Complement C3 Unit Converter
Complement C3 Unit Converter
Convert complement C3 between g/L, mg/dL and mg/L, and read it the way it should be read — as a pattern with C4, not as a number on its own.
Complement C3 converter
g/L ⇄ mg/dLComplement C3 1.15 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
- no molar unit
- C3 is reported by mass; molar reporting is not used clinically and none is offered here
- read with C4
- the C3 and C4 pattern is more informative than either value alone, because C4 participates in the classical pathway only
- sample stability
- C3 falls in serum left at room temperature; separate promptly and freeze if analysis will be delayed
Worked example
Complement C3 1.15 g/L
1.15 × 100 = 115 mg/dL
1.15 × 1000 = 1,150 mg/L
Within the adult reference interval of 0.9–1.8 g/L
The C3 and C4 pattern
| C3 | C4 | Pathway implicated | Typical causes |
|---|---|---|---|
| Low | Low | Classical | Active lupus, especially lupus nephritis; cryoglobulinaemia; serum sickness; endocarditis-associated immune complex disease |
| Low | Normal | Alternative | Post-streptococcal glomerulonephritis; C3 glomerulopathy; C3 nephritic factor |
| Normal | Low | Classical, without C3 consumption | Hereditary angioedema (C1 inhibitor deficiency); partial C4 gene deletion, which is common and benign |
| Normal | Normal | None detected | Does not exclude activation — C3 is an acute-phase reactant and consumption can be masked |
Reference interval in both conventions
| g/L | mg/dL | |
|---|---|---|
| Adult reference interval | 0.9 – 1.8 | 90 – 180 |
| Low — consumption or deficiency | < 0.9 | < 90 |
| Raised — acute-phase response | > 1.8 | > 180 |
Reading C3 as a pattern, and the traps that hide it
Complement C3 is reported in g/L in most laboratories and in mg/dL in the United States, differing by a factor of 100. The conversion is arithmetically trivial; the interpretation is not, and the single most important point is that C3 should not be read on its own. It is measured with C4, and it is the pattern of the pair, rather than either value, that carries the diagnostic information.
Low C3 with a low C4 indicates classical pathway activation, since C4 sits within that pathway only. It is the pattern of active lupus, particularly lupus nephritis, and of cryoglobulinaemia, serum sickness and the immune complex disease of subacute bacterial endocarditis. Low C3 with a normal C4 indicates activation confined to the alternative pathway, which C4 takes no part in: post-streptococcal glomerulonephritis, C3 glomerulopathy, and the C3 nephritic factor that drives it. A normal C3 with a low C4 raises hereditary angioedema and, more prosaically, partial C4 gene deletion, which is common and of no consequence.
Two traps blunt the test. The first is that C3 is itself an acute-phase reactant, so inflammation raises production at the same time as immune complexes consume it. The two effects can cancel, producing a normal C3 in a patient whose complement is being actively consumed. A normal result therefore does not exclude activation, and serial measurement in a known patient is more informative than any single value.
The second is pre-analytical. Complement proteins are unstable in serum left at room temperature: C3 is degraded by continuing in vitro activation and by proteolysis, and the measured level falls. A spuriously low C3 from a sample that sat on a bench or travelled slowly is a common cause of a confusing result, and worth excluding by repeating on a properly handled specimen before an investigation is started. Samples should be separated promptly and frozen if analysis will be delayed.
Frequently asked questions
How do I convert complement C3 from g/L to mg/dL?
Multiply by 100. A C3 of 1.15 g/L is 115 mg/dL. To go the other way, divide the mg/dL figure by 100.
What does a low C3 with a normal C4 mean?
It points to activation of the alternative pathway, which C4 takes no part in. Post-streptococcal glomerulonephritis, C3 glomerulopathy and C3 nephritic factor are the classic causes.
What does a low C3 with a low C4 mean?
It indicates classical pathway activation. Active lupus, particularly lupus nephritis, is the commonest cause, alongside cryoglobulinaemia, serum sickness and endocarditis-associated immune complex disease.
Can complement be consumed with a normal C3?
Yes. C3 is an acute-phase reactant, so inflammation increases hepatic production at the same time as immune complexes consume it. The two can offset, producing a deceptively normal result, which is why serial measurement is more useful than a single value.
Why can a sample give a falsely low C3?
Complement proteins are unstable in serum kept at room temperature, degrading through continued in vitro activation and proteolysis. A delayed or warm sample is a common cause of a spuriously low C3, so repeat on a properly handled specimen before investigating.
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References
- Fanouriakis A, Kostopoulou M, Andersen J, et al. EULAR recommendations for the management of systemic lupus erythematosus: 2023 update. Ann Rheum Dis. 2024;83(1):15–29.
- Kidney Disease: Improving Global Outcomes (KDIGO) Glomerular Diseases Work Group. KDIGO 2021 Clinical Practice Guideline for the Management of Glomerular Diseases. Kidney Int. 2021;100(4S):S1–S276.