INTRODUCTION

The Centers for Disease Control and Prevention (CDC) estimates that more than one million people in the United States are infected with HIV¹. Of these, 24 to 27 percent do not even know that they are infected.² The CDC also estimates that 56,300 people in the United States became infected with HIV in 2006.³ Unfortunately, HIV testing rates among adults have remained virtually flat over the last five year while surveys suggest that nearly 40% of adults believe that they had never been tested for HIV.⁴

Although the success of treatment depends, in part, on timely diagnosis of an HIV infection; HIV treatment with the combination of highly active antiretroviral therapy (HAART), and the improved management of opportunistic infections has markedly improved HIV survival rates.⁵
According to CDC data in 2005, 38% of people with AIDS had their initial positive HIV test less than one year before their AIDS diagnosis.⁶ In the HAART era, it is more important than ever to diagnose HIV disease earlier, so effective treatment can be provided.

HIV testing began in the mid 1980’s with the development of a first generation enzyme immunoassay (EIA) for HIV1. This was followed by FDA approval for the use of a supplemental Western blot (WB) confirmatory test in 1987 which was designed to improve the specificity of the diagnostic process. Early in the AIDS pandemic, laboratory tests were developed with a primary purpose of protecting the nation’s blood supply.^7,8 At that time the risk of HIV infection through transfusion was approximately 1 in 100. Estimates in 2004 placed the risk at approximately 1 in 1.9 million.⁹

Early enzyme-based immunoassays were relatively insensitive, and not very specific. A significant contributor to false positive results, seen with first generation HIV assays, was the necessity of propagating the HIV virus in tissue culture since the virus assembles and buds from the host cell membranes. The use of tissue culturing techniques and viral lysates in the manufacturing process led to the co-purification of host cellular proteins. As a result, assays occasionally responded to antibodies against the host cell antigens rather than antigens related to an HIV infection itself.¹⁰

Over the years, diagnostic manufacturers worked to maximize test sensitivity and improve the specificity. To minimize false positive results, the U.S. Food and Drug Administration (FDA) mandated that every test would need to be confirmed by an independent procedure (an immunofluorescence- based assay (IFA) or a Western blot (WB)) before an HIV result could be declared final and reported to a patient. In 1989, the CDC and the Association of Public Health Laboratories (APHL) recommended the combination of an EIA and a WB as the “gold standard” for the diagnosis of HIV infection.¹¹

In the 1990’s, a better understanding of the serologic evolution of an HIV infection led diagnostic companies to develop assays better tailored to the immunologic response and the serologic pattern of an HIV infection. At present, there are sensitive and specific antibody tests, tests that detect antigens present during the early stages of an HIV-1 infection, assays that respond to both IgG and IgM antibodies, as well as assays that target the molecular biology of the HIV infection itself.

HIV screening depends on detecting the presence of HIV antibodies. Determining the stage of an HIV infection is more complicated and depends on the pattern of antigens and antibodies. A known serologic window exists between the time that an HIV infection begins, and the time when diagnostic tests are able to detect the antigen or antibody responses to that infection.¹² Because of the limited duration of the antigenic response, HIV screening is usually conducted by testing for the presence of HIV antibodies which arrive a little later, but persist until an individual is severely immunodeficient. When there is clinical indication suggesting a recent HIV exposure, additional diagnostic testing may include tests for HIV p24 antigen, as well as nucleic amplification to look for molecular evidence of HIV-1 RNA.

During the past five years, the FDA has approved six rapid HIV tests (four of which are Clinical Laboratory Improvement Amendments of 1988 (CLIA)-waived for pointof- care-testing (POCT); two third-generation EIA tests that can detect HIV-1, HIV-2, and HIV-Group O; a qualitative diagnostic RNA assay (Aptima, Gen-Probe); and the new quantitative viral load assays. Additional HIV tests may soon be available in the United States, including a fourth generation EIA that recognizes elements of both the antigen and antibody response to an HIV infection and markedly reduces the serologic window.⁹

For more than 20 years, traditional antibody testing processes have relied upon approaches designed to maximize sensitivity without sacrificing specificity. This is achieved by a multi-step algorithm that is consistent with the operation of a traditional clinical laboratory. Unfortunately the traditional laboratory algorithm often took days to weeks to complete. The advent of point-of- care testing (POCT), combined with the realization of the important barrier that results from “test anxiety” and the significant numbers of individuals who consequently fail to return to receive a confirmatory result, have propelled the recent awareness that HIV screening and confirmation need to be accomplished simultaneously if public health authorities are to maximize the effectiveness of the HIV screening programs.

In conventional HIV testing, where clients must return several days later to learn their results, nearly 25% of the clients fail to return for the second meeting. This makes it clear that the conventional testing algorithm is neither cost-effective nor sufficient HIV screening.