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Hematology & Immunology

Diagnostic Complement Assays: CH50 and AH50

Core Principle of Complement Testing
🧷 The complement system consists of three activation pathways (classical, alternative, lectin) that converge at C3, leading to a common terminal pathway that forms the membrane attack complex (MAC).
🧷 CH50 (total hemolytic complement) tests the integrity of the entire classical pathway from C1 through C9, while AH50 tests the alternative pathway from factors B, D, and properdin through C9.
🧷 These functional assays measure the serum's ability to lyse antibody-coated sheep RBCs (CH50) or rabbit RBCs (AH50), reported as the dilution causing 50% hemolysis.
🧷 Board pearl: CH50 and AH50 are screening tests — an abnormal result indicates a complement deficiency somewhere in that pathway, requiring individual component testing for precise diagnosis.
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Understanding CH50: The Classical Pathway Screen
📍 CH50 requires all nine classical pathway components (C1q, C1r, C1s, C2, C3, C4, C5, C6, C7, C8, C9) to be present and functional.
📍 A normal CH50 essentially excludes deficiency of any classical or terminal pathway component — it's an excellent screening test.
📍 CH50 is reported in units/mL, with normal ranges varying by laboratory (typically 30–75 U/mL).
📍 Low CH50 indicates either complement consumption (active disease) or hereditary deficiency of one or more components.
📍 Board clue: CH50 of zero or near-zero suggests complete deficiency of a single component, while moderately reduced levels suggest consumption.
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Understanding AH50: The Alternative Pathway Screen
🔹 AH50 tests the alternative pathway components (Factor B, Factor D, properdin) plus the shared terminal pathway (C3, C5–C9).
🔹 This pathway provides innate immunity through spontaneous C3 hydrolysis and amplification on pathogen surfaces.
🔹 Normal AH50 with low CH50 localizes the deficiency to early classical pathway components (C1, C2, or C4).
🔹 Low AH50 with normal CH50 indicates isolated alternative pathway component deficiency (rare).
🔹 Board pearl: Both CH50 and AH50 low → deficiency in the shared components (C3 or C5–C9); consider C3 deficiency if infections are predominantly bacterial.
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Pre-analytical Considerations and Sample Handling
⭐ Complement proteins are thermolabile — improper handling leads to falsely low results.
⭐ Samples must be processed immediately or frozen at −70°C; avoid repeated freeze-thaw cycles.
⭐ Serum (not plasma) is required because EDTA or citrate chelates calcium and magnesium needed for complement activation.
⭐ Hemolysis interferes with the assay by releasing hemoglobin that affects optical density measurements.
⭐ Board pearl: A surprisingly low CH50 in a healthy patient → consider pre-analytical error before pursuing extensive workup for complement deficiency.
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Complement Consumption Patterns in Active Disease
✅ SLE: Low C3, C4, and CH50 during active disease, especially with nephritis. Levels normalize with treatment.
✅ Post-infectious glomerulonephritis: Low C3 with normal C4 (alternative pathway activation), normalizes in 6–8 weeks.
✅ C3 nephritic factor (membranoproliferative GN type II): Persistently low C3 and AH50 with normal C4 and CH50.
✅ Cryoglobulinemia and serum sickness: Low C4 with relatively preserved C3 (early classical pathway consumption).
✅ Board distinction: Transient complement consumption (post-infectious GN) versus persistent consumption (C3 nephritic factor) guides diagnosis.
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Hereditary Angioedema and C1 Inhibitor Testing
🧠 Hereditary angioedema (HAE) results from C1 inhibitor deficiency → unregulated classical pathway activation → low C4.
🧠 During attacks: very low C4, normal C3, low-normal CH50 (C4 depletion limits classical pathway function).
🧠 Between attacks: C4 remains chronically low (diagnostic clue), C3 normal, CH50 may normalize.
🧠 C1 inhibitor testing: both antigenic level and functional assay needed — Type I HAE has low levels, Type II has normal levels but defective function.
🧠 Board pearl: Recurrent angioedema without urticaria + low C4 → think HAE and check C1 inhibitor.
