Before starting a biologic, a thiopurine, rituximab, high-dose steroids or chemotherapy, check HBsAg and anti-HBc, not HBsAg alone. A patient who is HBsAg-negative but anti-HBc-positive has been infected and carries a real risk of reactivation, and that risk is highest with B-cell depleting therapy. Anti-HBs does not cancel it.
Reactivation presents late and badly — hepatitis, sometimes fulminant, weeks to months after the drug started, at which point stopping the immunosuppression does not reverse it. In a country with intermediate hepatitis B endemicity this is not an unusual scenario, and the test costs a fraction of the drug you are about to prescribe. Order both markers with the baseline bloods, write the result where the prescriber will see it, and record the plan for prophylaxis rather than leaving it to be decided if something happens.
- Order HBsAg and anti-HBc together — HBsAg alone misses occult infection
- Treat anti-HBc positivity as a reactivation risk regardless of anti-HBs status
- Assess risk by drug class; B-cell depleting therapy carries the highest
- Record the prophylaxis decision in the notes, not just the serology result
- Recheck before any change to a higher-risk immunosuppressive agent
Why it matters
Reactivation is preventable before the first dose and largely irreversible once it presents.
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