Complement (C3, C4, CH50)

The complement system is a cascade of plasma proteins that mediates innate immune defence, immune complex clearance and inflammation. The classical cascade is triggered by antibody antigen complexes (C1q, C1r, C1s, C4, C2, C3), the lectin cascade by mannose binding lectin, and the alternative cascade by spontaneous C3 hydrolysis (factors B, D, properdin, H and I).

All converge on C3, leading to C5 to C9 and the membrane attack complex. The routine investigation measures C3 and C4 concentrations (by nephelometry or turbidimetry) and total haemolytic complement activity (CH50, and AP50 for the alternative cascade). Low levels show consumption (immune complex disease), reduced synthesis or genetic deficiency.

Complement is used to monitor systemic lupus erythematosus and lupus nephritis, to assess membranoproliferative glomerulonephritis and C3 glomerulopathy, to diagnose hereditary angio oedema (low C4 and C1 inhibitor), to evaluate recurrent infections (especially encapsulated bacteria and Neisseria), and to investigate atypical haemolytic uraemic syndrome.

Complement deficiencies are rare (about 1 to 5% of primary immunodeficiencies). C3 and C4 are also acute phase reactants, rising with inflammation, which can mask consumption.

Indications. Known or suspected SLE (diagnosis, flare, nephritis). Glomerulonephritis with low C3 (post-infectious, membranoproliferative, C3 glomerulopathy). Recurrent angio oedema without urticaria (C4 first, then C1 inhibitor level and function). Recurrent neisserial infection or recurrent bacterial infections.

Suspected cryoglobulinaemia, vasculitis (urticarial vasculitis, hypocomplementaemic), infective endocarditis with immune complex disease, atypical haemolytic uraemic syndrome and paroxysmal nocturnal haemoglobinuria (eculizumab therapy monitoring).

Specimen. Serum 1 to 2 mL, separated promptly (within 1 hour) and tested fresh or frozen at minus 70 °C. Complement is labile, and in vitro activation by storage at room temperature lowers C3 and C4 and causes C3 breakdown products. CH50 needs serum frozen immediately. EDTA plasma is not used for CH50.

C3.

• Reference range 0.9 to 1.8 g/L (90 to 180 mg/dL). The most abundant complement protein.
• Low C3 (under 0.9 g/L): active SLE (especially nephritis), post-infectious glomerulonephritis (returns to normal in 6 to 8 weeks), membranoproliferative glomerulonephritis, C3 glomerulopathy (C3 nephritic factor), immune complex disease, sepsis, liver failure, malnutrition, factor H or I deficiency, and infection with encapsulated bacteria.
• Raised C3: acute phase response, inflammation, obesity.

C4.

• Reference range 0.1 to 0.4 g/L (10 to 40 mg/dL).
• Low C4: active SLE (often falls before C3), cryoglobulinaemia, hereditary angio oedema (types 1 and 2, C4 low between attacks in 80 to 95%), acquired C1 inhibitor deficiency, classical cascade deficiency, and C4A/C4B gene deletions (common, with partial deficiency in 15 to 20% of the population, a risk factor for lupus).
• Raised C4: inflammation.

CH50 (total haemolytic complement).

• Reference range 31 to 66 U/mL (or 100 to 300 units, method dependent). Measures the integrity of classical cascade and terminal components (C1 to C9).
• Low or absent CH50: deficiency of any classical or terminal component, or consumption. Zero CH50 with normal C3 and C4 suggests terminal component (C5 to C9) deficiency, with recurrent Neisseria infections.
• AP50: alternative cascade activity, low in factor B, D, properdin deficiency and C3 deficiency.

Pattern interpretation.

Hereditary angio oedema. C4 low with low C1 inhibitor antigen (type 1) or low function (type 2), and normal C1q. Acquired angio oedema: low C1q also. C4 is a screening test, and C1 inhibitor level and function confirm diagnosis.

Critical values and findings needing immediate action.

• Falling C3 and C4 with rising anti-dsDNA and active urinary sediment: lupus nephritis flare, urgent nephrology review and biopsy.
• Very low C3 with acute nephritic syndrome in a child after infection: post-streptococcal glomerulonephritis, with blood pressure and fluid management.
• Low C3 with haemolytic anaemia, thrombocytopenia and renal failure: atypical haemolytic uraemic syndrome, urgent plasma exchange or complement inhibitor.
• Facial, laryngeal or abdominal swelling with low C4: hereditary angio oedema attack, emergency treatment with C1 inhibitor concentrate or icatibant (adrenaline and antihistamines are ineffective).
• Recurrent meningococcal disease with zero CH50: terminal complement deficiency, vaccination and prophylactic antibiotics.

Interferences and pitfalls.

• Sample handling is the main source of error: delay, warm storage and repeated freezing activate complement in vitro and give falsely low values.
• C3 and C4 are acute phase proteins, and raised baselines can mask consumption, so a normal level in active disease is possible. Trends within the same patient are more informative than single values.
• Cryoglobulins precipitate in the cold and can lower C4 artefactually, so samples are kept at 37 °C when cryoglobulinaemia is suspected.
• Immunoassays can underestimate levels in the presence of C3 nephritic factor or paraproteins.
• A normal C3 and C4 does not exclude SLE or nephritis.
• Complement levels do not correlate perfectly with disease activity, and anti-dsDNA, urine sediment and clinical features are used together.
• Genetic C4 deficiency (null alleles) gives a persistently low C4 without disease.
• Eculizumab and similar drugs block terminal activity and CH50 is very low on treatment.
• CH50 requires fresh handling and is only done in specialised laboratories.

Special groups.

• Children: C3 is lower in neonates, and low C3 in nephritic syndrome with persistent low levels beyond 8 weeks suggests membranoproliferative disease or C3 glomerulopathy.
• Pregnancy: complement rises in normal pregnancy, and a normal level in lupus pregnancy may hide active disease.
• Older adults: raised baselines.
• Regions: post-infectious glomerulonephritis is more common in low income settings.

Follow up tests. Anti-dsDNA and ANA, C1 inhibitor antigen and function, C1q, C3 nephritic factor, factor H, factor I and factor B levels, anti-factor H antibodies, soluble C5b-9, cryoglobulins, hepatitis C serology, anti-streptolysin O and anti-DNase B, urinalysis, renal biopsy with immunofluorescence, and genetic testing for complement gene variants.