How Common Are False Positive Antibody Tests?
False positive antibody test results are particularly common in low-prevalence populations, where they can actually exceed the number of true positive results, making confirmatory testing essential in these settings. 1
Context-Dependent False Positive Rates
The frequency of false positive antibody tests varies dramatically based on disease prevalence in the tested population:
Hepatitis C Antibody Testing
- In low-prevalence populations (<10% HCV prevalence), false-positive anti-HCV results occur in 15-60% of screening-positive tests 1, 2
- The CDC documented that in populations with approximately 1% HCV prevalence (similar to the general U.S. population), 22% of anti-HCV screening-reactive results were confirmed false positives, with an additional 10% indeterminate (likely also false positive) 3
- In low-prevalence settings, the number of false positives may actually exceed the number of true positives 1
- Among blood donors (a very low-prevalence population), 97.8% of samples with very low signal-to-cutoff ratios (<4.5) were false positives 4
Syphilis Antibody Testing
- Biological false positives (BFPs) for nontreponemal tests occur but are relatively rare (<0.85% of those tested) 1
- The false positive rate for syphilis testing is substantially lower than for hepatitis C, though specific populations (pregnancy, autoimmune diseases, other infections) have higher rates 1
Why Low Prevalence Matters
The mathematical relationship between prevalence and false positives is critical to understand:
- Even with tests that have 98-99% specificity, false positives become problematic when disease prevalence drops below 10% 1
- In a population with 5% prevalence (typical for national surveys), using an antibody test with 98% specificity means that approximately 12 out of 1000 tested would be falsely positive, while only 4 would be missed 2
- At 50% prevalence (healthcare workers with respiratory symptoms), only 7 per 1000 would be falsely positive 2
Clinical Implications and Mandatory Confirmatory Testing
For Hepatitis C
All positive anti-HCV screening tests require confirmatory testing with either HCV RNA or supplemental antibody testing (RIBA) before reporting as positive 1, 2
The CDC explicitly states that:
- A person should only be considered to have serologic evidence of HCV infection after verification by more specific testing 1
- Positive screening results without confirmation lead to false diagnoses, unnecessary anxiety, and inappropriate clinical decisions 1
For Syphilis
- Automated nontreponemal tests should be confirmed with manual procedures for endpoint titration 1
- The false positive rate is lower than HCV but still requires clinical correlation 1
Common Pitfalls to Avoid
The most critical error is reporting or acting on screening antibody test results alone without confirmatory testing in low-prevalence populations 1, 2
Additional pitfalls include:
- Failing to consider the population prevalence when interpreting results - a positive test in an asymptomatic person with no risk factors has a much higher chance of being false positive 1, 3
- Not recognizing that very low signal-to-cutoff ratios (<4.5 for some HCV assays) predict false positives in >95% of cases 4
- Assuming all positive antibody tests represent current infection - even true positive antibody tests may reflect past resolved infection rather than active disease 2, 5
Practical Testing Algorithm
For suspected hepatitis C in low-prevalence settings:
- Initial anti-HCV screening test (EIA or CIA) 2
- If positive, immediately reflex to HCV RNA testing - this is now the preferred confirmatory approach 2, 5
- Positive HCV RNA = current infection requiring treatment 2, 5
- Negative HCV RNA = either false positive screening test, resolved past infection, or rarely early acute infection 2, 5
Alternative confirmation with RIBA is acceptable but does not distinguish current from past infection, making HCV RNA the superior confirmatory test 1, 2