Detecting acute infection

Viral loads tests may be used to detect the presence of HIV in the body prior to the development of antibodies. The standard means of identifying HIV-positive people is through the use of antibody tests. The appearance of antibodies in the blood is often accompanied by a seroconversion illness. Seroconversion is often preceded by a burst in viral replication, and high viral load is common among people recently exposed to HIV.

If HIV infection is suspected (for example if you know that you may have been exposed to HIV from a sexual partner known to be HIV-positive), doctors may conduct an HIV p24 antigen test, which can detect HIV infection seven days prior to the appearance of HIV antibodies. Viral load testing is even more sensitive than p24 antigen testing; HIV RNA can be detected in the blood two or three days prior to p24 antigen. The benefit of HIV RNA testing during suspected acute infection is to reduce the period of diagnostic uncertainty and facilitate very early treatment.

A person who tests positive for HIV in the blood, despite negative or indeterminate antibody results, is said to have primary HIV infection. However, false positive results are fairly common. A viral load result of less than 5,000 copies/ml during primary infection is probably suggestive of a false positive, because most people have viral loads above 100,000 during this time.

See Exposure and primary infection in The immune system and HIV: How HIV damages the immune system for further discussion of testing.

Pros and cons

There is disagreement among experts about the appropriate use of viral load testing during the early days of HIV infection. Some doctors recommend testing for people with symptoms of seroconversion illness who may have been exposed to HIV. Many doctors are less enthusiastic about the uses of viral testing for asymptomatic individuals with a possible exposure to HIV. A leading researcher in this area, Dr Eric Daar, has found that viral screening in such situations produces very few cases of HIV infection and should be performed with caution, due to the risk of false positive results.

Other studies of viral testing during primary infection have led experts to advise against viral load testing to screen possible acute HIV exposure, due to the high rate of false positives seen in studies so far. However, despite the caution about viral testing in adults, infants born to HIV-positive mothers are commonly tested for HIV RNA during the early days of life.

Viral load during seroconversion has been linked to prognosis, as it is in chronically infected individuals. However, the huge fluctuations in viral load during the first weeks of infection make the time of testing crucial, and experts have not yet standardised this type of early testing. Consequently, this makes interpretation of these early viral load results very difficult.

The main benefit of viral load testing to confirm HIV infection is the facilitation of early antiretroviral therapy. The immunological benefits of early therapy are discussed in When to start treatment: Treatment during primary infection and Restoring HIV-specific immunity in Anti-HIV therapy: Restoring the immune system.

References

Carr A et al. Diagnosis and treatment of primary HIV infection. The journal of HIV combination therapy 2(3):49-54, 1997.

Daar ES et al. Virology and immunology of acute HIV type 1 infection. AIDS Research and Human Retroviruses 14(3):S229-S34, 1998.

Rich JD et al. Misdiagnosis of HIV infection by HIV-1 plasma viral load testing: a case series. Annals of Internal Medicine 130:37-39, 1999.