Diagnosis

Laboratory criteria for defining a confirmed case now accommodate new multi-test algorithms, including criteria for recognizing early HIV infection and differentiating between HIV-1 and HIV-2 infection (Figure 1). In 2023, the CDC updated its recommendations for HIV testing to provide guidance on the use of HIV NATs (nucleic acid tests) with a diagnostic claim in the third step of the current recommended algorithm for laboratory testing.¹³

Figure 1. Recommended Laboratory HIV Testing Algorithm for Serum or Plasma Specimens.¹³

A confirmed case of HIV infection is now classified in one of five stages (0, 1, 2, 3, or unknown).⁹ Early infection, i.e., a negative HIV test within 6 months of HIV diagnosis, is classified as stage 0. If the criteria for stage 0 are not met, the stage is classified as 1, 2, 3, or unknown depending on CD4+ T-lymphocyte test results or whether an opportunistic illness was diagnosed >180 days after the diagnosis of HIV infection (Table 3). Acquired immunodeficiency syndrome (AIDS) is classified as stage 3.⁹

Table 3. Case classification of HIV infections (persons ≥6 years of age).⁹

Stage 0	No opportunistic illnesses	If >180 days have elapsed after stage 0 at diagnosis, the stage at the later date is classified as 1, 2, 3, or unknown
Stage 1	No opportunistic illnesses	Either CD4+ T-lymphocyte count of ≥500 cells/μL or CD4+ T-lymphocyte percentage of total lymphocytes ≥26
Stage 2	No opportunistic illnesses	Either CD4+ T-lymphocyte count of 200-499 cells/μL or CD4+ T-lymphocyte percentage of total lymphocytes of 14-25
Stage 3 (AIDS)	At least one opportunistic illness	Either CD4+ T-lymphocyte count of <200 cells/μL or CD4+ T-lymphocyte percentage of total lymphocytes of <14.*
Stage unknown	No information on opportunistic illnesses	No information on CD4+ T-lymphocyte count or percentage.

*Documentation of opportunistic illnesses (Table 1) supersedes a CD4+ T-lymphocyte count of >200 cells/μL or CD4+ T-lymphocyte percentage of total lymphocytes of >14.

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