Hepatitis B Surface Antibody (Anti-HBs), Quantitative: Proving Immunity and What the 10 mIU/mL Threshold Means

At a Glance

This test measures how much hepatitis B surface antibody — anti-HBs — is in your blood, as a number in mIU/mL. Your immune system makes it after a hepatitis B vaccine series or after recovering from infection, so it marks immunity rather than infection.

The number that matters is 10. CDC treats anti-HBs at 10 mIU/mL or above, one to two months after finishing a series, as the marker of vaccine-induced protection. Below that, protection is not documented by this test — a different statement from being unprotected.

What "Qn" means on your report

Qn is not part of the test's name. It is the scale: quantitative, meaning a number rather than a word. The LOINC concept for this observation carries the scale value Qn with example units of mIU/mL.

The same antibody is also run qualitatively, classified as Reactive or Non-Reactive. A quantitative result supplies the concentration, which allows a direct comparison against a threshold. A qualitative classification has a cutoff behind it too, and some reports carry both — the University of Iowa reports reactivity, immune status and a quantitative concentration together — so Reactive alone does not establish that no number exists. Read the whole report.

The 10 mIU/mL threshold, and the grey zone just underneath it

CDC's guidance for health-care personnel states that immunocompetent adults and children with vaccine-induced anti-HBs of 10 mIU/mL or above are seroprotected, and its 2023 screening recommendations use the same figure, measured one to two months after a series.

Laboratories do not all draw the line identically, which is where results near 10 get confusing. Labcorp reports immunity above 10.0 mIU/mL. The University of Washington states that per the manufacturer's insert the protective level is 12 or above. The University of Iowa reports the same antibody by electrochemiluminescence immunoassay in three tiers: non-reactive below 8.5, indeterminate from 8.5 up to 11.5, reactive at 11.5 or above.

So a result of 9, or 11, can be called non-reactive, indeterminate or reactive depending on the laboratory. Within a few units of 10, the reference information on your own report decides.

How to read your result

Read the number against your own report's reference interval, then against your question.

At or above 10 mIU/mL, with a documented complete series and no immune globulin in the last six months, documents protection; for a healthy adult that is the end of the question. Well above 100 is the same answer more emphatically: no guidance behind this page states an upper threshold, so a figure in the hundreds or above a thousand is a strong response, and a high flag comes from the stored bound rather than a clinical ceiling.

Just under 10 is the ambiguous zone above — indeterminate or non-reactive by assay.

Clearly below 10 means protection is not documented by this test. The reasons — never vaccinated, an incomplete series, measurement too soon, antibody declined over years, or a genuine failure to respond — differ in what follows, and the section below separates them.

Any result leaves one question open: this test does not tell you whether you have hepatitis B. Only the full panel does.

One framing note: CDC's Pink Book states routine serologic testing of vaccinated people is not recommended, directing post-vaccination testing at named groups — health-care personnel, hemodialysis patients, immunocompromised people, infants of surface-antigen-positive mothers — but what happens within them differs. For health-care personnel it is documenting a response once. For hemodialysis patients the need for boosters "should be assessed by annual testing", with "a booster dose ... when antibody levels decline below 10 mIU/mL". Immunocompromised people "might need annual testing". What follows about a declining number needing no action applies to immunocompetent documented responders.

If your result is below 10

What happens next is a clinical decision with more than one acceptable route. Where a response was never documented, ACIP describes two options — a second complete series followed by anti-HBs testing, or a single additional dose followed by testing one to two months later. Route and schedule depend on the product: a series is two doses at 0 and 1 month for one vaccine and three at 0, 1 and 6 for others, and the two-dose product may be used even after a non-response to another's.

Non-response is defined as anti-HBs below 10 mIU/mL after two complete series, not after a dose count — and current infection has to be excluded before settling on it. One low result after one series is nowhere near that point.

If you were never vaccinated or did not finish, ACIP has recommended vaccination for adults 19 to 59 since 2022, and for those 60 and over with risk factors.

A number below 10 in someone who previously documented a response — an anti-HBs at or above 10 after a completed series — is a different situation, and the reason is immune memory. For immunocompetent people CDC's Pink Book states immune memory remains intact for more than thirty years and that those whose antibody has declined remain protected against significant infection. Protection is not an open question across the timescales studied; the endpoint is.

The distinction that matters is between completing a series and documenting a response to it: without a post-series result at or above 10 on record, a low number cannot be read as waning.

Proving immunity for work, school or travel

This is why a quantitative anti-HBs is often ordered: it supplies a concentration that can be compared against whatever threshold is asked for. A qualitative result carries a cutoff too and some reports give both, so the practical question is what the requesting institution specifies.

What makes the documentation hold up is the pairing: the result, and the vaccination history it is read against. A figure at or above 10 with a documented series is consistent with vaccine-induced protection; the same figure with no vaccination record is equally consistent with past infection, and only core antibody separates them.

One more thing can supply it. Hepatitis B immune globulin transfers anti-HBs passively rather than provoking it, and CDC states it "can provide anti-HBs for 4-6 months after administration; therefore, testing for anti-HBs ≤6 months after HBIG administration is not an accurate measure of a person's immune status." A protective-looking figure within six months of immune globulin — after an exposure, or at birth to a surface-antigen-positive mother — does not document a vaccine response, whatever the vaccination record says. At the other end, measuring sooner than one to two months after the last dose can understate it.

