Failures in the Detection of HIV p24 Antigen With the Determine HIV-1/2 Ag/Ab Combo Rapid Test

Correspondence Failures in the Detection of HIV p24 Antigen With the Determine HIV-1/2 Ag/Ab Combo Rapid Test TO THE EDITOR—We read with interest the report by Rosenberg et al [1], showing that the Determine HIV-1/2 Ag/Ab Combo rapid test (Combo RT), in a field evaluation using whole blood collected from at-risk persons, performed poorly in the detection of acute human immunodeficiency virus (HIV) infection. The authors mentioned 2 possible explanations for this low sensitivity: the time course of individuals included in the study and failures in the detection of p24 antigen (Ag), owing either to p24 Ag/antibody (Ab) bound forms, which are unsuitable for detecting p24 Ag in a rapid-test strip, or to a reduced ability to detect clade non-B strains. We conducted an evaluation study of the Combo RT in 119 dilutions of 66 cell-derived supernatant samples of various HIV-1 and HIV-2 subtypes and in 20 archived plasma samples from individuals with acute-phase HIV-1 infection (Table 1). Supernatant dilutions were prepared to obtain 3 p24 Ag concentration levels (100, 200, and 400 pg/ mL) and were then tested with a p24 Ag–specific assay (Innotest HIV Antigen mAB assay; Innogenetics) (threshold, 10 ng/mL p24 Ag) to determine the real p24 Ag concentrations. When the upper concentration of a supernatant gave a negative result, a specimen with the highest p24 Ag level (>1000 pg/mL) was tested when available. In addition, the limit of detection of the Combo RT was estimated using 2 reference standards: (1) the World Health Organization Ag standard (HIV-1 isolate, code NIBSC 90/636), prepared in 5 serial dilutions (2–5 IU/mL), and (2) the French national reference p24 Ag panel (SFTS 2007), including 5 dilutions ranging from 25 to 100 pg/mL. All dilutions were prepared in human negative plasma. The test's limit of detection was established after 6 independent readings, between 2.5 and 3 IU/mL with the World Health Organization standard as the reference, and between 40 and 50 pg/mL with the French standard, which is at least 2-fold higher than the limit of detection reported in the package insert (12.5–25 pg/mL). The overall sensitivity in supernatant samples was 21%, ranging from 0% for 6 HIV-1 group M Cell-Derived Supernatants by Subtype No. All Tested Samples 72–127 pg/mL Samples 169–290 pg/mL Samples 310–640 pg/mL Samples 3600–12 800 pg/mL Samples Positive, % Tested Positive Tested Positive Tested Positive Tested Positive A 5 9 3 0 1 0 4 2 1 …