Interpreting complement profiles
Reading C3 and C4 (with CH50, AH50, C1q and C1 inhibitor when needed) by pattern, separating consumption from low synthesis and inherited deficiency, using complement to narrow glomerulonephritis and track lupus activity, and the acute phase, sample and drug effects that mislead.
Tier 2Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Most laboratories report C3 and C4 by nephelometry. Typical adult ranges are C3 0.9 to 1.8 g/L and C4 0.1 to 0.4 g/L; use the local range.
- Low levels mean consumption (immune complexes, alternative pathway activation), reduced synthesis (liver failure, malnutrition) or inherited deficiency. The pattern of C3 against C4 tells you which pathway is active.
- C3 and C4 are acute phase proteins. Inflammation raises them, which can hide consumption.
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Sources
- RCPA Manual (complement C3, complement C4, classical and alternative pathway function)AU/NZRoyal College of Pathologists of Australasia
- Primary Immunodeficiencies (PID) Clinical UpdateAU/NZASCIA (Australasian Society of Clinical Immunology and Allergy), 2025
- Aringer M et al. 2019 European League Against Rheumatism/American College of Rheumatology classification criteria for systemic lupus erythematosus. Ann Rheum Dis 2019;78:1151-9EULAR/ACR, 2019
- KDIGO 2021 Clinical Practice Guideline for the Management of Glomerular Diseases (Kidney International 2021;100(4S):S1-S276)Kidney Disease Improving Global Outcomes, 2021 (lupus nephritis updated 2024, IgA nephropathy 2025)
- Complement deficiencies and infections (El Sissy C et al. Curr Opin Immunol 2026;98:102711)Current Opinion in Immunology, 2026
- Last updated
- 14 November 2026
What changed
- 14 November 2026 Personal emails, About story and Australian-style copy