Eligibility for enrollment was assessed using available test results to satisfy 1 of 6 study-specific criteria for acute and early HIV infection, each of which was mapped to an estimated Fiebig stage group (defined by protocol as I/II, III/IV, or V). Criteria were designed to support HIV diagnosis in early stages of infection and establish relative recency of infection (including minimizing the risk of progression to Fiebig VI by the time of enrollment) while accommodating differences in testing procedures at study sites. Time periods refer to the number of days that a test could be collected prior to screening for study entry. Criteria D–F were designed to capture participants in the earliest stages of human immunodeficiency virus infection, with low-level viremia and before development of antibody responses to HIV. The S/CO was used as a surrogate for HIV RNA based on the understanding that p24 antigen detection at an S/CO of 0.5–10 (and a negative immunoglobulin M/immunoglobulin G immunoassay) corresponds roughly to an HIV RNA of 4.0–5.5 log₁₀ copies/mL but is also the most common range for false-positive tests [30]. A recent increase of S/CO from below to within this range was considered indicative of new infection (criterion E). To minimize false-positive diagnoses, participants with this range of S/CO but no prior S/CO assessment required HIV RNA testing prior to enrollment (criterion F). The concomitant requirement for a nonreactive HIV antibody test within the preceding 7 days was designed a priori to exclude participants in Fiebig stages III–V.

Abbreviations: AEHI, acute and early HIV infection; HIV, human immunodeficiency virus; S/CO, signal-to-cutoff ratio (from ARCHITECT HIV Antigen/Antibody Combo assay or GS HIV Combo Antigen/Antibody enzyme immunoassay); WB, Western blot.