What are the next steps if a patient's Human Immunodeficiency Virus (HIV) Antibody/Antigen (AB/AG) screening test is reactive, considering the possibility of a false positive result?

Yes, False Positive HIV AB/AG Screening Tests Are Possible and Require Confirmatory Testing

A reactive HIV AB/AG screening test can absolutely be a false positive, and you must never diagnose HIV based on screening results alone—all reactive screening tests require confirmatory testing before making a diagnosis. 1, 2

Understanding False Positive Rates

False positive results occur more commonly than many clinicians realize, particularly in low-prevalence populations:

• In low-risk populations, false positives can occur in up to 20-29% of reactive screening tests 3
• Among cancer patients specifically, 29% of reactive 4th generation tests were false positives 3
• However, when BOTH screening AND confirmatory tests are reactive, false positives are extremely rare—in one study of 290,000 blood donors, no false-positive Western blot results were detected 2

Primary Causes of False Positive Results

Technical and Laboratory Factors

• Heterophilic antibody interference is a major cause, particularly with the Abbott ARCHITECT HIV Ag/Ab combo assay, where 95% of false-reactive specimens became negative when retested on different platforms 4
• Specimen-handling errors and laboratory errors are the most common causes of incorrect results 2
• Mislabeled samples must always be considered, especially in patients with no identifiable HIV risk factors 2

Patient-Related Factors

• Nonspecific reactions occur more frequently in pregnant or parous women 2
• Cross-reactivity can occur with other infections (prior schistosomiasis) or rare conditions like ameloblastoma 5
• Recent vaccination, particularly HIV vaccine trial participants, may produce false positives 2
• Certain malignancies and chemotherapy (particularly plant alkaloids) increase false positive risk 3

High-Risk Demographics for False Positives

Multivariate analysis identified these predictors of false-reactive results 3:

• Age >60 years (adjusted OR 6.983)
• Female sex (adjusted OR 6.060)
• Non-Black, non-Hispanic race/ethnicity
• Patients receiving plant alkaloid chemotherapy (adjusted OR 2.870)

Mandatory Confirmatory Testing Algorithm

Follow the CDC-recommended testing algorithm strictly 6, 1:

Step 1: Initial Reactive Screening Test

• If 4th generation HIV-1/2 antigen/antibody combination assay is reactive, proceed immediately to Step 2
• Never diagnose or counsel a patient as HIV-positive at this stage 1, 2

Step 2: HIV-1/HIV-2 Antibody Differentiation Immunoassay

• If differentiation assay is positive: Confirms HIV infection—proceed with CD4 count, viral load, and resistance testing 6, 1
• If differentiation assay is negative: Proceed immediately to Step 3 to rule out acute infection 6, 1
• If differentiation assay is indeterminate: Proceed to Step 3 7

Step 3: HIV RNA Testing (NAAT)

• If HIV RNA is positive: Confirms HIV infection (likely acute infection) 6
• If HIV RNA is negative: The screening test was a false positive—patient is HIV-negative 6
• If HIV RNA is negative but clinical suspicion remains high: Repeat testing in 4 weeks 6

Special Circumstances Requiring Extra Vigilance

Discordant Results Pattern

When you encounter a reactive rapid test (like OraQuick) but non-reactive 4th generation test 6:

• Perform HIV RNA testing immediately to rule out acute infection
• This pattern suggests either false-positive rapid test, very early infection, or technical error
• If RNA is negative, the rapid test was likely false positive

Very High Signal-to-Cutoff Ratios

Even extremely high S/CO ratios (>400) can be false positives 5:

• One case report documented S/CO >400 that was ultimately false positive due to cross-reactivity with synthetic peptide components
• High S/CO does not eliminate the need for confirmatory testing

Patients on PrEP

• PrEP use can attenuate or delay HIV seroconversion in diagnostic assays 7
• Reactive results may represent true infection or false positives—these scenarios are challenging to differentiate 7
• Management depends on pre-test probability: may range from stopping PrEP for 3-4 weeks and retesting, to intensifying to full ART if clinical suspicion is high 7

Critical Pitfalls to Avoid

1. Never diagnose HIV without confirmatory testing—false positives have devastating psychological and social consequences 1, 2

2. Don't assume high S/CO ratios guarantee true positivity—even values >400 can be false positive 5

3. Don't rely on a single assay platform—heterophilic interference can cause persistent false reactivity on the same platform 4

4. Don't forget the window period—antibody tests cannot rule out infection within 6 months of exposure 1

5. Avoid tie-breaker algorithms in resource-limited settings—these can perpetuate false-positive diagnoses when tests are susceptible to the same interference 8

Clinical Management During Workup

While awaiting confirmatory results 6:

• For patients with recent high-risk exposure: Consider post-exposure prophylaxis
• For high-risk individuals once HIV is ruled out: Consider Pre-Exposure Prophylaxis (PrEP)
• Document all test results clearly in the medical record
• Provide appropriate counseling about the meaning of preliminary results without causing undue alarm

Follow-Up Testing Requirements

If initial confirmatory testing suggests false positive 6:

• Repeat testing in 4 weeks to definitively rule out infection
• For high-risk individuals, additional serologic testing at 6,12, and 24 weeks may be warranted
• Consider testing on alternative platforms if heterophilic interference is suspected 4