New HIV algorithm holds promise

April 1, 2012

Clinical laboratories can now adopt a new algorithm that detects acute as well as established human immunodeficiency virus (HIV) infections and differentiates between HIV-1 and HIV-2. The algorithm recommends a fourth-generation HIV-1/2 Ag/Ab immunoassay as the initial screening test in order to maximize the algorithm sensitivity.

The Association of Public Health Laboratories (APHL), which developed the new algorithm in collaboration with the Centers for Disease Control and Prevention (CDC), created the previous algorithm for HIV testing with CDC back in 1989. That algorithm called for a specimen repeatedly reactive by a screening immunoassay to be confirmed by a supplemental HIV antibody test, either a Western blot or indirect immunofluorescence assay.

As the years progressed, technology began to outpace the algorithm. “You could detect positive cases with an immunoassay, but the lower sensitivity of the Western blot didn’t allow you to confirm the positivity of the patient,” says Kelly Wroblewski, MPH, MT, director of infectious diseases at APHL. “Another problem is that the Western blot doesn’t identify HIV-2.”

If an immunoassay test was positive and a Western blot was negative or indeterminate, extensive follow-up was necessary. “You could either request a new specimen, or send a specimen to the CDC or a reference laboratory,” Wroblewski explains.

HIV-2 incidence rates are low in the United States. “Most laboratories identify very few HIV-2 infections per year, making it impractical to maintain a testing platform solely for these specimens,” Wroblewski notes.

From 1988 to June 2010, there were 242 HIV-2 cases reported to CDC. Of these, 166 met case definition criteria. These HIV-2 cases were concentrated in the Northeast (66%, including 46% in New York City) and occurred primarily among persons born in West Africa (81%).1 Some HIV-2 infections may be hidden, however, and are falsely categorized as HIV-1 due to cross reactivity. The new algorithm should correctly identify these infections.

Algorithm development

During the past several years, APHL and CDC gathered data to support a testing algorithm that would identify the greatest number of new HIV cases. In 2009, the organizations released a status report that described a host of proposed HIV testing algorithms that would achieve their goal.

Next, the two organizations convened the 2010 HIV Diagnostics Conference, where the new HIV diagnostic algorithm was proposed. Attendees included federal, state, and local health officials; Department of Defense officials; HIV test manufacturers; public health, clinical, and hospital laboratorians; and academicians.

The new algorithm uses advanced FDA-approved testing technologies including a fourth-generation HIV-1/2 Ag/Ab immunoassay, an HIV-1/HIV-2 antibody differentiation assay, and nucleic acid amplification tests (NAATs), with the goal of identifying more infections earlier and differentiating HIV-1 from HIV-2.

Data show that HIV transmission often occurs very early after infection. “Attention has been shifted to narrow the window period between the time of infection and diagnosis,” says Mark Pandori, PhD, director, San Francisco Public Health Laboratory, who serves on a joint committee with the APHL and CDC. The new algorithm, using assays that are more sensitive and specific, makes this possible. Fourth-generation assayscan identify infection prior to seroconversion by detecting the viral p24 antigen.

In 2011, The Clinical and Laboratory Standards Institute (CLSI) published Criteria for Laboratory Testing and Diagnosis of Human Immunodeficiency Virus Infection; Approved Guideline (M53-A), the first formal publication of the new algorithm. This document provides guidance for laboratory professionals performing HIV testing and for interpreting results by health care providers in advanced diagnostic laboratories.

How it works

Antigen/antibody combination tests entered U.S. markets in 2010. A positive screening result should be followed with a rapid supplemental antibody test that differentiates between HIV-1 and HIV-2 antibodies. Currently, the multispot is the only test on the market with this capability.

If a result is negative or inconclusive, the algorithm suggests that laboratorians perform a NAAT to detect HIV RNA, which may be present during the window period between infection and the development of detectable antibodies.

Urgency of adoption

Clinical laboratories should consider adopting the algorithm to ensure that they are running the most up-to-date tests. “If we continue to run old lab tests, we are missing out on opportunities to detect the most infectious individuals,” Dr. Pandori says. “We have the responsibility to look for recent HIV infection.”

“This is an opportunity to enhance early diagnosis and get people into care while reducing the risk of transmission,” says Barbara Werner, PhD, infectious disease consultant, Massachusetts Department of Public Health, Boston, who served on the APHL/CDC HIV steering committee and was co-author of CLSI document M53-A. “People with acute HIV infection tend to have high viral loads. By diagnosing these individuals early on, we hope transmission can be stopped. So there are both clinical and public health reasons to detect HIV early.”

Pros and cons

Most clinical laboratories are already familiar with the technologies required to perform the new algorithm. “Implementing the new algorithm is like implementing any new test according to CLIA recommendations,” Wroblewski says.

HIV-1/HIV-2 differentiation assays are much easier to use than previous testing methods. In high-incidence settings, testing for acute infection has increased the number of detected cases by 3% to 10%.

Timely results are another benefit. The new algorithm allows for clearer results and a faster turnaround time. The multispot assay can be done in 20 to 30 minutes.

The new algorithm also reduces the number of inconclusive and indeterminate results, which decreases tech time and ultimately is a cost savings for laboratories.

Cost of testing is a concern, however. CDC is currently conducting an analysis to determine whether the proposed algorithm might offer cost savings, despite the higher price of HIV-1/2 Ag/Ab assays compared to antibody-only tests. Few assays are currently available for testing using the proposed algorithm.

Another potential downside is that a low volume of specimens will require NAAT, creating the challenge of maintaining a low-volume assay or sending HIV NAAT to a reference lab.

Fourth generation assay offerings

Two assays currently on the U.S. market can be used for screening in the new HIV algorithm. The Abbott HIV-1/2 Ag/Ab immunoassay system has been available as an FDA-approved method for HIV screening since June 2010. An immunoassay from Bio-Rad has also received FDA approval. Both have been shown in studies to be sensitive and specific assays, and both detect acute and long-standing infections.

Abbott’s ARCHITECT HIV Ag/Ab Combo assay is automated and requires some floor space, while BioRad’s GS HIV Combo Ag/Ab EIA comes in a manual and an automated version. A benchtop model is available.


  1. Centers for Disease Control and Prevention. HIV-2 Infection Surveillance-United States, 1987-2009. MMWR. 2011;60(29):985-988. Accessed January 23, 2012.

Karen Appold, BA, is an editorial consultant working in collaboration with the Association of Public Health Laboratories. She has covered the clinical laboratory industry since 1999.