Hepatitis B

01Pathophysiology

Hepatitis B virus is a partially double-stranded DNA virus belonging to the Hepadnaviridae family. Unlike many hepatotropic viruses, HBV is not directly cytopathic. The liver injury you see in symptomatic hepatitis B is driven entirely by the host immune response. CD8+ cytotoxic T lymphocytes recognize viral antigens displayed on infected hepatocyte surfaces and mount an attack, destroying the hepatocytes in the process. This immune-mediated mechanism is the single most important concept linking pathophysiology to clinical presentation.

This explains a critical clinical paradox that exam writers love to test: neonates and immunosuppressed individuals do not mount a vigorous immune response, so they rarely develop symptomatic acute hepatitis. However, precisely because they fail to clear the virus, they have the highest rates of progression to chronic infection. Perinatal transmission results in chronic HBV in approximately 90% of infected neonates, compared to less than 5% of immunocompetent adults infected in adulthood. The weaker the immune response, the less acute illness but the greater the chronicity risk.

Once chronic infection is established, HBV has a unique ability among hepatitis viruses: it integrates its DNA directly into the host hepatocyte genome. This genomic integration is the mechanism behind the association between chronic HBV and hepatocellular carcinoma (HCC). A high-yield distinction is that HBV can cause HCC even in the absence of cirrhosis, whereas hepatitis C virtually always requires cirrhosis as an intermediary step before HCC develops. This is because HBV's oncogenic potential comes from insertional mutagenesis and expression of the HBx protein, which disrupts tumor suppressor pathways independently of the cirrhotic process.

The extrahepatic manifestations of HBV are driven by immune complex deposition. Circulating HBsAg-anti-HBs complexes deposit in vessel walls and glomeruli, producing conditions such as polyarteritis nodosa (PAN) and membranous nephropathy. The serum sickness-like prodrome (arthralgias, urticaria, fever) seen before the onset of jaundice in acute HBV is also an immune complex phenomenon. When a vignette describes a patient with joint pain and a rash that then progresses to jaundice, the test writer is pointing you toward acute hepatitis B.

02Classification and Clinical Manifestation

Phases of Chronic HBV Infection

Clinical Manifestation by Disease Stage

03Diagnostic Workup

Serologic Interpretation Table

Diagnostic Tests Summary

The best initial test when you suspect hepatitis B is HBsAg. If it is positive, the patient has active HBV infection. The next step is to determine whether this is acute or chronic by ordering anti-HBc IgM. If anti-HBc IgM is positive, the infection is acute. If it is negative and anti-HBc IgG is positive with HBsAg persisting beyond 6 months, the infection is chronic.

The window period is one of the most commonly tested serologic scenarios. During this phase, HBsAg has been cleared by the immune system, but anti-HBs has not yet appeared. The only detectable marker is anti-HBc IgM. A vignette describing a patient with clinical hepatitis and negative HBsAg, negative anti-HBs, but positive anti-HBc IgM is testing your knowledge of the window period. This is a diagnosis you can only make if you order the right test.

Differentiating vaccination from resolved infection is straightforward but frequently tested. A vaccinated individual has anti-HBs as the sole positive marker because the vaccine contains only recombinant HBsAg and does not expose the patient to core antigen. A patient with resolved natural infection will have both anti-HBs and anti-HBc IgG positive.

For chronic HBV, the HBV DNA quantitative viral load is the most accurate test for assessing viral replication and is the primary tool for monitoring treatment response. HBeAg status further refines the picture: HBeAg-positive chronic HBV indicates wild-type virus with high replication, while HBeAg-negative chronic HBV with elevated HBV DNA suggests a precore or basal core promoter mutant that cannot produce HBeAg but is still actively replicating.

Fibrosis assessment via liver biopsy or non-invasive elastography (FibroScan) is performed in chronic HBV to determine the extent of liver damage and guide treatment initiation, particularly in patients with normal or borderline-elevated ALT.

04Management and Treatment

Acute HBV in immunocompetent adults requires only supportive care. The overwhelming majority will clear the infection spontaneously and develop protective anti-HBs antibodies. Antiviral therapy is reserved for the rare cases of fulminant hepatitis (presenting with coagulopathy and encephalopathy), where entecavir or tenofovir should be started immediately alongside urgent referral for liver transplant evaluation.

