CDC HIV Classification System
The CDC HIV Classification System is based on two key parameters: CD4+ T-lymphocyte count (or CD4 percentage) and clinical stage of disease, creating a matrix of nine categories (A1-C3) that guide clinical management and define AIDS.
Classification Structure
The CDC system combines three immunologic categories with three clinical categories to create a comprehensive staging framework 1:
Immunologic Categories (Based on CD4 Count)
- Category 1: CD4 count ≥500 cells/mm³ (or CD4 percentage ≥29%) 1
- Category 2: CD4 count 200-499 cells/mm³ (or CD4 percentage 14-28%) 1
- Category 3: CD4 count <200 cells/mm³ (or CD4 percentage <14%) 1, 2
Clinical Categories
Category A includes 1:
- Asymptomatic HIV infection
- Persistent generalized lymphadenopathy
- Acute (primary) HIV infection with accompanying illness or history of acute HIV infection
Category B includes symptomatic conditions not in Category C, such as 1:
- Bacillary angiomatosis
- Persistent or recurrent oral/vaginal candidiasis
- Cervical dysplasia (moderate to severe)
- Constitutional symptoms (fever >38.5°C or diarrhea >1 month)
- Oral hairy leukoplakia
- Herpes zoster (≥1 episode or >1 dermatome)
- Idiopathic thrombocytopenic purpura
- Pelvic inflammatory disease
- Peripheral neuropathy
Category C consists of AIDS-indicator conditions 1, 2
AIDS Definition
AIDS is defined as any HIV-infected person with CD4 count <200 cells/mm³ (or CD4% <14%), OR any person in clinical category C, regardless of CD4 count 2, 3. This expanded definition was implemented in 1993 and includes all categories A3, B3, C1, C2, and C3 2.
Clinical Implications
The classification directly determines management decisions 1:
- Categories A1, A2, B1: Observation period with monitoring every 3-4 months 1
- Categories A3, B2, B3: Initiation of prophylaxis for opportunistic infections 1
- Categories C1, C2, C3: Active treatment period for AIDS-defining conditions 4
Pediatric Considerations
Children have age-specific CD4 thresholds that differ substantially from adults 1:
- <12 months: Category 3 (severe suppression) = CD4 <750 cells/μL or <15%
- 1-5 years: Category 3 = CD4 <500 cells/μL or <15%
- 6-12 years: Category 3 = CD4 <200 cells/μL or <15%
Important Caveats
CD4 percentage may provide different staging than absolute CD4 count, particularly in certain populations 5. When discordant, CD4 percentage-based staging may indicate need for earlier antiretroviral therapy initiation 5. The system emphasizes that CD4 count is critical for determining prophylaxis timing and monitoring treatment efficacy 1.