CSF IgG Unit Converter
CSF IgG Unit Converter
Convert CSF IgG between mg/L, mg/dL, g/L and µg/mL — mass units only, because polyclonal IgG has no single molar mass, and with the 26% gap between Mayo’s interval and ARUP’s.
CSF IgG converter
Mass units onlyCSF IgG reported as 55 mg/L
The conversion, and why there is no molar column
g/L = mg/L ÷ 1000
µg/mL = mg/L × 1 — the same quantity written two ways
No µmol/L or nmol/L. Polyclonal IgG has no single molar mass
- four subclasses
- IgG1 and IgG4 are around 146 kDa, IgG3 around 170 kDa because of its extended hinge — a 16% spread before glycosylation is counted, and glycosylation itself varies
- a changing mixture
- the subclass proportions differ between patients and between diseases, and intrathecal synthesis changes them, so CSF IgG is not even the same mixture as the serum IgG it is divided by in the index
- mg/L to mg/dL
- a pure SI prefix relation, involving no mass at all, which is why it is the one conversion this page can make honestly
Worked example
CSF IgG reported as 55 mg/L
55 ÷ 10 = 5.50 mg/dL, the unit both Mayo and ARUP print
55 mg/L = 55 µg/mL = 0.0550 g/L
Inside Mayo's 0.0–8.1 mg/dL and inside ARUP's 0.0–6.0 mg/dL
A CSF IgG of 7 mg/dL would be inside Mayo's interval and outside ARUP's — the same number, two verdicts
CSF IgG and the index it feeds: Mayo against ARUP
| Parameter | Mayo (SFIG) | ARUP (0050676) | They disagree by |
|---|---|---|---|
| CSF IgG | 0.0 – 8.1 mg/dL | 0.0 – 6.0 mg/dL | 26% at the upper limit |
| CSF IgG, in mg/L | 0 – 81 | 0 – 60 | the same 26% |
| CSF albumin | 0.0 – 27.0 mg/dL | 0 – 35 mg/dL | 30%, in the opposite direction |
| IgG index | 0.00 – 0.70 | 0.28 – 0.66 | ARUP sets a lower bound; Mayo does not |
| IgG synthesis rate | 0 – 12 mg/24 h | ≤ 8.0 mg/d | 50% at the upper limit |
| CSF IgG/albumin ratio | 0.00 – 0.21 | not published | — |
Why CSF IgG takes mass units and CSF albumin does not
| Measurand | Molar mass | Units offered here |
|---|---|---|
| CSF albumin | 66,500 Da, a single defined protein | mass and µmol/L — see the albumin converter |
| CSF IgG | none — four subclasses, 146–170 kDa, variably glycosylated | mass only |
| CSF total protein | none — a mixture of hundreds of proteins | mass only |
Reading a CSF IgG, and the two things it cannot tell you alone
A raised CSF IgG is one of the most over-read numbers in neurology, because three entirely different situations produce it and the value alone cannot separate them. The first is intrathecal synthesis — B cells inside the central nervous system making antibody, which is the finding in multiple sclerosis, neurosarcoidosis, neuroborreliosis and chronic viral infection. The second is a leaking blood–CSF barrier passively admitting serum IgG, which happens in bacterial meningitis, Guillain-Barré syndrome and spinal block. The third is simply a high serum IgG — myeloma, chronic infection, liver disease — passing through an entirely normal barrier.
The IgG index exists to separate the first from the other two. It divides the CSF:serum IgG ratio by the CSF:serum albumin ratio, so the barrier term cancels and what is left is production. The companion measurement is oligoclonal banding on isoelectric focusing, which is more sensitive than the index and is what current diagnostic criteria actually ask for; the index survives because it is numeric and trendable where bands are a yes or a no.
The unit question on this page has a real answer rather than a conventional one. IgG is not one molecule. It is four subclasses whose heavy chains differ in length and hinge structure — IgG1 and IgG4 near 146 kDa, IgG3 near 170 kDa — each variably glycosylated, in proportions that differ between people, between diseases, and between the two compartments being compared. Dividing a milligram figure by any single number to get micromoles produces a quantity whose value depends on an assumption about subclass mix, not on how much antibody is present. So this page offers mass units only, and says so, because a reader who has seen a molar figure quoted elsewhere deserves to know what it rests on.
One practical note on the arithmetic: because the index divides a ratio by a ratio, the units cancel and it does not matter whether all four inputs are in mg/dL or all four in mg/L — provided they are all the same. Mixing them is what goes wrong, and it goes wrong by factors of ten.
Frequently asked questions
How do I convert CSF IgG from mg/L to mg/dL?
Divide by 10. A CSF IgG of 55 mg/L is 5.5 mg/dL. Mayo and ARUP both report in mg/dL, while European laboratories and the Reiber tradition generally use mg/L.
What is a normal CSF IgG?
Mayo publishes 0.0–8.1 mg/dL (0–81 mg/L) and ARUP publishes 0.0–6.0 mg/dL (0–60 mg/L). The 26% gap at the upper limit is genuine, so a CSF IgG of 7 mg/dL is within the interval at Mayo and above it at ARUP.
Can CSF IgG be converted to µmol/L or nmol/L?
No, and a figure quoted in those units should be treated with suspicion. IgG is four subclasses spanning roughly 146 to 170 kDa, variably glycosylated, in proportions that change with the disease and differ between CSF and serum. Any molar value therefore depends on an assumed subclass mix rather than on the amount of antibody present. This is the same reason CSF total protein takes mass units only, and it is different from albumin, which is one defined protein and does have a molar mass.
Does the IgG index depend on which unit I use?
No, provided every input uses the same one. The index is a ratio of ratios, so the units cancel and four values in mg/dL give the same answer as four values in mg/L. The albumin quotient is the opposite case — it deliberately mixes mg/L with g/L and is therefore unit-sensitive.
Is a raised CSF IgG enough to diagnose multiple sclerosis?
No. A raised CSF IgG is produced just as readily by a leaking blood–CSF barrier or by a high serum IgG passing through an intact one. The IgG index removes the barrier term, and oligoclonal banding is more sensitive still and is what current criteria ask for.
Related calculators
References
- Mayo Clinic Laboratories. Cerebrospinal fluid IgG index profile, serum and spinal fluid (SFIG) — reference values for CSF IgG, CSF albumin, the IgG index, the IgG synthesis rate and the albumin quotient. Test catalogue, test ID 610783; 2026.
- ARUP Laboratories. Immunoglobulin G, CSF index — reference intervals for CSF IgG, CSF albumin, serum IgG and serum albumin, the IgG index and the IgG synthesis rate. Laboratory Test Directory 0050676; 2026.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/