The CDC has been testing a new HIV diagnostic algorithm due to the prevalence of false-negative results in early testing, according to Morbidity and Mortality Weekly Report.
The algorithm replaces the supplemental Western blot test with an HIV-1/HIV-2 antibody differentiation test that was approved by the FDA in March. Because the immunoassay of the current algorithm can detect HIV earlier than the Western blot and immunofluorescence assay supplemental tests, infections in the early stages are often incorrectly interpreted as negative upon supplemental testing. The new algorithm also includes an RNA test to resolve the discrepancy.
When the new algorithm was tested at a Phoenix HIV testing program, 37 undiagnosed HIV infections were detected. Specimens with a reactive immunoassay were tested with both a Western blot test and the HIV-1/HIV-2 antibody differentiation assay. Twenty-five of the 37 cases were detected with a Western blot test, an antibody differentiation assay test, or both. Those with negative Western blot and antibody differentiation assay were tested for HIV-1 RNA, which resulted in the detection of 12 cases of acute HIV.
In the ongoing STOP study, the algorithm was also tested at sites in New York, San Francisco and North Carolina. Of the 37,876 patients tested, 654 were positive according to the immunoassay. Of those, 554 were positive for HIV-1 according to antibody differentiation assay, and one was positive for both HIV-1 and HIV-2. Among the 99 patients with negative or indeterminate antibody differentiation assay, 55 had detectable HIV-1 RNA. However, 37 of those were negative when tested with Western blot or immunofluorescence assay. Researchers reported that overall, the new algorithm accounted for 9% of what would otherwise have been misdiagnoses.
“This report demonstrates that acute HIV infections detected with third- or fourth-generation [immunoassays] often are misclassified as HIV-negative by [Western blot] or [immunofluorescence assay], potentially leading to adverse clinical outcomes for patients and further HIV transmission within the community,” researchers wrote.