Complement C4 Unit Converter
Complement C4 Unit Converter
Convert complement C4 between g/L, mg/dL and mg/L — and read it the opposite way round to most tests, because complement is consumed when it is activated, so it is a LOW C4 that is the abnormal finding.
Complement C4 converter
g/L ⇄ mg/dLComplement C4 0.25 g/L, reported with a C3 of 1.15 g/L
The conversion, and the direction of the test
mg/dL = g/L × 100
and there is no molar unit: C4 is reported as a mass concentration against a reference preparation
- g/L ⇄ mg/dL
- a factor of 100. A C4 of 0.25 g/L is 25 mg/dL and 250 mg/L. North American reports use mg/dL, most laboratories elsewhere use g/L
- no nmol/L
- C4 is a large multi-chain glycoprotein circulating as a pro-protein that is cleaved on activation, and it is measured immunochemically against a standard. Molar reporting is not used clinically
- low is the abnormal result
- the point that inverts the usual reading of a laboratory test. Complement components are consumed when the cascade is activated, so activation lowers the measured concentration. A low C4 is the finding; a high C4 is an acute-phase artefact of inflammation
- C4 is measured with C3
- the pair localises the activation. Low C4 with low C3 fits classical-pathway activation by immune complexes; low C4 with a normal C3 points to the classical pathway proximally, most importantly in hereditary angioedema; low C3 with a normal C4 fits alternative-pathway activation
- the commonest cause of a spurious low C4
- the specimen. Complement is consumed in vitro in a warm or delayed sample and by repeated freeze–thaw cycles, so a low result on a sample that travelled badly should be repeated on a fresh, promptly separated one before it is investigated
Worked example
Complement C4 0.25 g/L, reported with a C3 of 1.15 g/L
0.25 g/L = 25 mg/dL = 250 mg/L — the same concentration written three ways
25 mg/dL sits mid-interval against Mayo's 14–40 mg/dL, and the C3 beside it is normal too, so there is no evidence of complement consumption on this sample
Now move the C4 down and keep the C3 normal. A C4 of 0.05 g/L (5 mg/dL) with a normal C3 is the classical-pathway pattern, and in a patient with recurrent angioedema without urticaria it is the result that prompts C1-inhibitor level and function — the C4 stays low between attacks, which is what makes it useful as a screen
Move both down — C4 0.08 g/L (8 mg/dL) with a C3 of 0.55 g/L — and the pattern fits active lupus or another immune-complex disease. In the 2019 EULAR/ACR lupus classification criteria a low C3 or C4 scores 3 points and both together score 4
And going up means almost nothing: a C4 of 0.45 g/L (45 mg/dL) is an acute-phase response, not a complement disorder
Reading C3 and C4 as a pair
| C3 | C4 | What the pattern indicates |
|---|---|---|
| Low | Low | Classical-pathway activation by immune complexes. Active systemic lupus erythematosus is the classic example, and the fall tracks disease activity closely enough to be used in monitoring; also mixed cryoglobulinaemia, post-infectious and membranoproliferative glomerulonephritis, and severe sepsis |
| Normal | Low | Activation or deficiency confined to the proximal classical pathway. Hereditary angioedema is the diagnosis not to miss — uncontrolled C1 esterase activity consumes C4 continuously, so the C4 is low between attacks as well as during them. Also partial C4 gene deletion, which is common and harmless, and heterozygous C4 deficiency |
| Low | Normal | Alternative-pathway activation — C3 nephritic factor and C3 glomerulopathy, atypical haemolytic uraemic syndrome, and the alternative-pathway consumption of some infections |
| Low | Low, with a low CH50 or absent components | Suggests a true complement component deficiency rather than consumption. Recurrent encapsulated-organism or neisserial infection makes this worth pursuing with a functional total complement assay |
| Normal | Normal | No evidence of consumption on this sample — but neither protein is sensitive enough for a normal pair to exclude it, because both are acute-phase reactants and synthesis can offset consumption |
C4 in hereditary angioedema — useful, but not a rule-out
| Question | Answer | Source |
|---|---|---|
| Is C4 low between attacks? | In the great majority of patients with C1-inhibitor deficiency, yes — C1 esterase activity is unopposed continuously, not only during attacks, so C4 is consumed all the time. That is what makes it usable as a first-line screen | Standard complement physiology; the basis of the traditional screening algorithm |
| How sensitive is a low C4? | About 81% in untreated patients in one systematic evaluation, with a specificity of 85%. Normal C4 levels were recorded on nine separate occasions in five genetically confirmed HAE patients | Tarzi et al, Clin Exp Immunol 2007;149:513–516 |
| So can a normal C4 exclude hereditary angioedema? | No. Where clinical suspicion is real — recurrent angioedema without urticaria, unresponsive to antihistamines and adrenaline, often with abdominal attacks or a family history — C1-inhibitor antigenic level and C1-inhibitor function must be measured regardless of the C4 | Tarzi et al, and the WAO/EAACI guideline |
| What confirms the diagnosis? | Low C1-inhibitor function, with the antigenic level low in type I and normal or raised in type II. C1q is normal in hereditary disease and often low in acquired C1-inhibitor deficiency, which is the distinction that matters in an older patient with no family history | WAO/EAACI hereditary angioedema guideline |
A test read in the opposite direction
The arithmetic is a factor of a hundred and a factor of a thousand: a C4 of 0.25 g/L is 25 mg/dL and 250 mg/L. There is no molar unit, because C4 is a large multi-chain glycoprotein measured immunochemically against a reference preparation rather than as an amount of substance.
