Immunoglobulin M (IgM) Unit Converter
Immunoglobulin M (IgM) Unit Converter
Convert IgM between g/L and mg/dL — and read it knowing the one thing IgM tells you that IgG cannot: a raised specific IgM points to recent infection, while a raised specific IgG points to past exposure or immunity.
Immunoglobulin M (IgM) converter
g/L ⇄ mg/dLIgM 1.2 g/L, reported with IgG 11.0 g/L and IgA 2.0 g/L
The conversion, and why there is no molar unit
mg/dL = g/L × 100
and there is no nmol/L or µmol/L, because polyclonal IgM has no single molecular weight
- g/L ⇄ mg/dL
- a factor of 100, and the only arithmetic on this page. An IgM of 1.2 g/L is 120 mg/dL. North American reports use mg/dL, most of the rest of the world uses g/L
- no molar unit
- circulating IgM is mostly a pentamer of five immunoglobulin units held together by a J chain, around 900 kDa, variably glycosylated and polyclonal. There is no single mass to divide by, so a mass concentration against a reference preparation is the only meaningful report. Any IgM quoted in nmol/L has been invented
- total IgM is not specific IgM
- the number on this page is every IgM molecule in the sample. An "IgM positive" result for measles, mumps, Epstein–Barr virus, hepatitis A or Toxoplasma is a different assay measuring antibody directed at that one organism, reported as an index or a titre and never in g/L
- why the interval is so wide
- the adult upper limit is nearly eight times the lower one on Mayo's nephelometric assay (37–286 mg/dL). IgM is the most variable of the three main immunoglobulin classes between healthy people, so modest deviations matter less than they would for IgG
Worked example
IgM 1.2 g/L, reported with IgG 11.0 g/L and IgA 2.0 g/L
1.2 g/L = 120 mg/dL — the same concentration, and that is the whole of the arithmetic
120 mg/dL sits mid-interval against Mayo's adult 37–286 mg/dL, and the IgG and IgA alongside it are normal too, so there is no immunoglobulin abnormality here
Now the question this page exists for. If the same patient's specific IgM to a named organism came back positive, that is a different assay and a different unit entirely — and it would suggest infection within roughly the last few weeks, because IgM is the first antibody made in a primary response and it wanes over weeks to a few months
A positive specific IgG with a negative specific IgM would mean the opposite: past exposure or vaccination, and immunity rather than current infection
And a polyclonal total IgM of, say, 6.0 g/L (600 mg/dL) is a different problem again — twice the upper limit, and with hepatosplenomegaly it is electrophoresis and free light chains that are needed, not a repeat IgM
Specific IgM against specific IgG — the pairing most readers actually want
| Pattern on paired serology | Usual interpretation | Caveats |
|---|---|---|
| IgM positive, IgG negative | Early primary infection — IgM is the first antibody class made in a primary response, typically detectable within days and before IgG | Very early infection can be negative for both; repeat in one to two weeks. A single IgM is also the result most vulnerable to false positives |
| IgM positive, IgG positive | Recent infection, with the response maturing as IgG appears and IgM begins to wane | IgM can persist for months after some infections, and can reappear in reactivation or be raised polyclonally by an unrelated acute illness |
| IgM negative, IgG positive | Past exposure or vaccination — immunity, not current infection. This is the pattern that answers “am I protected?” | For some organisms IgG avidity or a quantitative titre is needed to judge protection rather than mere exposure |
| IgM negative, IgG negative | Susceptible — no evidence of past or present infection | Immunosuppressed and very young patients may fail to mount a detectable response |
| False-positive IgM | Commoner than clinicians expect: rheumatoid factor, polyclonal B-cell stimulation by an unrelated virus, cross-reacting antibody and an acute Epstein–Barr virus infection all produce spurious IgM positives | This is why a positive IgM in a low-prevalence setting is confirmed — by a second method, a paired convalescent sample, or nucleic acid testing |
What a high or low total IgM means
| Finding | Causes worth naming | Next step |
|---|---|---|
| Raised, polyclonal | Acute infection of almost any kind, chronic liver disease, and primary biliary cholangitis — in which a raised IgM is characteristic and was part of the older diagnostic description of the disease | Usually none beyond treating the cause; a persistent unexplained rise warrants electrophoresis |
| Raised, monoclonal | Waldenström macroglobulinaemia — an IgM paraprotein with lymphoplasmacytic marrow infiltration, classically with hepatosplenomegaly, lymphadenopathy, anaemia and, at high concentrations, hyperviscosity. Also IgM monoclonal gammopathy of undetermined significance, and chronic lymphocytic leukaemia | Serum protein electrophoresis with immunofixation, serum free light chains, blood count and film, and haematology referral. Symptomatic hyperviscosity is uncommon below about 30 g/L of IgM but is a medical emergency when it occurs |
| Low, with low IgG | Common variable immunodeficiency, other primary antibody deficiencies, and secondary causes — rituximab and other B-cell depletion, haematological malignancy, long-term corticosteroids | Immunology referral with specific antibody responses; the total is only the start |
| Low, with raised IgG and IgA | Hyper-IgM syndrome — a class-switch recombination defect, so IgM is made but cannot be switched to the other classes | Specialist immunology and genetic assessment |
| Low, in isolation | Selective IgM deficiency: normal IgG and IgA with a persistently low IgM. A recognised entity with no agreed definition, no single cut-off and an uncertain clinical significance — many of those identified are well, a minority have recurrent sinopulmonary or encapsulated-organism infection | Confirm it persists on a repeat sample before pursuing it, and assess specific antibody responses rather than acting on the total alone |
The antibody that dates an infection
The conversion is a factor of one hundred: grams per litre times a hundred gives milligrams per decilitre, so an IgM of 1.2 g/L is 120 mg/dL. No molar unit is offered, and none exists. Circulating IgM is predominantly a pentamer — five immunoglobulin units joined by a J chain, around 900 kilodaltons, variably glycosylated and, in a total measurement, polyclonal. There is no single molecular weight to divide by, so the result is a mass concentration reported against a reference preparation. An IgM quoted in nanomoles per litre has been invented somewhere along the way.
