Immunoglobulin G (IgG) Unit Converter
Immunoglobulin G (IgG) Unit Converter
Convert IgG between g/L, mg/dL and mg/L, and see why a raised total cannot tell a polyclonal rise from a paraprotein.
Immunoglobulin G (IgG) converter
g/L ⇄ mg/dLImmunoglobulin G 11.2 g/L
Formula and conversion factors
g/L = mg/dL ÷ 100
mg/L = g/L × 1000
- × 100
- g/L is the SI convention and mg/dL the US one; a gram is 1000 mg and a litre is 10 dL, so the factor is 100
- no molar unit
- IgG is a polyclonal mixture of four subclasses of differing mass, so a molar concentration has no single meaningful value and none is offered
- subclasses
- IgG1 to IgG4; a subclass deficiency, most often IgG2, can exist behind a normal total because IgG1 makes up most of it
Worked example
Immunoglobulin G 11.2 g/L
11.2 × 100 = 1,120 mg/dL
11.2 × 1000 = 11,200 mg/L
Within the adult reference interval of 7.0–16.0 g/L
Adult reference interval and thresholds
| g/L | mg/dL | |
|---|---|---|
| Adult reference interval | 7.0 – 16.0 | 700 – 1600 |
| Hypogammaglobulinaemia — investigate | < 7.0 | < 700 |
| Commonly seen in primary antibody deficiency | < 4.0 | < 400 |
| Raised — polyclonal or monoclonal | > 16.0 | > 1600 |
What a raised or low IgG points to
| Pattern | Typical causes | Next test |
|---|---|---|
| Polyclonal rise | Chronic infection, chronic liver disease, autoimmune disease | Electrophoresis showing a broad-based rise |
| Monoclonal rise (paraprotein) | MGUS, myeloma, lymphoproliferative disease | Electrophoresis with immunofixation, serum free light chains |
| Low IgG | Primary immunodeficiency, nephrotic or gut protein loss, rituximab, corticosteroids | Full immunoglobulin panel, urinary protein, vaccine responses |
| Normal IgG, recurrent infection | IgG subclass or specific antibody deficiency | IgG subclasses and test immunisation responses |
Why IgG has no molar unit, and what the total hides
Immunoglobulin G is reported in g/L under the SI convention and in mg/dL in the United States, and the two differ by a factor of 100: an IgG of 11.2 g/L is 1,120 mg/dL. Some laboratories, particularly when reporting alongside subclasses, use mg/L instead, a further factor of ten. No molar unit is offered here, and that is deliberate. IgG is not a single molecule but a polyclonal mixture of four subclasses of differing mass produced by countless B-cell clones, so a molar concentration would have no single meaningful value.
A raised total is where interpretation gets difficult, because the number itself cannot tell you what produced it. A polyclonal rise — many clones responding at once — suggests chronic infection, chronic liver disease or autoimmune disease, and needs no haematological work-up in itself. A monoclonal rise is a paraprotein, and points to MGUS, myeloma or a lymphoproliferative disorder. The total IgG looks identical in both cases; only serum protein electrophoresis with immunofixation, and serum free light chains, will separate them.
A low IgG raises three broad possibilities. Primary immunodeficiency, most commonly common variable immunodeficiency, is the one not to miss, particularly in an adult with recurrent sinopulmonary infection. Protein loss through nephrotic syndrome or protein-losing enteropathy lowers IgG alongside albumin. And drug effect is now probably the commonest cause seen in practice: rituximab and other B-cell depleting agents, and long-term corticosteroids, all lower it, sometimes for years after the last dose.
A normal total does not exclude an antibody deficiency. IgG subclass deficiency — most often IgG2, which carries the response to polysaccharide antigens — can coexist with a total in the reference range, because IgG1 makes up the bulk of the total and masks a deficit in the others. Specific antibody deficiency, in which the total and the subclasses are normal but the response to test immunisation is poor, is likewise invisible here. Where the clinical picture suggests antibody deficiency, subclasses and vaccine responses are the next step regardless of the total.
Frequently asked questions
How do I convert IgG from g/L to mg/dL?
Multiply by 100. An IgG of 11.2 g/L is 1,120 mg/dL. To go the other way, divide the mg/dL figure by 100.
Why is there no molar unit for IgG?
IgG is a polyclonal mixture of four subclasses with different molecular masses, produced by many different B-cell clones. There is no single molar mass that would make a µmol/L figure meaningful, so laboratories report it by mass only.
Can a total IgG distinguish a paraprotein from a polyclonal rise?
No. The two look identical as a number. Serum protein electrophoresis separates a broad-based polyclonal rise from a discrete monoclonal band, and immunofixation with serum free light chains characterises the band.
What lowers IgG apart from immunodeficiency?
Protein loss through nephrotic syndrome or protein-losing enteropathy, and drug effect — rituximab and other B-cell depleting agents, and long-term corticosteroids. Drug effect is now a common cause and can persist for years after treatment.
Does a normal IgG exclude antibody deficiency?
No. IgG subclass deficiency, most often IgG2, can hide behind a normal total because IgG1 dominates it. Specific antibody deficiency, with poor responses to test immunisation, also occurs with normal levels.
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References
- Bonilla FA, Khan DA, Ballas ZK, et al. Practice parameter for the diagnosis and management of primary immunodeficiency. J Allergy Clin Immunol. 2015;136(5):1186–1205.
- Bird J, Behrens J, Westin J, et al. UK Myeloma Forum and Nordic Myeloma Study Group guidelines for the investigation of newly detected M-proteins. Br J Haematol. 2009;147(1):22–42.
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.