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Terminal Complement Deficiencies and Neisserial Infections
⚡ Deficiencies of C5, C6, C7, C8, or C9 → inability to form MAC → increased susceptibility to Neisseria (meningococcus and gonococcus).
⚡ Classic presentation: recurrent or severe neisserial infections, often with unusual serotypes or at older ages than typical.
⚡ CH50 and AH50 are both zero or very low because both pathways require terminal components to complete hemolysis.
⚡ Individual component assays (C5–C9) needed to identify the specific deficiency.
⚡ Board pearl: Teenager with second episode of meningococcal meningitis → screen for terminal complement deficiency with CH50/AH50.
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Early Classical Pathway Deficiencies
📌 C1q, C1r, C1s deficiency: Strong association with SLE-like syndrome (>90% develop autoimmunity).
📌 C4 deficiency: Two genes (C4A and C4B) — complete deficiency rare but associated with SLE; partial deficiency common.
📌 C2 deficiency: Most common complement deficiency in Caucasians; 10–30% develop SLE-like disease.
📌 Mechanism: Impaired clearance of immune complexes and apoptotic cells → loss of tolerance → autoantibody production.
📌 Board pearl: Young patient with lupus-like disease and CH50 of zero but normal AH50 → suspect early classical pathway component deficiency.
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C3 Deficiency: The Central Component
📣 C3 deficiency is rare but severe — C3 is the convergence point of all pathways and the most abundant complement protein.
📣 Clinical features: Recurrent severe bacterial infections (especially encapsulated organisms), membranoproliferative glomerulonephritis, and sometimes SLE-like disease.
📣 Laboratory: Both CH50 and AH50 are zero (C3 required for both pathways), very low or absent C3 level.
📣 Mechanism: Loss of opsonization (C3b), chemotaxis (C3a), and immune complex clearance.
📣 Board clue: Infant with recurrent pneumococcal sepsis and both CH50 and AH50 of zero → C3 deficiency until proven otherwise.
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Alternative Pathway Deficiencies
🔸 Factor B deficiency: Rare, presents with recurrent pyogenic infections similar to C3 deficiency.
🔸 Factor D deficiency: Extremely rare, also causes recurrent neisserial infections.
🔸 Properdin deficiency: X-linked, increased susceptibility to meningococcal disease, especially in young males.
🔸 Laboratory pattern: Low AH50 with normal CH50 and normal C3, C4 levels.
🔸 Board pearl: Male child with meningococcal sepsis and family history of males dying from meningitis → consider properdin deficiency (X-linked).
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Complement Testing in Glomerulonephritis
🧷 Post-streptococcal GN: Low C3, normal C4, low AH50, normal to slightly low CH50. Normalizes in 6–8 weeks.
🧷 Lupus nephritis: Low C3, low C4, low CH50. Levels correlate with disease activity.
🧷 Membranoproliferative GN type I: Variable — may have low C3 and/or C4 depending on triggering disease.
🧷 MPGN type II (dense deposit disease): Persistently low C3, very low AH50, normal C4 and CH50 due to C3 nephritic factor.
🧷 Board distinction: Transient hypocomplementemia (post-infectious) versus persistent (MPGN II, active lupus).
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Paroxysmal Nocturnal Hemoglobinuria and Complement
📍 PNH results from acquired deficiency of GPI-anchored proteins including CD55 (DAF) and CD59 on RBC surfaces.
📍 Loss of complement regulatory proteins → increased sensitivity to complement-mediated lysis.
📍 CH50 and AH50 are typically normal because the defect is on the cell surface, not in the fluid phase.
📍 Diagnosis: Flow cytometry showing loss of CD55/CD59 on RBCs, not complement levels.
📍 Board pearl: Hemolytic anemia with normal CH50/AH50 but positive Ham test (acidified serum) → PNH, not systemic complement deficiency.
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Acquired Complement Deficiencies
🔹 Sepsis and DIC: Consumption of multiple components → low CH50, variable individual components.
🔹 Liver disease: Decreased synthesis of most complement proteins (made in liver) → proportionally low levels.
🔹 Malnutrition: Reduced synthesis, particularly affecting C3.
