HIV p24 Antigen

Immunity

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Detect infection with Human Immunodeficiency Virus

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HIV p24 Antigen Test Information Guide

Why is it done?
- What it measures: Detects HIV p24 antigen, a protein produced by HIV virus in the blood during early infection before antibodies develop
- Early detection of HIV: Can detect HIV infection within 1-3 weeks of exposure, before antibodies are present (window period)
- Acute retroviral syndrome evaluation: Used when evaluating patients with symptoms of acute HIV infection (fever, rash, lymphadenopathy)
- Post-exposure prophylaxis (PEP) evaluation: Performed after potential HIV exposure to initiate early treatment if needed
- Screening in high-risk populations: Part of fourth-generation HIV testing (combines p24 antigen and antibody detection)
- Monitoring viral load: Assessing HIV disease progression and response to antiretroviral therapy
Normal Range
- Negative result: Less than 5 pg/mL or non-detectable
- Positive result: Greater than 5 pg/mL (picograms per milliliter), indicating detectable HIV p24 antigen
- Unit of measurement: pg/mL (picograms per milliliter)
- Normal interpretation: Negative result indicates no detectable HIV p24 antigen; however, does not completely exclude HIV infection if tested during window period
- Abnormal interpretation: Positive result indicates HIV infection is likely; requires confirmatory testing and should be followed by HIV antibody testing and viral load assessment
- Borderline/Indeterminate results: Values near cutoff (2-5 pg/mL) may require repeat testing or confirmatory assays such as Western blot or HIV RNA testing
Interpretation
- Positive p24 antigen with negative antibodies: Suggests acute HIV infection during window period (1-3 weeks post-exposure); patient is highly infectious; immediate ART initiation recommended
- Positive p24 antigen with positive antibodies: Confirms established HIV infection; p24 levels help assess viral burden and disease stage
- Negative p24 antigen with negative antibodies: Likely HIV-negative; however, may represent window period or very early infection; repeat testing recommended at 1-3 months if exposure suspected
- High p24 antigen levels: Indicates high viral load; associated with acute infection, AIDS, or uncontrolled disease; requires urgent clinical intervention
- Low/undetectable p24 antigen: In treated patients indicates effective antiretroviral therapy; generally correlates with improved immune function and reduced transmission risk
- Factors affecting results:
  - Timing since exposure (window period sensitivity)
  - Antiretroviral therapy effectiveness
  - HIV strain variability
  - CD4 count level
  - Immunological status of patient
  - Laboratory testing methodology
- Clinical significance: p24 antigen becomes detectable at approximately the same time as HIV RNA but before antibodies develop; presence indicates infectious disease; serves as surrogate marker for viral replication and disease stage
Associated Organs
- Primary organ systems affected:
  - Immune system (CD4+ T cells, lymphocytes, macrophages) - primary target
  - Blood and lymphoid tissues
  - Central nervous system (CNS)
  - Gastrointestinal tract
  - Respiratory system
- Conditions associated with abnormal results:
  - Human Immunodeficiency Virus (HIV) Type 1 and Type 2 infection
  - Acute retroviral syndrome
  - AIDS (Acquired Immunodeficiency Syndrome)
  - Opportunistic infections (Pneumocystis pneumonia, tuberculosis, toxoplasmosis)
  - HIV-associated malignancies (Kaposi sarcoma, lymphomas)
  - HIV dementia/cognitive disorder
- Potential complications of positive results:
  - Progressive immunosuppression and CD4 depletion
  - Development of opportunistic infections
  - Increased risk of transmission to others
  - Development of antiretroviral drug resistance if ART non-adherence occurs
  - Chronic inflammation with long-term organ damage
  - Cardiovascular and metabolic complications
Follow-up Tests
- Confirmatory tests for positive p24 antigen:
  - HIV antibody testing (ELISA, rapid tests)
  - Western blot or HIV differentiation immunoassay
  - HIV RNA (viral load) testing via RT-PCR
- Initial assessment tests after positive diagnosis:
  - CD4 count (T-cell enumeration)
  - CD4 percentage
  - HIV viral load (quantitative HIV RNA)
  - HIV genotype resistance testing
  - Complete blood count (CBC)
  - Comprehensive metabolic panel
- Screening tests for opportunistic infections:
  - Tuberculosis skin test (TST) or interferon-gamma release assay (IGRA)
  - Toxoplasma serology if CD4 < 100 cells/μL
  - Cytomegalovirus (CMV) serology
  - Syphilis serology (RPR/VDRL and treponemal test)
  - Hepatitis B and C serology
- Monitoring frequency during ART:
  - Viral load and CD4 count: Every 3 months initially, then every 6 months if stable and suppressed
  - p24 antigen: Used to assess treatment response; should become undetectable with effective ART
  - Metabolic and organ function panels: Annually or as indicated by ART regimen
- Complementary tests providing additional information:
  - Immune activation markers (CD8+, HLA-DR+)
  - D-dimer and other inflammatory markers
  - Lipid panel and glucose testing
Fasting Required?
- Fasting requirement: NO - Fasting is not required for HIV p24 antigen testing
- Sample collection: Blood draw (serum) - can be collected at any time of day
- Medications to avoid: None specific; however, continue current antiretroviral medications as prescribed
- Special preparation instructions:
  - No special preparation needed
  - Inform healthcare provider of any recent illnesses or infections
  - Inform provider of all current medications including supplements
  - Normal daily activities may be continued
  - Timing of test critical in acute infection evaluation (test too early may give false negative)

How our test process works!