鈥�
