With fourth-generation HIV screening assays on the U.S. market for the past year and a new testing algorithm being proposed by the CDC, more clinical laboratories are moving to the new assays in order to take advantage of the assays’ ability to detect HIV antigen as well as HIV 1 and HIV 2 antibodies.
Outside the United States, the fourth-generation HIV assays have been used for more than 10 years. The U.S. had only third-generation assays available for a primary screen. The third-generation HIV assays detect only antibody and other supplemental assays, such as nucleic acid-based tests (NAT), and must be used for antigen detection. NAT has typically been used for confirming an acute infection but only when the patient had reason to believe he or she had been exposed or when a high-risk patient showed signs of early infection.
The body’s immunological response to the HIV virus is to produce antibodies—hopefully at detectable levels—but that only happens after a sharp increase in the viral load has occurred. It is during this window of high-level viral antigen that the patient is most infectious. Thus, the disadvantage of using a third-generation assay as a primary screen is that the assay is unable to detect the infection at a point in time that the patient is most likely to transmit the virus. This is a distinct disadvantage considering that, if they know their HIV status, patients are more likely to abstain from high-risk behaviors, thereby reducing the likelihood of transmission to partners.
Given that the fourth-generation assays have antibody sensitivity that is equal to the third-generation assays and have the ability to detect the presence of HIV antigen, these new assays give the clinical laboratory an opportunity to significantly decrease the serologic window, aid the physician in diagnosing acute HIV infections, and increase the opportunity to link the patient with care earlier in his or her infection.
Ginger Weeden, MLS(ASCP), is USSD Division Marketing Manager for Bio-Rad Laboratories
