What blood tests are used to assess for HIV?

Blood Tests for HIV Assessment

The primary test for HIV assessment is the conventional serum test consisting of a repeatedly reactive immunoassay followed by confirmatory Western blot or immunofluorescent assay, which has sensitivity and specificity greater than 99.5%. 1

Primary Screening Tests

Fourth-Generation Combination Tests (Recommended First-Line)

• Combination antigen/antibody assays that simultaneously detect:
  • HIV antibodies (anti-HIV-1 and anti-HIV-2)
  • HIV p24 antigen 1
• These tests significantly reduce the diagnostic window period compared to antibody-only tests 2
• Available as:
  • Instrument-based laboratory assays (preferred for routine screening)
  • Rapid point-of-care tests (though these have lower sensitivity for p24 antigen) 3

Alternative Screening Options

• Rapid HIV testing using blood or oral fluid specimens
  • Results available in 5-40 minutes
  • Sensitivity and specificity >99.5%
  • Important: Initial positive results require confirmation with conventional methods 1
• Qualitative HIV-1 RNA testing
  • Particularly useful in suspected acute infection when antibodies may not yet be detectable 1

Confirmatory Testing

For any reactive screening test, confirmation is essential:

• Western blot or immunofluorescent assay - traditional confirmatory method 1
• HIV viral load (HIV RNA) testing - recommended before ART initiation due to possibility of false-positive screening results 1

Special Situations

Suspected Acute HIV Infection

• HIV RNA testing or combination antibody + p24 antigen tests are essential for:
  • Persons with ongoing condomless sexual exposures
  • Those sharing needles or works
  • Individuals with signs/symptoms of acute HIV infection 1
• HIV RNA testing is more sensitive during the acute phase before antibody development 1

Baseline Testing After Diagnosis

After confirming HIV infection, additional tests should be ordered:

• HIV genotype to assess transmitted NRTI and NNRTI resistance 1
• CD4 cell count to assess immune status 1
• HIV viral load to establish baseline viral activity 1

Screening Frequency Recommendations

• All persons who have ever been sexually active: at least once in their lifetime 1
• High-risk individuals (MSM, transgender women, people who inject drugs): at least annually, and as frequently as every 3 months 1
• Pregnant women: during each pregnancy 1

Common Pitfalls to Avoid

1. False negatives during window period: Fourth-generation tests can detect infection approximately 4.9 days after a positive PCR result, but there's still a window period when tests may be negative 2

2. False positives: All HIV tests can have false-positive results, which is why confirmatory testing is essential before initiating treatment 1

3. Relying solely on antibody tests: Using antibody-only tests may miss acute infections when p24 antigen or HIV RNA would be detectable 3

4. Inadequate follow-up testing: For high-risk individuals with negative results, regular retesting is essential as risk for HIV often changes over a person's lifetime 1

By using the appropriate HIV testing strategy based on clinical context and risk factors, clinicians can ensure timely diagnosis, which is critical for reducing morbidity, mortality, and preventing further transmission.