For chronic HBV, treatment is indicated when the patient meets criteria suggesting active disease with risk of progression: HBV DNA >2,000 IU/mL combined with elevated ALT (greater than 2 times the upper limit of normal) or evidence of significant fibrosis on biopsy or elastography. The two first-line agents are tenofovir disoproxil fumarate (TDF) 300 mg daily and entecavir 0.5 mg daily. Both are nucleos(t)ide analogues with a high barrier to resistance and are taken indefinitely in most patients. The choice between them depends on the clinical scenario. TDF is preferred in lamivudine-experienced patients because entecavir has cross-resistance with lamivudine. Tenofovir alafenamide (TAF) 25 mg daily is a newer formulation with less renal and bone toxicity, making it the preferred tenofovir option in patients with chronic kidney disease, osteoporosis, or those on other nephrotoxic agents.

Pegylated interferon-alpha 2a (180 mcg subcutaneously weekly for 48 weeks) offers the advantage of a finite treatment course and the possibility of HBsAg seroconversion. However, it has more side effects (flu-like symptoms, cytopenias, depression) and is contraindicated in decompensated cirrhosis and pregnancy. It is best suited for young, motivated patients with HBeAg-positive chronic HBV, high ALT, low viral load, and no advanced fibrosis.

A frequently tested management concept is HBV reactivation prophylaxis. Before initiating immunosuppressive therapy (particularly rituximab, other anti-CD20 agents, high-dose corticosteroids, or TNF-alpha inhibitors), all patients must be screened with HBsAg and anti-HBc. If either is positive, prophylactic antiviral therapy with entecavir or tenofovir should be started before immunosuppression and continued for 6 to 12 months after its completion. Failure to screen is a classic exam trap.

For perinatal transmission prevention, the infant born to an HBsAg-positive mother must receive both hepatitis B immune globulin (HBIG) and the first dose of the HBV vaccine within 12 hours of birth. If the mother has a high viral load (>200,000 IU/mL or >10^6 copies/mL), she should be started on tenofovir at 28 to 32 weeks of gestation to reduce the viral load before delivery and decrease the risk of vertical transmission despite immunoprophylaxis.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students get questions on hepatitis B wrong is by misreading the serologic pattern. The entire diagnosis of HBV, its stage, and its management hinge on the correct interpretation of a handful of serologic markers. The trap is not that the markers are inherently difficult, but that students often memorize them in isolation rather than understanding the immunologic logic behind the sequence. For example, a vignette presenting a patient with clinical hepatitis, negative HBsAg, negative anti-HBs, and positive anti-HBc IgM is testing the window period. Students who have only memorized "HBsAg positive equals infection" will miss this entirely and may incorrectly conclude the patient does not have HBV.

A second common trap is confusing vaccination with resolved infection. When a vignette gives you a patient with only anti-HBs positive and all other markers negative, the answer is vaccination, not prior infection. Resolved infection always shows anti-HBc IgG positivity alongside anti-HBs because the patient was exposed to the whole virus, including the core antigen.

The third trap involves HBV reactivation in the setting of immunosuppression. A classic vignette presents a patient with resolved HBV (anti-HBs positive, anti-HBc positive, HBsAg negative) who begins rituximab or chemotherapy and then develops acute hepatitis with reappearance of HBsAg. Students who believe "anti-HBs means fully protected forever" will miss this. The tested concept is that all patients must be screened for HBV serologies before immunosuppressive therapy, and prophylactic antivirals should be initiated when indicated.

Finally, do not forget that HBV can cause hepatocellular carcinoma without cirrhosis as an intermediary step. This distinguishes it from HCV, where the pathway to HCC almost invariably passes through cirrhosis. A vignette showing a non-cirrhotic patient with chronic HBV and a liver mass should raise concern for HCC, and the question is testing whether you know that cirrhosis is not a prerequisite for HBV-associated HCC.

The core competency being tested across HBV questions is serologic interpretation, the ability to sequence the correct diagnostic workup, and recognition of clinical scenarios requiring prophylactic or therapeutic intervention before complications develop.