Skip to content

Hepatitis B Surface Antigen (HBsAg)

Published: July 27, 2026Updated: August 25, 2026

HBsAg is a protein on the surface of the hepatitis B virus (HBV) that is detected in the blood during acute or chronic HBV infection.

Quick summary

HBsAg is a key diagnostic marker for detecting current hepatitis B infection.

Medically reviewed by WellAlly Editorial TeamLast reviewed August 25, 2026Editorial policy

Key facts at a glance

Units
S/CO
Clinical utility
diagnostic
Result kind
quantitative
FDA-cleared
Roche HBsAg, Abbott HBsAg, Siemens HBsAg, Ortho HBsAg
interpretation: {"text":"Indicates current infection with the hepatitis B virus.","label":"Positive"}; {"text":"Suggests no current infection, but clinical correlation is needed.","label":"Negative"}

The presence of HBsAg in the blood indicates that the person is currently infected with the hepatitis B virus. It is one of the first markers to appear after infection and can be detected as early as 1-2 weeks post-exposure.

HBsAg testing is crucial for diagnosing acute and chronic hepatitis B infections, monitoring the progression of the disease, and assessing the need for antiviral treatment.

A negative HBsAg result generally means that the person is not currently infected with HBV, but it does not rule out the possibility of infection in the early stages or in immunocompromised individuals.

Frequently asked questions

What does a positive HBsAg test result mean?
A positive result indicates that the person is currently infected with the hepatitis B virus.
Can HBsAg be negative in early infection?
Yes, HBsAg may not be detectable in the very early stages of infection.
  1. 1
    Centers for Disease Control and Prevention (CDC) - Hepatitis B TestingSource
  2. 2
    World Health Organization (WHO) - Hepatitis B FactsheetSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 25, 2026. · Read our editorial policy