🔹 Autoantibodies: C3 nephritic factor (stabilizes C3 convertase), C1q antibodies in lupus, C1 inhibitor antibodies.
🔹 Board clue: Critically ill patient with new-onset low complement → think consumption from sepsis before considering hereditary deficiency.
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Complement Genetics and Inheritance Patterns
⭐ Most complement deficiencies are autosomal recessive — both alleles must be affected for complete deficiency.
⭐ Exception: Properdin deficiency is X-linked recessive (affects males).
⭐ C4 exists as two genes (C4A and C4B) — complete deficiency requires loss of all four alleles (rare).
⭐ Heterozygotes typically have 50% of normal levels and are usually asymptomatic.
⭐ Board pearl: Complement deficiencies presenting in childhood with severe infections suggest homozygous deficiency; family screening may identify asymptomatic heterozygotes.
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Clinical Associations by Component
✅ C1q, C1r, C1s, C4, C2 deficiency → SLE-like autoimmunity (impaired immune complex clearance).
✅ C3 deficiency → Severe pyogenic infections + glomerulonephritis.
✅ C5–C9 deficiency → Neisserial infections (impaired bacterial lysis).
✅ Properdin deficiency → Neisserial infections (X-linked pattern).
✅ Factor H or I deficiency → Uncontrolled alternative pathway activation → low C3, atypical HUS, macular degeneration.
✅ Board summary: Early components → autoimmunity; late components → Neisseria; regulatory proteins → uncontrolled activation.
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Interpreting Combined CH50/AH50 Results
🧠 Both low → Terminal pathway deficiency (C5–C9) or C3 deficiency.
🧠 CH50 low, AH50 normal → Early classical pathway deficiency (C1, C2, C4).
🧠 CH50 normal, AH50 low → Alternative pathway deficiency (Factor B, D, properdin) — rare.
🧠 Both normal → No significant complement deficiency; consider other causes of patient's symptoms.
🧠 Board approach: Use the pattern to localize the deficiency, then order specific component levels for confirmation.
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Follow-up Testing After Abnormal Screening
⚡ Abnormal CH50 → measure C3, C4 first (most common abnormalities), then C1q, C2 if those are normal.
⚡ Abnormal AH50 with normal CH50 → measure Factor B, properdin.
⚡ Both CH50 and AH50 abnormal → start with C3; if normal, test terminal components (C5–C9).
⚡ Consider functional assays for suspected regulatory protein defects (C1 inhibitor, Factor H).
⚡ Board pearl: The screening tests (CH50/AH50) localize the problem; specific component assays make the diagnosis.
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Monitoring Disease Activity with Complement
📌 SLE: C3 and C4 levels correlate with disease activity, especially renal flares. Rising levels suggest improving disease.
📌 Serial measurements more useful than single values — track trends during treatment.
📌 Normalization of complement may lag behind clinical improvement by weeks.
📌 Persistently low levels despite clinical remission may indicate ongoing subclinical activity.
📌 Board application: Patient with lupus nephritis on treatment with improving proteinuria but persistently low C4 → consider inadequate immunosuppression.
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Board Question Stem Patterns
📣 Recurrent meningococcal infections in a teenager → terminal complement deficiency, check CH50 and AH50.
📣 Young woman with malar rash and CH50 of zero → early classical pathway deficiency with secondary SLE.
📣 Child with recurrent sinopulmonary infections and undetectable C3 → C3 deficiency.
📣 Post-streptococcal GN with C3 not normalizing after 8 weeks → consider MPGN or C3 glomerulopathy.
📣 Angioedema without urticaria and low C4 → hereditary angioedema, check C1 inhibitor.
📣 Male infant with fatal meningococcemia and maternal uncle who died similarly → properdin deficiency (X-linked).
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One-Line Recap
🔸 CH50 and AH50 are functional screens for classical and alternative complement pathways respectively — abnormal results localize deficiencies that predispose to autoimmunity (early components), pyogenic infections (C3), or neisserial disease (terminal components), with patterns guiding diagnosis of hereditary deficiencies or consumption in active disease.
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