If you remember only one thing

Ten is a floor, not a target, and this number answers one question. At or above 10 after a complete series documents protection, unless immune globulin in the last six months supplied the antibody. Below 10 means protection is not documented — not that you are unprotected, and not that you have hepatitis B, which is a different test on the same panel.

Two people, same result

Two people both get 4 mIU/mL.

The first is 24, finished a three-dose series six weeks ago for a nursing placement, with no other history. Her result is a first post-vaccination measurement below the threshold, so she is at the start of a defined protocol — one route is a single further dose and a retest in one to two months — not at the end of one.

The second is 58, vaccinated in his thirties, with a record showing both the completed series and an anti-HBs of 140 mIU/mL measured two months afterwards. His 4 mIU/mL today is a declined level in a responder whose response was documented at the time, and for an immunocompetent person CDC's Pink Book puts immune memory at more than thirty years.

Change one detail and the second becomes the first: had his record shown the doses but no post-series result, his 4 would be an undocumented response, not a waned one. The number is identical in all three; what separates them is not in the result.

What this result cannot tell you

Whether you have hepatitis B. Anti-HBs is the immunity marker; the infection markers are HBsAg and core antibody, and CDC's 2023 recommendations put all adults aged 18 and over on once-in-a-lifetime screening with all three together. This test is one leg of that panel.

Whether antibody came from a vaccine, a past infection, or immune globulin. Total anti-HBc separates vaccination from resolved infection; recent HBIG is a question of timing.

Whether you are protected, as against whether protection is documented. A figure below 10 in a documented responder is not evidence of susceptibility.

And it is not absolute even when high: Labcorp states on its own assay that the presence of anti-HBs is not an absolute indicator of resolved hepatitis infection nor of protection from future infection.

Common interpretation mistakes

Reading a high number as too much antibody. There is no upper threshold in the guidance; a high flag comes from a stored bound.

Treating a figure a unit or two below 10 as settled. Assays differ — one reports an indeterminate band from 8.5 to 11.5.

Concluding non-response from one low result. The definition requires anti-HBs below 10 after two complete series, with infection excluded.

Reading this test as a hepatitis B screen. It cannot detect infection, and a normal result here says nothing about HBsAg.

Taking the antibody as proof of vaccination without the record. Past infection produces anti-HBs too, and core antibody is what separates them.

Reading a low number as waning without a documented prior response. Without a post-series result at or above 10 on record, there is nothing established to have waned from.

Measuring too soon. CDC's window is one to two months after the last dose.

Questions your doctor may ask

Have you had a hepatitis B vaccine series, how many doses, and do you have the dates? Was your antibody ever checked after that series, and what was the result? Have you had hepatitis B immune globulin in the last six months, after an exposure or at birth? Are you on dialysis, immunosuppressed, or living with HIV? Are you being asked for proof of immunity, and does the requester specify a threshold? Have you ever been told you had hepatitis B? Were you born in, or have you lived in, a country where hepatitis B is common?

Read together with

The other two legs answer what this one cannot. HBsAg marks current infection. Total anti-HBc indicates past or present infection and separates vaccine-induced antibody from antibody left by a resolved one; IgM core antibody then distinguishes a recent infection from an older one. CDC's 2023 recommendation is to run all three together. If this antibody was also run qualitatively, that is the same antibody classified as Reactive or Non-Reactive.

Clinical pearls

The threshold belongs to the assay, not the antibody. Ten, twelve, and an indeterminate band from 8.5 to 11.5 are all in use, and a result near the line is interpreted by the report it arrived on.

Measurable antibody and protection are not the same quantity, and one can decline while the other persists — in immunocompetent people CDC puts immune memory at more than thirty years. What is under study is the endpoint, not whether decades of protection exist.

Clinical Takeaway

Anti-HBs indicates immunity to hepatitis B, not infection with it. CDC treats 10 mIU/mL or above, one to two months after a completed series, as documenting vaccine-induced protection and states no upper limit, so a high figure is a strong response and a high flag reflects a stored bound. Results within a unit or two of 10 depend on the assay, one reporting an indeterminate band from 8.5 to 11.5. Where no response was ever documented, a result below 10 leads to clinician-managed revaccination — a further series or a single dose — then retesting; non-response is defined only after two complete series with infection excluded. A declined level in a documented responder is separate: for an immunocompetent person CDC puts immune memory at more than thirty years, while dialysis responders have a recommended annual test with a booster below 10 and immunocompromised people "might need annual testing". Recent immune globulin can supply the antibody, making a result within six months uninterpretable. And whatever the number, this test cannot tell you whether you have hepatitis B.

In one sentence

This number says whether protection is documented, not whether you are infected, and 10 mIU/mL is a floor rather than a target.

Bottom line

At or above 10 with a record of a complete series, you have what a proof-of-immunity request asks for — unless you had immune globulin within the last six months, which supplies the antibody itself and makes the result uninterpretable. Below 10, bring your vaccination dates and any previous antibody result to your clinician; what follows depends on those more than on the number.

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