The Hepatitis B Surface Antibody Quantitative test measures the level of hepatitis B surface antibodies (anti-HBs) in your blood in mIU/mL (milli-International Units per milliliter). It may appear on your lab report as Hep B Surf Ab Quant, Hepatitis B Surface Ab, QN, or Hepatitis B Surface Ab Immunity, QN.
This level tells you whether you have sufficient immunity against hepatitis B virus (HBV), either from vaccination or past infection. A result flagged as low means your antibody level is below the protective threshold of 10 mIU/mL — the standard cutoff used by the CDC, WHO, and most clinical guidelines to define immunity. It does not mean you have hepatitis B; it means your current antibody level is not considered sufficient to protect against infection.
At a Glance
- The protective threshold for hepatitis B surface antibody is ≥10 mIU/mL
- A result below 10 mIU/mL is considered non-immune — insufficient protection against HBV
- The most common cause is waning immunity after vaccination — a normal decline in antibody levels over time
- The specific values <3.5 and <3.1 are LabCorp reporting thresholds, not your actual measured values — they mean antibodies were undetectable on that assay
- A low result does not mean you currently have hepatitis B — further testing would be needed to determine that
- If you are a healthcare worker or at ongoing exposure risk, a level below 10 mIU/mL should be treated as non-immune and addressed promptly
- Next steps typically involve hepatitis B vaccination or a booster dose
What does "Hep B Surf Ab Quant Low" mean?
A low result means the quantitative level of anti-HBs antibodies in your blood was below 10 mIU/mL — the level considered protective against hepatitis B infection. The result is reported as low because it falls below the laboratory's reference range.
The most common cause is waning immunity after vaccination — a normal decline in antibody levels over time. This does not necessarily mean you are unprotected: immune memory cells can still mount a rapid response to a booster dose or real exposure even when circulating antibody is low. For clinical purposes, however, a level below 10 mIU/mL is treated as non-immune and vaccination is usually recommended.
Other reasons for a low result include never having been vaccinated, being a vaccine non-responder (a small proportion of people do not develop adequate antibody after a standard vaccine series), or being immunocompromised.
What do Hep B Surf Ab Quant 3.5, 3.1, or values with "<" mean?
If your result shows <3.5 mIU/mL, <3.1 mIU/mL, or a similar value prefixed with "<", this is a LabCorp reporting convention — not an actual measured antibody level. It means hepatitis B surface antibodies were below the lower limit of quantification for the assay. LabCorp uses 3.5 mIU/mL as the detection floor on one assay and 3.1 mIU/mL on another. The "<" symbol means "less than" this limit.
In all of these cases, antibodies were not detected at a quantifiable level — equivalent to a negative or non-immune result, and well below the protective threshold of 10 mIU/mL. The clinical meaning is identical regardless of whether your report shows <3.5, <3.1, or any similar below-limit value.
Understanding the reference range
| Result | Interpretation |
|---|---|
| ≥10 mIU/mL | Protective — sufficient immunity against hepatitis B |
| 5–9.9 mIU/mL | Borderline — may warrant repeat testing or a booster dose (often reported as low) |
| <5 mIU/mL | Non-immune — no meaningful antibody protection detected (often reported as negative or low) |
| <3.5 or <3.1 mIU/mL | LabCorp below-limit-of-detection reporting — equivalent to non-immune |
The 10 mIU/mL threshold is the internationally accepted minimum protective level defined by the CDC and WHO. Some laboratories use 12 mIU/mL as their cutoff — always refer to your specific lab's reference range. The HealthMatters optimal range for this test is set at 9.9–1000 mIU/mL, consistent with clinical guidelines.
What is "Hepatitis B Surface Ab Immunity, QN"?
This is a LabCorp test name variant for the same quantitative anti-HBs test. "Immunity" reflects its purpose — assessing hepatitis B immune status. "QN" means quantitative, indicating the result is a number in mIU/mL rather than just reactive or non-reactive. A low result on this test carries the same clinical meaning as described above.
Why does my portal say "Warning Low" or "Patient does not have immunity to hepatitis B virus"?
It does not mean you have hepatitis B — it means your current antibody level is below the protective threshold. "Warning Low" or "WarningLow Hepatitis B Surf Ab Quant" is a standard lab flag for any result below the reference range. The phrase "patient does not have immunity to hepatitis B virus" is an interpretive comment added by some labs and portals to clarify the significance of a sub-threshold result. Vaccination or a booster dose is typically the next step.
What are the next steps after a low result?
If you have been vaccinated previously: A low result after vaccination usually means antibody levels have declined over time — a normal pattern. A single booster dose is often sufficient to restore a protective level. Your doctor will determine whether a booster or a full re-vaccination series is appropriate based on your vaccination history.
If you have never been vaccinated: A primary hepatitis B vaccine series (three doses over six months) is recommended. Testing after completing the series confirms that a protective level has been achieved.
If you are a vaccine non-responder: A small proportion of people — roughly 5–10% of healthy adults — do not develop adequate antibody after a standard vaccine series. A repeat series is usually tried; those who still do not respond are considered non-responders. This reflects individual immune variation, not a harmful effect of the vaccine. Non-responders in high-risk settings require specific management guidance from their doctor.
If you are a healthcare worker or at ongoing exposure risk: A level below 10 mIU/mL should be treated as non-immune and addressed promptly. The CDC recommends that healthcare workers who do not achieve ≥10 mIU/mL after two full vaccine series be counselled on susceptibility and the need for post-exposure prophylaxis if an exposure occurs.
Hepatitis B Surf Ab Quant
Every lab result you’ve ever had, charted and explained.
Healthmatters is a tracker for your lab results. Upload a report from any lab, any country — every marker charted and explained.
Start with my report →
Frequently Asked Questions
What does "Hep B Surf Ab Quant Low" mean?
What do Hep B Surf Ab Quant 3.5, 3.1, or values with "<" mean?
What is the normal range for Hep B Surf Ab Quant?
What is "Hepatitis B Surface Ab Immunity, QN"?
Why does my portal say "patient does not have immunity to hepatitis B virus"?
Why would my hep B antibodies be low after vaccination?
Does a low result mean I have hepatitis B?
What is a protective level of hepatitis B surface antibody?
All our content is backed by peer-reviewed studies, academic research, and trusted medical sources. We’re committed to accuracy and transparency — see our editorial policy for details.
-
Hepatitis B Surface Antibody, Quantitative (Test 006530).
-
Hepatitis B Virus Surface Antibody, Qualitative/Quantitative, Serum.
-
Hepatitis B Surface Antibody (Quantitative).
-
CDC guidance for evaluating health-care personnel for hepatitis B virus protection and for administering postexposure management
-
Hepatitis B Vaccination, Screening, and Linkage to Care: Best Practice Advice From the American College of Physicians and the Centers for Disease Control and Prevention