Reflex to Hepatitis B Core IgM antibody
Other names: Rfx to HBc IgM
Rfx to HBc IgM (Reflex to Hepatitis B Core IgM Antibody)
What this test measures
The Rfx to HBc IgM test looks for IgM antibodies to the Hepatitis B core antigen, which are produced by your immune system during the early phase of a Hepatitis B infection.
“Rfx” means reflex testing — this test is automatically performed only if an initial Hepatitis B screening test (usually HBsAg) comes back abnormal or positive. It helps confirm whether an infection is recent or acute, rather than past or chronic.
Why this test is important
Hepatitis B is a viral infection that affects the liver. Early diagnosis is essential to prevent liver inflammation and long-term complications, such as:
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Liver damage (hepatitis)
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Cirrhosis (scarring of the liver)
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Liver cancer (hepatocellular carcinoma)
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Liver failure
When Hepatitis B enters the body, HBc IgM antibodies appear early in infection. Detecting them helps doctors determine that a person has new, active Hepatitis B rather than a long-standing infection or immunity from vaccination.
Because this is a reflex test, you may see the test name listed even if it was not actually performed. Many labs only run it if screening results suggest Hepatitis B infection.
Common symptoms of acute Hepatitis B
Some individuals have no symptoms, but possible signs include:
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Fatigue and weakness
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Loss of appetite, nausea, or vomiting
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Abdominal discomfort, especially on the right side
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Dark urine and pale stools
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Yellowing of the skin or eyes (jaundice)
Even without symptoms, early identification matters to protect your liver and reduce transmission to others.
When this test is ordered
This test is commonly ordered when:
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A Hepatitis B surface antigen (HBsAg) test is positive
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There are symptoms of acute hepatitis
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Liver enzymes (ALT/AST) are elevated
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A doctor suspects recent Hepatitis B exposure
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Someone has risk factors such as unprotected sex, IV drug use, or exposure to infected blood
Next steps if your HBc IgM is positive
Your healthcare provider may recommend:
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Additional Hepatitis B testing (HBsAg, anti-HBs, HBV DNA)
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Antiviral treatment, if needed
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Monitoring liver function
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Avoiding alcohol to reduce liver stress
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Vaccination or testing for household/sexual contacts
Early follow-up is important for protecting liver health and preventing transmission.
Key takeaways
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Rfx to HBc IgM is a reflex test for recent/acute Hepatitis B.
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It measures early-phase immune response antibodies.
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A positive result typically means recent infection.
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It helps distinguish new infection vs. chronic infection or vaccination immunity.
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