Antibody Screen — Hepatitis C

Test Characteristics

Metric	Value
False-negative rate	2% (sensitivity 98%)
False-positive rate	0.5% (specificity 99.5%)
Bayes factor (positive test)	100×
Bayes factor (negative test)	1/50×
Base rate	2% of US adults

Interpreting Results

Scenario	Prior	+ Result	− Result
Average US adult	2%	100 × 2% ≥ 100%2%× 100100%	1/50 × 2% = 0.04%2%÷ 500.04%
Baby boomer (born 1945–1965)	5%	100 × 5% ≥ 100%5%× 100100%	1/50 × 5% = 0.1%5%÷ 500.1%
History of IV drug use	40%	100 × 40% ≥ 100%40%× 100100%	1/50 × 40% = 0.8%40%÷ 500.8%

100 × \textprior ≥ 100%\textprior× 100100% for all rows: exact posteriors are 67% (average), 84% (baby boomer), 99% (IV drug use). Positive antibody may indicate cleared past infection — confirmatory HCV RNA always required.

• + result: risk goes from red hair (2%) to 67%
• − result: risk drops to near fatal car crash (0.04%)

Sources:

• Systematic review, Sci Rep, 2019. Nature.
• PMC. Hepatitis C screening.
• CDC. Laboratory guidelines for HCV antibody testing.