Causes of False Positive HIV Serology
False-positive HIV serology results occur primarily due to laboratory errors (specimen-handling, mislabeling, failure to follow testing algorithms), but also from nonspecific antibody reactions in certain populations, particularly pregnant/parous women, and rarely from cross-reactivity with other antigens or recent vaccinations. 1
Primary Causes
Laboratory and Technical Errors
Specimen-handling errors, laboratory errors, and failure to follow the recommended testing algorithm are the most common causes of incorrect HIV test results. 1
Sample contamination on high-throughput automated platforms can cause false-positive results, particularly when HIV samples are processed on shared clinical chemistry analyzers alongside high viral load specimens. 2
Mislabeled samples must always be considered, especially for patients with no identifiable HIV risk factors. 1
Nonspecific Immunologic Reactions
Nonspecific reactions producing indeterminate or false-positive results occur more frequently among pregnant or parous women than other low-prevalence populations. 1
Cross-reactivity to synthetic peptide components of screening assays can produce very high signal-to-cutoff ratios (>400) despite absence of true HIV infection. 3
Heightened CD5+ and early B-lymphocyte polyclonal cross-reactivity may be a major cause of false positivity in certain geographical areas and populations. 4
Recent Vaccination or Immune Triggers
HIV vaccine-induced antibodies may be detected by current tests and cause false-positive results in vaccine trial participants. 1
Tdap booster vaccination can trigger false-positive results on both screening and confirmatory antibody tests, with rapid seroreversion occurring within days, suggesting an acute immunologic trigger. 5
COVID-19 infection and vaccination increase the risk of false-positive HIV ELISA results, with rates rising from the expected 0.4% to 1.8% in individuals with detectable anti-SARS-CoV-2 antibodies, likely due to homology between HIV-1 and SARS-CoV-2 surface glycoproteins. 6
Other Medical Conditions
False-positive heterophile antibody tests (which can cross-react in some HIV assays) may occur in patients with leukemia, pancreatic carcinoma, viral hepatitis, or CMV infection. 1, 7
Prior schistosomiasis infection or rare germ cell tumors may cause cross-reactivity to specific assay components. 3
Autoimmune diseases have been associated with false-positive HIV results. 5
Critical Context: True vs. False Positives
False-positive results when both screening AND confirmatory tests are reactive are extremely rare. 1
In a study of approximately 290,000 blood donors, no false-positive Western blot results were detected. 1
Among military applicants from low-prevalence populations (<1.5 infections/1,000), only one false-positive result occurred among 135,187 persons tested. 1
Common Pitfalls to Avoid
Never diagnose HIV based on screening test alone—all reactive screening tests must be confirmed with supplemental testing (Western blot, IFA, or HIV-1/HIV-2 antibody differentiation assay) before diagnosis. 8
Do not ignore the possibility of false positives even with very high signal-to-cutoff ratios on screening assays, as cross-reactivity can produce markedly elevated results. 3
Consider recent vaccination history (within 7-14 days) when evaluating unexpected positive results, and retest at a short interval if suspected. 5
In high-prevalence settings with shared automated platforms, ensure HIV samples are tested on dedicated instruments to prevent contamination-related false positives. 2
Weak positive test lines should not be recognized as valid except in blood donor screening, as they frequently represent false positives requiring confirmatory testing. 4