HIV testing technology in US needs to change or risk missing acute infections

A fourth-generation HIV testing assay detected almost two-thirds of individuals with acute HIV infection, investigators report in an article published in the online edition of the Journal of Acquired Immune Deficiency Syndromes. The researchers believe that their results show the ARCHITECT HIV Ag/Ab Combo Assay to have significant advantages, including the time needed to obtain a result compared to the current pooled HIV RNA testing strategy used to diagnose acute HIV infection. Such assays are already routinely used in the United Kingdom.

A separate US study published in the August 1^st edition of Clinical Infectious Diseases has found that using only HIV antibody tests will mean that a significant number of recent HIV infections in gay men will be missed.

In the US, HIV antibody tests are the routine technology used to diagnose HIV infections. These tests detect the antibodies that develop in response to infection with HIV, typically detectable within three to five weeks of infection.

Antibody tests are therefore unable to detect infections in the acute phase, defined as being the interval between infection with the virus and the development of antibodies.

Most people with acute HIV infection are unaware of their status, and there is accumulating evidence that people in that situation are a major driver of the HIV epidemic. The identification of individuals with acute infection could therefore help curtain the spread of the virus.

Diagnosis of acute HIV infection relies on detection of HIV virus using viral load tests, or p24 antigen. Pooled HIV viral load testing has been shown to be an effective means of diagnosing acute infections. However, it is slow (typically taking between seven and 21 days to obtain the results), cumbersome and labourious. It is therefore not a realistic technology for resource-limited settings, nor for addressing the problem of ongoing transmission from people during acute infection.

An alternative could be the use of a test that combines testing for HIV antibodies with an ultra-sensitive p24 assay, which can detect HIV infection several weeks earlier than previous generations of antibody tests in people recently infected. Such assays are now routinely used in some countries, for example the UK.

Investigators therefore studied samples obtained from gay men in the US enrolled in the EXPLORE study. This study involved HIV-negative men who were enrolled between 1999 and 2001. Over 24 months of follow-up, they were tested at six-monthly intervals for HIV using antibody tests.

The investigators tested samples using two HIV viral load assays, as well as the fourth-generation ARCHITECT HIV Ag/Ab Combo assay and three other testing technologies, including a third-generation assay.

Analysis of 217 samples obtained from men at their last HIV-negative study visit showed that 21 (10%) had detectable levels of HIV RNA. The median viral load was 130,000 copies. These infections were categorised as acute.

To evaluate the performance of the ARCHITECT Combo assay the investigators tested 334 samples. These included:

• 21 acute infections.
• 83 samples obtained at enrolment to the study which were HIV-negative.
• Eleven samples collected before seroconversion with indeterminate antibody tests, but a median viral load of 57,000 copies/ml.
• 217 HIV antibody-positive samples.

The ARCHITECT Combo assay was positive for 13 of the 21 acute samples. The median viral load of individuals testing positive with this technology was significantly higher than that of individuals testing negative (662,000 copies/ml vs 3576 copies/ml).

All the HIV-negative controls tested negative with the ARCHITECT Combo assay, and all the indeterminate antibody tests were positive. The assay also accurately diagnosed all the antibody-positive samples.

Finally, the investigators found that the ARCHITECT Combo assay detected all HIV infections missed by second- and third-generation testing assays.

“In this study, HIV Combo detected 13 of 21 acute HIV infections (62%) in a high-risk population”, comment the investigators.

The investigators note that some of the samples detected as HIV-infected using the ARCHITECT Combo assay would most likely have been missed using the current pooled HIV RNA testing technology.

They also note several other potential advantages of the ARCHITECT Combo assay, including the availability of the first test result in as little as 28 minutes and the assay’s ability to handle up to 200 tests an hour. Moreover, the assay only needs to analyse each sample on one occasion. Pooled testing, however, requires three or four stages of analysis using ever smaller samples to find how many samples in a positive initial pool are HIV viral load-positive. “These types of algorithms result in delays in reporting results and increase the overall cost of testing”, write the authors.

“Our results suggest that ARCHITECT HIV Ag/Ab Combo may be useful for detection of acute HIV infection,” conclude the investigators.

Antibody tests alone not enough to detect acute HIV infection in gay men

A second US study has also shown the value of fourth-generation tests. Conducted in Seattle, it showed that only 80% of HIV-positive gay men had their infection diagnosed using rapid, oral HIV tests. Although the remaining infections were detected using pooled testing, it took a median of 19 days for the results to be available.

“The failure to diagnose acute HIV infection represents an important public health problem”, note the investigators, “persons with primary infection may be up to 10 times more likely to transmit HIV per sexual act than are individuals with established infections”.

They note “fourth-generation combination assays detect both anti-HIV antibodies and p24 antigen” and are therefore an alternative to pooled testing as a way of detecting individuals recently infected with HIV.