Hepatitis B Serology: HBsAg, Anti-HBc & Window Period

Creator: Progress Note
Published: 2026-08-27
Keywords: hepatitis B, hepatitis b serology usmle, hbsag anti hbs anti hbc, window period hepatitis b, hbeag infectivity, hepadnavirus reverse transcriptase, hepatitis d superinfection, hbv hcc without cirrhosis, polyarteritis nodosa hepatitis b, hbig newborn

bolt The 60-second version

Hepatitis B virus (HBV) is the most heavily tested hepatitis virus on Step 1 because its serologic markers tell a story: who is infected, who is immune, who is contagious, and who is in the window period.

check_circleHBV is a hepadnavirus: enveloped, partially double-stranded circular DNA that replicates through an RNA intermediate using its own reverse transcriptase.
check_circleHBsAg = current infection (acute or chronic; > 6 months = chronic). Anti-HBs = immunity. HBeAg = active replication and high infectivity.
check_circleAnti-HBc IgM marks acute infection and is the only positive marker in the window period; anti-HBc IgG marks past or chronic infection. Vaccination produces anti-HBs only.
check_circleChronicity depends on age at infection: about 90% of infected newborns, but fewer than 5% of healthy adults.
check_circleHBV can cause hepatocellular carcinoma even without cirrhosis because viral DNA integrates into the host genome.
check_circleHepatitis D (a defective RNA virus) needs HBsAg to infect; superinfection of a chronic HBV carrier is more severe than coinfection.

target What you'll be able to do · aligned to USMLE Step 1 · Gastrointestinal; Microbiology

MICRODescribe HBV structure, replication by reverse transcription, and transmission.
MICROInterpret HBV serologic panels, including the window period and vaccination status.
PATHRelate age at infection to chronicity, and identify cirrhosis, HCC, and extrahepatic complications.
PHARMDescribe vaccination, HBIG, hepatitis D, and antiviral treatment.

How hepatitis B is tested on Step 1

Most questions give a serology panel and ask for the patient's status. Others test virology, transmission, or associated diseases.

HBsAg	Anti-HBs	Anti-HBc	HBeAg	Interpretation
+	−	IgM	+	Acute infection, highly infectious
−	−	IgM	−	Window period (recovering acute infection)
−	+	IgG	−	Recovered, immune from natural infection
−	+	−	−	Immune from vaccination
+	−	IgG	+ or −	Chronic infection (HBeAg+ = high replication)

HBcAg itself is not detected in serum; only antibodies to it are measured.

Window period. After HBsAg disappears and before anti-HBs appears, anti-HBc IgM is the only positive marker. Timing varies between patients.

Virology and transmission

HBV is a DNA virus that uses reverse transcriptase.

Structure: enveloped hepadnavirus with partially double-stranded circular DNA; DNA polymerase with reverse transcriptase activity.
Transmission: blood, sexual contact, and perinatal (mother to child). It is far more infectious by needlestick than HCV or HIV.
Histology: "ground-glass" hepatocytes from accumulated HBsAg; liver damage is mainly from the cytotoxic T-cell response, not the virus itself.
Chronicity by age: about 90% of perinatal infections, 25–50% of infections at ages 1–5, and fewer than 5% of adult infections become chronic.

Hepatitis D is a defective RNA virus that uses HBsAg as its envelope. Preventing HBV prevents HDV.

Complications and extrahepatic disease

Chronic HBV causes liver and immune-complex disease.

Chronic HBV complications. Surveillance for HCC is recommended in high-risk chronic carriers.

Prevention and treatment

Know the vaccine and the antiviral classes.

Vaccine: recombinant HBsAg (a subunit vaccine) → anti-HBs only. Given to all infants starting at birth.
Infants of HBsAg-positive mothers: hepatitis B vaccine plus hepatitis B immune globulin (HBIG) within 12 hours of birth.
Chronic HBV drugs: nucleos(t)ide reverse transcriptase inhibitors such as tenofovir and entecavir (suppress replication); pegylated interferon-α in selected patients.
Needlestick from an HBsAg-positive source: unvaccinated people receive HBIG and the vaccine series.

Worked example

Read the panel step by step.

functions Worked example — the confusing panel

A 30-year-old had jaundice 3 months ago that has resolved. Labs: HBsAg negative, anti-HBs negative, anti-HBc IgM positive, HBeAg negative. What is her status?

HBsAg negative: no detectable surface antigen now.
Anti-HBs negative: no protective antibody yet.
Anti-HBc IgM positive: recent acute infection.
This combination is the window period between HBsAg clearance and anti-HBs appearance.

Answer. Window period of a resolving acute hepatitis B infection.

Next in the roadmap

Hepatitis C

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3 real questions on Hepatitis B

Straight from the Progress Note question bank, each with a cited explanation. No account needed.

A liver biopsy from a patient with chronic hepatitis B infection shows ground-glass hepatocytes on histology. Which intracellular process is most directly responsible for this appearance?

Image for question 1 — Mark ong, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

A 52-year-old woman has fatigue and pruritus. Laboratory studies show an elevated alkaline phosphatase with normal transaminases and a positive antimitochondrial antibody. Liver biopsy is shown below. Which statement about the target of the antimitochondrial antibody and its relationship to the biopsy findings is correct?

Image for question 2 — Nephron, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

A 58-year-old man with chronic hepatitis B undergoes contrast-enhanced CT of the abdomen for surveillance. Imaging reveals a 3 cm hepatic lesion that demonstrates arterial phase hyperenhancement with subsequent venous phase washout. Alpha-fetoprotein is elevated. Which underlying pathophysiologic process most directly explains these imaging findings?

Image for question 3 — Zhenyu Pan, Guozi Yang, Tingting Yuan, Lihua Dong, Lihua Dong, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Frequently asked questions

What marker is positive in the hepatitis B window period?expand_more

Anti-HBc IgM; HBsAg and anti-HBs are both negative.

How do you tell vaccination from natural immunity?expand_more

Vaccination gives anti-HBs alone; natural immunity gives anti-HBs plus anti-HBc IgG.

What does HBeAg indicate?expand_more

Active viral replication and high infectivity.

Why can HBV cause liver cancer without cirrhosis?expand_more

HBV DNA integrates into the host genome and can disrupt growth-control genes.

References & further reading

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