What makes IgM worth its own page is its position in time. IgM is the first antibody class a B cell produces in a primary response, before class-switch recombination allows the same clone to make IgG. It is therefore the class that dates an exposure. A raised specific IgM to a named organism suggests infection within roughly the last few weeks; a raised specific IgG with a negative IgM suggests past exposure or vaccination, and in most cases immunity. That pairing is what a clinician usually wants from serology, and it is worth being explicit that it belongs to the specific-antibody assays rather than to the total IgM this page converts — the two are reported in entirely different ways, the total in g/L or mg/dL and the specific antibody as an index, a titre or a qualitative result. It is also worth knowing that a lone positive specific IgM is the least reliable serological result there is: rheumatoid factor, polyclonal B-cell stimulation by an unrelated virus and cross-reacting antibodies all produce false positives, which is why a positive IgM in a low-prevalence setting is confirmed rather than acted on.
The total IgM has its own uses, and they turn on one distinction it cannot itself make. A polyclonal rise accompanies acute infection of almost any kind, chronic liver disease and, characteristically, primary biliary cholangitis. A monoclonal rise is a different disease: an IgM paraprotein with hepatosplenomegaly, lymphadenopathy, anaemia or hyperviscosity raises Waldenström macroglobulinaemia, and the total immunoglobulin cannot tell the two apart. Serum protein electrophoresis with immunofixation and serum free light chains make that separation, and an unexplained persistent rise should go to them rather than to a repeat IgM.
In the other direction, a low IgM is read with IgG and IgA. Low IgM with low IgG suggests common variable immunodeficiency or another antibody deficiency, whether primary or secondary to B-cell depletion, haematological malignancy or corticosteroids. A low IgM with a raised IgG and IgA suggests a hyper-IgM syndrome, in which IgM is made but cannot be class-switched. An isolated low IgM with normal IgG and IgA is selective IgM deficiency — a real but poorly defined entity, with no agreed cut-off, identified in people who are often entirely well and in a minority with recurrent sinopulmonary or encapsulated-organism infection. It should be confirmed on a repeat sample and assessed with specific antibody responses rather than acted on from a single total.
Frequently asked questions
How do you convert IgM from mg/dL to g/L?
Divide by 100. An IgM of 120 mg/dL is 1.2 g/L, and 1.2 g/L is 120 mg/dL going the other way. There is no molar conversion: polyclonal IgM is a pentamer of variable mass with no single molecular weight, so a mass concentration against a reference preparation is the only meaningful report.
What does a raised IgM mean compared with a raised IgG?
For antibody directed at a named organism, the class dates the infection. IgM is the first antibody made in a primary response and wanes over weeks to a few months, so a raised specific IgM suggests recent infection. IgG appears later and persists for years, so a raised specific IgG with a negative IgM indicates past exposure or vaccination — in most cases immunity rather than current infection. Note that this is the specific-antibody assay, not the total IgM in g/L that this page converts.
Can a single positive IgM be trusted?
Not on its own, particularly when the pre-test probability is low. Rheumatoid factor, polyclonal B-cell stimulation by an unrelated virus, cross-reacting antibody and acute Epstein–Barr virus infection all cause false-positive IgM results. A positive IgM is usually confirmed with a second method, a paired convalescent sample showing a rise in IgG, or nucleic acid testing for the organism itself.
What causes a very high total IgM?
The important division is polyclonal against monoclonal, and the total cannot make it. A polyclonal rise accompanies acute infection, chronic liver disease and primary biliary cholangitis, in which a raised IgM is characteristic. A monoclonal IgM — especially with hepatosplenomegaly, lymphadenopathy, anaemia or hyperviscosity symptoms — raises Waldenström macroglobulinaemia. Serum protein electrophoresis with immunofixation and serum free light chains are the tests that separate them.
Is a low IgM on its own a problem?
Often not. An isolated low IgM with a normal IgG and IgA is selective IgM deficiency, which has no agreed definition or cut-off; many people found to have it are well, though a minority have recurrent sinopulmonary or encapsulated-organism infection. It should be confirmed on a repeat sample. A low IgM alongside a low IgG is a different matter and points to common variable immunodeficiency or another antibody deficiency, including secondary causes such as rituximab.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: IGM — Immunoglobulin M (IgM), Serum. Nephelometry. Reference values: ≥18 years 37–286 mg/dL; 16–<18 years 49–201 mg/dL; 2–<4 years 37–184 mg/dL.
- Louis AG, Gupta S. Primary selective IgM deficiency: an ignored immunodeficiency. Clin Rev Allergy Immunol. 2014;46(2):104–111. doi:10.1007/s12016-013-8375-x
- Owen RG, Treon SP, Al-Katib A, et al. Clinicopathological definition of Waldenström’s macroglobulinemia: consensus panel recommendations from the Second International Workshop on Waldenström’s Macroglobulinemia. Semin Oncol. 2003;30(2):110–115.
- Cunningham-Rundles C. How I treat common variable immune deficiency. Blood. 2010;116(1):7–15. doi:10.1182/blood-2010-01-254417
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.