Hepatitis B is diagnosed with a simple blood test, most commonly a panel that checks for three markers at once: a surface antigen (HBsAg), a surface antibody (anti-HBs), and a core antibody (anti-HBc). Together, these three results tell your doctor whether you have an active infection, had one in the past, or are immune from vaccination. Results are typically available within a few days, and the test requires only a single blood draw.
The Three-Marker Blood Panel
The standard hepatitis B screening panel looks for three things in your blood, and each one answers a different question.
HBsAg (surface antigen): This is a protein that sits on the surface of the virus. If it shows up in your blood, it means the virus is present and you are currently infectious. A positive result can indicate either a new (acute) or long-standing (chronic) infection. One important note: HBsAg can show up as a false positive within 30 days of receiving a hepatitis B vaccine dose, since the vaccine itself contains this protein.
Anti-HBs (surface antibody): This is an antibody your immune system makes in response to the surface antigen. A positive result generally means you are immune to hepatitis B, either because you recovered from a past infection or because you were vaccinated. Antibody levels can decline over time after vaccination, but most people still mount a protective response if exposed to the virus again.
Anti-HBc (core antibody): This antibody appears when your body has been exposed to the actual virus, not the vaccine. It shows up at the onset of symptoms and persists for life. A positive result tells your doctor you were infected at some point, though it doesn’t specify when. Crucially, people who are immune only through vaccination will never test positive for this marker, which is what makes it useful for distinguishing vaccine immunity from natural immunity.
Reading Your Results
The combination of these three markers paints a clear picture. If all three are negative, you have never been infected and are not immune, meaning vaccination is recommended. If only anti-HBs is positive, you are immune from vaccination. If anti-HBs and anti-HBc are both positive but HBsAg is negative, you recovered from a past infection and are now immune.
The result that prompts further action is a positive HBsAg. This means you have an active infection. At that point, additional testing is needed to determine whether the infection is new or chronic, and how much damage, if any, has occurred.
Distinguishing Acute From Chronic Infection
A single positive HBsAg result doesn’t tell you how long you’ve been infected. To figure that out, your doctor will order a test for a specific type of core antibody called IgM anti-HBc. This particular antibody spikes early in infection and fades over several months. If IgM anti-HBc is positive, the infection is likely acute, meaning it developed recently. If it’s negative but HBsAg remains positive, the infection is chronic.
The distinction matters because most adults who get acute hepatitis B clear the virus on their own within six months. Chronic infection, on the other hand, means the virus has persisted and may require long-term monitoring or treatment. An infection is formally classified as chronic when HBsAg remains positive for longer than six months.
Viral Load Testing
Once a chronic infection is confirmed, your doctor will measure how much virus is circulating in your blood. This is done through a test that detects and quantifies the virus’s genetic material, giving a number reported in international units per milliliter (IU/mL).
This viral load number helps guide treatment decisions. Adults with a low viral load (under 2,000 IU/mL) and normal liver enzyme levels typically do not need antiviral treatment right away, because these numbers suggest the virus is not actively damaging the liver. Higher viral loads, especially combined with elevated liver enzymes, point toward active disease that may benefit from treatment. Pregnant women with viral loads above 200,000 IU/mL are recommended to start antiviral therapy during their third trimester to reduce the risk of passing the virus to their baby.
Liver Enzyme Levels
Alongside viral load, doctors check liver enzymes, particularly ALT, through a standard blood test. ALT is released into the bloodstream when liver cells are damaged, so elevated levels suggest the virus is causing inflammation. Most people with chronic hepatitis B are asymptomatic and have normal ALT, meaning the virus is present but not actively harming the liver. When ALT levels are persistently elevated, it signals that the infection is in a more active phase and the liver is under stress.
Viral load and ALT are typically monitored together over time, since both can fluctuate. A single set of normal results doesn’t guarantee the infection will stay quiet, which is why people with chronic hepatitis B need regular follow-up blood work, often every six to twelve months.
Assessing Liver Scarring
For people with chronic hepatitis B, especially those with elevated liver enzymes, doctors need to evaluate whether the liver has developed scarring (fibrosis) or more advanced scarring (cirrhosis). Two main tools are used for this.
A FibroScan is a noninvasive imaging test that measures liver stiffness using sound waves. It takes about ten minutes, feels similar to an ultrasound, and gives an immediate result. Studies show FibroScan has high accuracy for detecting cirrhosis, with diagnostic performance scores around 0.89 to 0.90 on a scale where 1.0 is perfect. It is less reliable for detecting moderate, intermediate-stage fibrosis.
A liver biopsy, where a small tissue sample is taken with a needle, remains the most definitive way to assess fibrosis at all stages. It’s more invasive and requires local anesthesia, but it provides a detailed look at the liver’s internal structure. Because FibroScan struggles with middle-range fibrosis, biopsy is still recommended when the degree of scarring is unclear and would change treatment decisions.
Testing Infants Born to Positive Mothers
Babies born to mothers with hepatitis B receive a vaccine dose and an immune globulin injection within 12 hours of birth. To confirm whether the baby was protected, follow-up blood testing is done at approximately 9 to 12 months of age. This test checks for both HBsAg (to see if the baby became infected) and anti-HBs (to confirm the vaccine produced immunity). Testing is done at this age rather than earlier to allow time for the vaccine series to be completed and for maternal antibodies to clear.
Who Should Get Tested
The CDC recommends that all adults be screened for hepatitis B at least once in their lifetime using the three-marker panel. This is a relatively recent shift from older guidelines that only recommended testing for people in higher-risk groups, such as those born in regions where hepatitis B is common, people who inject drugs, or healthcare workers exposed to blood. The rationale for universal screening is straightforward: most people with chronic hepatitis B have no symptoms and can unknowingly transmit the virus or develop liver damage over decades without knowing they’re infected.
Additional testing is recommended for pregnant women during each pregnancy, regardless of prior results, and for anyone with ongoing risk factors such as new sexual partners or occupational exposure to blood.