Infection / Serology
Hepatitis B surface antigenHBsAg
Marker of active HBV infection (acute or chronic).
Sample
Serum
Biology
What it measures
Envelope glycoprotein produced in massive excess by infected hepatocytes.
Clinical use
Why we order it
HBV screening; positive >6 mo defines chronic HBV.
Pitfalls
Pre-analytic & interpretation traps
- !Window period: HBsAg may be negative early — use HBV DNA or anti-HBc IgM for acute hepatitis workup.
References
Primary literature
- AASLD HBV Guidelines — AASLD 2018