The substance of this page is a direction. Nearly every analyte in clinical chemistry is abnormal when it is high, and complement is the conspicuous exception: the components are consumed when the cascade is activated, so activation lowers the concentration that is measured. A low C4 is therefore the abnormal finding, and a raised C4 is an acute-phase response of very little consequence. It is worth stating plainly, because a report showing a C4 just above the interval is routinely read as though something were wrong, while the result that genuinely matters looks like a deficiency.
C4 is interpreted with C3, and the pair localises the problem. A low C4 with a low C3 is classical-pathway activation by immune complexes: active systemic lupus erythematosus is the classic example, and the fall tracks disease activity closely enough to be used in monitoring — the 2019 EULAR/ACR classification criteria score a low C3 or a low C4 at 3 points and both together at 4. Mixed cryoglobulinaemia, post-infectious and membranoproliferative glomerulonephritis and severe sepsis do the same thing. A low C3 with a normal C4 points instead to the alternative pathway, as in C3 glomerulopathy or atypical haemolytic uraemic syndrome.
A low C4 with a normal C3 is the pattern with a specific and urgent meaning: it points to the proximal classical pathway, and in a patient with recurrent angioedema and no urticaria it should raise hereditary angioedema. Unopposed C1 esterase activity consumes C4 continuously rather than only during attacks, so the C4 is low between attacks too, which is what makes it the cheapest useful screening test for the condition. It is a screen and not a rule-out, though: in one systematic evaluation a low C4 had a sensitivity of only about 81% in untreated patients, with normal levels recorded repeatedly in genetically confirmed cases. Where the clinical picture fits, C1-inhibitor antigenic level and function are measured whatever the C4 shows. Two more mundane explanations for an isolated low C4 deserve a thought first: a warm, delayed or repeatedly frozen sample consumes complement in the tube, and partial C4 gene deletion is common in the population and clinically silent.
Frequently asked questions
How do you convert complement C4 between g/L and mg/dL?
Multiply g/L by 100 to get mg/dL, or divide mg/dL by 100 to get g/L. A C4 of 0.25 g/L is 25 mg/dL, which is also 250 mg/L. There is no molar unit for C4: it is a large multi-chain glycoprotein measured immunochemically against a reference preparation.
Is a high or a low C4 the abnormal result?
A low one. Complement components are consumed when the cascade is activated, so activation lowers the measured concentration — the opposite of the way most laboratory tests behave. A low C4 is the diagnostically useful finding. A raised C4 is simply an acute-phase response, carries no weight as evidence of a complement disorder and needs no complement-specific investigation.
What does a low C4 with a normal C3 mean?
It localises the problem to the proximal classical pathway. The diagnosis not to miss is hereditary angioedema: unopposed C1 esterase activity consumes C4 continuously, so the C4 is low between attacks as well as during them. The pattern also arises from partial C4 gene deletion, which is common in the population and harmless, and from heterozygous C4 deficiency. In a patient with recurrent angioedema without urticaria it should prompt measurement of C1-inhibitor level and function.
Can a normal C4 rule out hereditary angioedema?
No. A low C4 between attacks is the traditional first-line screen and points strongly at C1-inhibitor deficiency, but in one systematic evaluation its sensitivity in untreated patients was only about 81%, and normal C4 levels were recorded on nine separate occasions in five genetically confirmed patients. Where the clinical suspicion is real, C1-inhibitor antigenic level and C1-inhibitor function should be measured regardless of the C4 result.
Why are C3 and C4 always measured together?
Because the pattern carries more information than either number. Low C3 with low C4 suggests classical-pathway activation by immune complexes, as in active lupus; low C4 with a normal C3 suggests proximal classical-pathway consumption or deficiency, most importantly hereditary angioedema; low C3 with a normal C4 suggests alternative-pathway activation, as in C3 glomerulopathy. A normal pair does not exclude activation, because both proteins are acute-phase reactants and synthesis can offset consumption.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: C4 — Complement C4, Serum. Nephelometry. Reference value 14–40 mg/dL.
- Tarzi MD, Hickey A, Förster T, Mohammadi M, Longhurst HJ. An evaluation of tests used for the diagnosis and monitoring of C1 inhibitor deficiency: normal serum C4 does not exclude hereditary angio-oedema. Clin Exp Immunol. 2007;149(3):513–516 — low C4 sensitivity 81%, specificity 85% in untreated patients.
- Aringer M, Costenbader K, Daikh D, et al. 2019 European League Against Rheumatism/American College of Rheumatology classification criteria for systemic lupus erythematosus. Arthritis Rheumatol. 2019;71(9):1400–1412 — low C3 or low C4 scores 3 points, low C3 and low C4 scores 4, threshold ≥10.
- Maurer M, Magerl M, Betschel S, et al. The international WAO/EAACI guideline for the management of hereditary angioedema — the 2021 revision and update. Allergy. 2022;77(7):1961–1990. doi:10.1111/all.15214
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