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How do doctors distinguish between new HIV infections and old ones?

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Updated February 03, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Despite guidelines for universal HIV testing, many people do not get tested for HIV on a regular basis. That means that at the time someone is diagnosed with a new HIV infection, they may wonder if there is any way of knowing when they were infected or who infected them.

Well, It Is Possible...

There are several ways for doctors to determine whether a person who has been newly diagnosed as HIV positive is also newly infected. In rare cases, when testing is done very early, doctors will find people who test positive for viral RNA or the p24 antigen but who are not yet antibody positive. These are the individuals in the earliest stages of HIV infection, but they will not be picked up on many standard antibody based HIV tests and therefore such straightforward early HIV infection diagnoses are somewhat unusual.

More often, doctors are trying to determine whether a person who has tested positive on a standard HIV test is recently infected or not. These people already have antibodies against the virus, and so their doctors are left to look at the specific characteristics of those antibodies that will be different in people with new HIV infections than in people with established HIV infections.

Antibody characteristics that doctors examine in order to detect incident HIV infections include:

  • The type of antibodies present. Different isotypes of antibodies appear at different times after exposure to a pathogen, which for many diseases can be useful for determining whether a person is newly infected or not. However, since the IgM antibodies which are usually used to detect new infections can also be present in people who have been infected with HIV for a while, this is not always as useful for staging HIV infections as doctors might like. (However, it can be used to distinguish between chronic and recent herpes infections.)
     
  • The amount of antibodies present. After antibodies first begin to appear in response to an HIV infection, they increase over a period of several months before leveling out. If these changes can be detected, it may be a sign of a relatively recent infection.
     
  • Which HIV proteins the antibodies bind to. As HIV infection progresses, the relative amounts of antibodies to different HIV antigens changes. It is possible to use this to determine whether someone has an early or late HIV infection.
     
  • How strongly antibodies bind to HIV. Newly HIV infected individuals generally have antibodies that bind less tightly to HIV than people with long established infections. However, people who began treatment quite soon after infection may also have antibodies with similarly low avidity.
     

... But It Probably Won't Happen.

In conclusion, it is possible for doctors to determine if your new HIV diagnosis is the result of a new infection or an older infection; however, this information is not part of standard HIV testing. Determining whether a newly diagnosed infection is a new HIV infection is most often done by people working in HIV surveillance rather than on patient care. Therefore, if you have recently been diagnosed as HIV positive and are concerned about when you seroconverted, you may need to talk to an infectious disease specialist about additional testing. Not every doctor will be aware of, or have access to, the necessary technology.

In fact, most of the time, doctors will not do these forms of testing on newly diagnosed patients - even if they are requested. A patient is usually considered to have an acute (i.e. newly transmitted) infection only when they had been tested and found negative within the previous year. Newly diagnosed HIV positive patients who have not been tested on a regular basis are often simply unable to find out if they were recently infected.

It is worth mentioning that there are a few other circumstances in which individuals end up categorized as newly infected instead of just newly diagnosed.

  • If a person has anti-HIV antibodies and is RNA positive, but their Western blot is indeterminate, that is thought to be a sign of an acute infection. These cases may be caught through the confirmatory testing algorithms used to detect HIV.
     
  • As mentioned earlier, when a person tests positive for viral RNA but doesn't yet produce anti-HIV antibodies, they are also considered to be newly infected. However, not all HIV testing includes an RNA test, so these cases are easily missed.

Why Detection of New HIV Infections is Important

Diagnosing new HIV infections when they are still new is important because doing so may significantly reduce the spread of HIV. For several reasons, people are at very high risk of transmitting HIV to their sexual partners in the weeks, months, or years before they know they are HIV positive.

The first reason that people with HIV may be more dangerous to their partners before they've tested positive is that if they are unaware of their risk, they may be unmotivated to practice safer sex. The second is that newly infected individuals often have higher viral loads and are more infectious than people who have been infected a longer time - making the chance of passing on the virus higher during any particular encounter. The third is that if you haven't been tested, then you aren't being treated, and treatment reduces the risk of infecting your partner.

Sources:

Cohen MS, Gay CL, Busch MP, Hecht FM. The detection of acute HIV infection. J Infect Dis. 2010 Oct 15;202 Suppl 2:S270-7.

Lee LM, McKenna MT. Monitoring the incidence of HIV infection in the United States. Public Health Rep. 2007;122 Suppl 1:72-9.

Parekh BS, McDougal JS. Application of laboratory methods for estimation of HIV-1 incidence. Indian J Med Res. 2005 Apr;121(4):510-8.

Schüpbach J, Gebhardt MD, Tomasik Z, Niederhauser C, Yerly S, Bürgisser P, Matter L, Gorgievski M, Dubs R, Schultze D, Steffen I, Andreutti C, Martinetti G, Güntert B, Staub R, Daneel S, Vernazza P. Assessment of recent HIV-1 infection by a line immunoassay for HIV-1/2 confirmation. PLoS Med. 2007 Dec;4(12):e343.

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