When should I test for Human Immunodeficiency Virus (HIV) after exposure?

HIV Testing Timeline After Exposure

After potential HIV exposure, testing should be conducted at baseline, 2-4 weeks, and 9 weeks (64 days) post-exposure, with a 9-week negative result on a 4th generation test considered conclusive for most exposures. 1

Recommended Testing Schedule

The Centers for Disease Control and Prevention (CDC) recommends the following testing timeline after potential HIV exposure:

1. Baseline testing - Immediately after exposure
2. 2-4 weeks post-exposure - 4th generation tests can detect most infections at this point
3. 9 weeks (64 days) post-exposure - Final conclusive test for most exposures
4. 6 months post-exposure - Only in special circumstances, particularly after Post-Exposure Prophylaxis (PEP) 1

This schedule represents a significant update from older guidelines that recommended testing at baseline, 6 weeks, 3 months, and 6 months 2.

Testing Methods and Window Periods

• 4th generation tests are now preferred as they detect both HIV antibodies and p24 antigen, with a window period of typically 14-21 days 1
• 5th generation tests provide separate antigen and antibody results, potentially detecting infection approximately 2 weeks post-exposure 3
• Older 3rd generation tests that only detect antibodies have longer window periods (up to 3 weeks or more) and are no longer preferred 1

Special Considerations

After Post-Exposure Prophylaxis (PEP)

If PEP was administered after exposure, it's particularly important to complete the full testing schedule through 6 months, as PEP may delay seroconversion and extend the window period for antibody detection 1.

For High-Risk Exposures

For known exposures to HIV-positive sources or high-risk exposures, some clinicians may still recommend the more conservative approach of testing at 6 months, though this is becoming less common with modern testing methods 1, 4.

Common Pitfalls to Avoid

1. Stopping testing too early - A single negative test before 9 weeks is insufficient to rule out infection 1
2. Using outdated testing algorithms - Relying solely on 3-month testing with older generation tests can miss early infections 1
3. Inconsistent recommendations - Lack of standardized protocols leads to variable practice and potentially inappropriate testing intervals 4
4. Delayed diagnosis - Late HIV testing is associated with increased morbidity, mortality, and healthcare costs 5, 6

Practical Implementation

For healthcare providers:

• Routinely ascertain HIV risk behaviors and testing histories
• Make strong recommendations for patients to be tested
• Incorporate testing into routine medical care 5
• Use 4th or 5th generation tests whenever available 1, 3
• Ensure patients understand the importance of completing the full testing schedule

For patients:

• Understand that modern HIV tests can detect infection much earlier than in the past
• Complete the full recommended testing schedule
• Consider PrEP (Pre-Exposure Prophylaxis) if at ongoing risk for HIV exposure 1

By following these updated guidelines, HIV infection can be detected earlier, leading to better treatment outcomes and reduced transmission risk.