Adult HIV Diagnosis

Standard Approach to HIV Diagnosis in Adults

In individuals over 18 months of age, HIV diagnosis is typically made by detecting HIV-specific antibodies in blood, using:

• 4th generation ELISA tests that simultaneously detect:
  • HIV antibodies
  • HIV p24 antigen

Benefits of 4th generation ELISA:

• Extremely sensitive
• Near 100% negative predictive value (except in window period infections)
• Reduced diagnostic window period

Drawback:

• False positives can occur in a small percentage of cases
• Hence, all reactive screening tests must be confirmed with a second, independent test

Ampath’s HIV Testing Algorithm (Since 2020)

Previous approach (before 2020):

• HIV-1 viral load was used to confirm reactive ELISA results
• If viral load <5000 copies/mL, Western blot was performed

Why the algorithm changed:

• Increased detection of undetectable viral loads in known HIV-positive patients already on ART
• Cost-saving and diagnostic efficiency

Current Ampath HIV Testing Algorithm

1. Screening Test:
   • 4th generation HIV-1/2 ELISA
2. If Non-reactive (Negative):
   • Patient is considered HIV-uninfected
   • Retesting only advised if there are ongoing HIV risk factors
3. If Reactive (Positive):
   • Proceed to a confirmatory 4th generation HIV-1/2 ELISA using a different platform
4. Confirmatory Test Results:
   • Non-reactive/Negative:
     • Result is discrepant
     • Further HIV testing recommended
     • Consult a virologist if necessary
   • Reactive/Positive:
     • Patient is HIV-infected
     • Confirm diagnosis using a second specimen, by:
       • Baseline HIV-1 viral load, or
       • HIV PCR, or
       • A third ELISA on a different platform‍

Special Considerations

If a newly diagnosed, ART-naïve patient has an undetectable HIV-1 viral load:

• Consider:
  • Elite controller (rare immune control of HIV)
  • False-reactive ELISA result
  • HIV-2 infection

Further diagnostic options should be discussed with an Ampath pathologist.

Specimen Collection Requirements

Dedicated Serum Tubes for HIV Testing:

• Introduced: 28 October 2017
• A separate clotted tube must be submitted for HIV testing

Why?

• Prevents contamination from:
  • Opening/recapping tubes
  • Sample use on other analysers (e.g., chemistry platforms)
• Reduces:
  • Risk of false positive results
  • Need for unnecessary confirmatory testing
  • Associated costs

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Note: These recommendations align with the 2023 South African HIV Clinicians Society Guidelines.