Immunoglobulin A (IgA) Unit Converter
Immunoglobulin A (IgA) Unit Converter
Convert IgA between g/L, mg/dL and mg/L — and check it before you trust a coeliac screen, because IgA deficiency makes tissue transglutaminase IgA uninterpretable.
Immunoglobulin A (IgA) converter
g/L ⇄ mg/dLImmunoglobulin A 2.1 g/L in an adult
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
- serum IgA is a polyclonal mixture of monomeric and dimeric molecules from countless B-cell clones, so it has no single molecular mass and no meaningful molar concentration
- 0.07 g/L
- the conventional threshold defining selective IgA deficiency, equivalent to 7 mg/dL — the lower limit of detection of most nephelometric assays
- secretory IgA
- the dimeric form at mucosal surfaces is a different measurement from serum IgA and is not covered by this interval
Worked example
Immunoglobulin A 2.1 g/L in an adult
2.1 g/L = 2.10 g/L
2.1 × 100 = 210 mg/dL
2.1 × 1000 = 2,100 mg/L
Within the adult reference interval of 0.70–4.00 g/L, so an IgA-based coeliac screen can be interpreted
Reference interval and thresholds across the units
| g/L | mg/dL | mg/L | |
|---|---|---|---|
| Adult reference interval | 0.7 – 4.0 | 70 – 400 | 700 – 4,000 |
| Partial IgA deficiency | 0.07 – 0.7 | 7 – 70 | 70 – 700 |
| Selective IgA deficiency, with normal IgG and IgM | < 0.07 | < 7 | < 70 |
| Raised — polyclonal or monoclonal | > 4.0 | > 400 | > 4,000 |
IgA deficiency and the coeliac screen
| Situation | What the IgA tTG result means | What to do |
|---|---|---|
| Normal total IgA | Interpretable in the usual way | Read the tTG IgA as reported |
| Selective IgA deficiency (< 0.07 g/L) | Uninterpretable — a negative result is meaningless | Request an IgG-based test: tTG IgG, deamidated gliadin peptide IgG or endomysial IgG |
| Partial deficiency (0.07 – 0.7 g/L) | Sensitivity is reduced and a negative result is unreliable | Discuss with the laboratory; many will add an IgG-based test |
| Total IgA never measured | Unknown — roughly 1 in 600 people is deficient | Measure total IgA alongside, or use a laboratory that does so automatically |
Why a coeliac screen should never be read without it
Immunoglobulin A is reported in g/L under the SI convention and in mg/dL in the United States, a factor of 100 apart, with mg/L appearing on some panels. No molar unit is offered here and none exists: serum IgA is a polyclonal mixture of monomeric and dimeric molecules produced by countless B-cell clones, so there is no single molecular mass to divide by. Any pmol/L figure quoted for IgA has been invented.
Selective IgA deficiency is the commonest primary immunodeficiency, affecting roughly one person in 600 in European populations, and it is defined as a serum IgA below 0.07 g/L with normal IgG and IgM in someone over four years of age. Most people who have it are entirely well, which is precisely why it goes unrecognised. A minority have recurrent sinopulmonary or gastrointestinal infection, and there is a raised prevalence of autoimmune disease.
The consequence that matters on a day-to-day basis is diagnostic rather than immunological. Coeliac serology rests on tissue transglutaminase IgA, and an IgA-deficient person cannot produce that antibody whatever their gut is doing. Their tTG IgA comes back negative, the result looks reassuring, and coeliac disease is excluded when it has not been. This is not a rare coincidence: coeliac disease is two to three times commoner among people with selective IgA deficiency than in the general population. Total IgA should therefore be measured alongside any IgA-based coeliac screen, and where it is deficient the test must be repeated with an IgG-based assay — tTG IgG, deamidated gliadin peptide IgG or endomysial IgG.
A raised IgA carries the same ambiguity as a raised IgG: the total cannot distinguish a polyclonal rise from a paraprotein. Chronic infection, alcohol-related and other chronic liver disease, inflammatory bowel disease, IgA nephropathy and IgA vasculitis all produce a polyclonal increase; IgA myeloma produces a monoclonal one, and only serum protein electrophoresis with immunofixation and serum free light chains will separate them. A partial reduction, rather than true deficiency, is also worth recognising as a drug effect — phenytoin, sulfasalazine and captopril are the usual culprits.
Frequently asked questions
How do I convert IgA from g/L to mg/dL?
Multiply by 100. An IgA of 2.1 g/L is 210 mg/dL, or 2,100 mg/L. Going the other way, divide the mg/dL figure by 100.
Why is there no molar unit for IgA?
Serum IgA is a polyclonal mixture of monomeric and dimeric molecules made by countless different B-cell clones, so it has no single molecular mass. Dividing a mass concentration by a molecular weight that does not exist would produce a number with no meaning, so none is offered.
What is selective IgA deficiency?
A serum IgA below 0.07 g/L with normal IgG and IgM in someone over four years old. It is the commonest primary immunodeficiency, affecting roughly one person in 600, and most people who have it are well — which is why it is usually found incidentally rather than through illness.
Why does IgA deficiency matter for a coeliac test?
Coeliac serology relies on tissue transglutaminase IgA. Someone who cannot make IgA cannot make that antibody, so their result is negative regardless of whether they have coeliac disease. Since coeliac disease is two to three times commoner in IgA deficiency, an IgG-based test is needed instead.
What raises IgA?
A polyclonal rise occurs in chronic infection, chronic liver disease — especially alcohol-related — inflammatory bowel disease, IgA nephropathy and IgA vasculitis. A monoclonal rise means IgA myeloma or a related plasma cell disorder. The total looks the same either way; electrophoresis with immunofixation separates them.
Related calculators
References
- Yel L. Selective IgA deficiency. J Clin Immunol. 2010;30(1):10–16.
- Al-Toma A, Volta U, Auricchio R, et al. European Society for the Study of Coeliac Disease (ESsCD) guideline for coeliac disease and other gluten-related disorders. United European Gastroenterol J. 2019;7(5):583–613.
- Husby S, Koletzko S, Korponay-Szabó IR, et al. European Society Paediatric Gastroenterology, Hepatology and Nutrition guidelines for diagnosing coeliac disease 2020. J Pediatr Gastroenterol Nutr. 2020;70(1):141–156.