Hepatitis B Surface Antigen
HBsAg· also: HepBsAg, Hepatitis B surface antigen
Clinical Overview
HBsAg (Hepatitis B Surface Antigen) is the first serological marker to appear after Hepatitis B virus (HBV) infection, detectable 1–10 weeks after exposure — before symptoms or liver enzyme abnormalities appear. A positive HBsAg indicates that Hepatitis B virus is present and the person is infectious. It is the standard screening test for Hepatitis B infection worldwide.
Why This Test Matters
HBsAg distinguishes active Hepatitis B infection (acute or chronic) from past resolved infection or vaccination. If HBsAg persists beyond 6 months, the infection is classified as chronic. Chronic HBV affects over 290 million people worldwide and is a leading cause of cirrhosis and hepatocellular carcinoma (liver cancer). All HBsAg-positive patients require further workup: HBeAg/Anti-HBe status, HBV DNA quantification (viral load), liver function tests (ALT, AST), and assessment for treatment eligibility. HBV DNA >2,000 IU/mL with elevated ALT is the threshold for treatment with oral antivirals (tenofovir, entecavir).
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| Negative / Non-reactive | qualitative | Negative = uninfected; Positive = active infection |
Also reported in: S/CO ratio.
What Causes Abnormal Results?
High HBsAg Causes
- Acute Hepatitis B infection (recent exposure within 1–6 months)
- Chronic Hepatitis B infection (HBsAg positive >6 months)
- Vertical transmission from mother to newborn (most common route in endemic areas)
- Unprotected sexual contact with infected person
- Shared needles (IV drug use)
- Blood transfusion (rare in countries with screening programs)
Low HBsAg Causes
- A negative HBsAg means no active Hepatitis B infection (does not distinguish between never infected, resolved infection, or successfully vaccinated)
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting required. A reactive (positive) HBsAg result on initial testing should be confirmed with a repeat HBsAg test on the same sample using a neutralization assay or a second method, per standard laboratory protocols.
Factors That Can Affect Results
- HBsAg can be transiently positive for 1–18 days after HBV vaccination (very low-level, vaccine-induced)
- Some immunocompromised patients may have false-negative HBsAg despite HBV infection (test HBV DNA if clinical suspicion is high)
- High-dose biotin supplementation can interfere with some biotin-streptavidin immunoassays
Related Topics
Frequently Asked Questions
What does a positive HBsAg mean and what should I do next?
A positive HBsAg means Hepatitis B virus is present in your blood and you are infectious to others. You need further evaluation: HBeAg (measures viral replication activity), HBV DNA viral load, liver function tests (ALT, AST), and abdominal ultrasound. If HBsAg remains positive for more than 6 months, it is classified as chronic Hepatitis B. Treatment with oral antivirals (tenofovir or entecavir) may be recommended depending on your viral load and liver status. All household members and sexual partners should be tested and vaccinated if not immune.
I was vaccinated against Hepatitis B — can I still test positive for HBsAg?
Very rarely. The HBsAg from vaccination can be transiently detectable for up to 18 days after receiving the vaccine, but only in some sensitive assay systems and only at very low levels. After this window, vaccination does not cause a positive HBsAg. What vaccination does produce is Anti-HBs (protective antibody) — a positive Anti-HBs with negative HBsAg confirms successful vaccination immunity.
Can Hepatitis B be cured?
Acute Hepatitis B resolves spontaneously in over 95% of immunocompetent adults — the immune system clears the virus. Chronic Hepatitis B (persisting >6 months) currently cannot be fully cured (the virus integrates into liver DNA), but modern antivirals (tenofovir alafenamide, entecavir) can suppress viral replication to undetectable levels, preventing liver damage, cirrhosis, and cancer. The goal of treatment is functional cure: HBsAg clearance with Anti-HBs seroconversion, which occurs in ~10–15% of treated patients after years of therapy.