What is the recommended approach for interpreting Hepatitis C (Hep C) serology?

Hepatitis C Serology Interpretation

Hepatitis C serology interpretation requires a two-step approach with initial antibody testing followed by confirmatory testing with either supplemental serologic tests or nucleic acid tests (NAT) for HCV RNA to accurately determine infection status. 1

Initial Screening

• Anti-HCV antibody testing using enzyme immunoassay (EIA) or chemiluminescence immunoassay (CIA) is the recommended first step for HCV screening 1
• A positive anti-HCV screening test alone is insufficient for diagnosis, as false-positive results occur in approximately 35% (range: 15%-60%) of cases in low-prevalence (<10%) populations 1
• Screening test results should always be verified with supplemental testing with high specificity 1

Supplemental Testing Options

Serologic Supplemental Testing

• Strip immunoblot assay (RIBA) can be used to verify antibody status with results reported as positive, negative, or indeterminate 1
• A positive RIBA confirms anti-HCV positivity but doesn't distinguish between current or past infection 1
• A negative RIBA indicates a false-positive screening test result 1
• An indeterminate RIBA result requires additional testing, either repeat anti-HCV testing (>1 month later) or HCV RNA testing 1

Nucleic Acid Testing (NAT)

• HCV RNA testing by NAT is considered the gold standard for confirming active HCV infection 2, 3
• A positive HCV RNA result in an anti-HCV positive person confirms current HCV infection 1
• A negative HCV RNA result in an anti-HCV positive person may indicate:
  • Resolved infection (15-25% of older adults, 40-45% of younger adults) 1
  • Early acute infection (before detectable viremia) 1
  • Intermittent viremia in chronic infection 1
  • False-positive antibody result (requires RIBA to verify) 1

Recommended Interpretation Algorithm

1. Initial screening: Test for anti-HCV antibodies using EIA or CIA 1
2. For positive screening results: Perform reflex testing for HCV RNA to identify current infection 1
3. If HCV RNA positive: Current HCV infection confirmed; proceed with counseling and linkage to care 1
4. If HCV RNA negative: Consider supplemental antibody testing (like RIBA) to distinguish between resolved infection and false-positive antibody result 1

Signal-to-Cut-Off (S/CO) Ratio Approach

• The S/CO ratio from screening tests can be used to minimize unnecessary supplemental testing 1
• High S/CO ratios strongly correlate with true positivity and viremia 4
• Low S/CO ratios often indicate false-positive results 4
• Using S/CO ratios to guide reflex testing is cost-effective and improves diagnostic accuracy 5

Special Considerations

• False-negative anti-HCV results can occur during early infection (1-2 weeks after exposure) when HCV RNA may be detectable but antibodies haven't developed 1
• Immunocompromised patients may have false-negative antibody results despite active infection 1
• In persons with suspected recent exposure, testing for HCV RNA is recommended even with negative antibody results 1
• Indeterminate results may indicate recent infection with ongoing seroconversion 1

Common Pitfalls to Avoid

• Relying solely on antibody screening results without confirmatory testing 1
• Failing to consider the population prevalence when interpreting results (higher false-positive rates in low-prevalence settings) 1
• Not recognizing that a single negative HCV RNA test doesn't definitively rule out infection in all cases 1
• Missing HCV infection in immunocompromised patients who may not produce detectable antibodies 1