# Complement Deficiency

Use CH50 and AH50 as the first branch point in patients with recurrent invasive infection, then confirm the implicated component and distinguish inherited deficiency from complement consumption, protein loss, impaired synthesis, or C1-inhibitor–mediated angioedema.

**Clinical question:** How should physicians use CH50, AH50, and targeted assays to identify and manage suspected complement deficiency?

Updated: 2026-09-15T17:30:07.279400+00:00

## What matters in practice
- Order CH50 and AH50 together when complement deficiency is suspected; the paired pattern directs component-level testing. [10][11][14]
- Low CH50 with normal AH50 localizes to the classical pathway and should prompt assessment of C1, C2, C4, and C1 inhibitor. [10][11]
- Low AH50 with normal CH50 suggests an alternative-pathway defect; measure factor B, factor D, factor H, and factor I. [10][11]
- Concurrently low CH50 and AH50 suggests a shared terminal-pathway defect involving C3 or C5-C9 and warrants quantitative and functional component testing. [10][11][14]
- For recurrent nonurticarial angioedema, measure C4 plus C1-inhibitor antigen and function; normal antigen does not exclude type 2 hereditary angioedema. [13][20]

## Use paired functional assays to localize the defect

Functional pathway testing determines the next component assays.

In a patient with recurrent invasive or unusually severe bacterial infection, obtain CH50 and AH50 concurrently as the initial complement screen. CH50 evaluates classical-pathway function and AH50 evaluates alternative-pathway function; abnormal results should drive targeted quantitative and functional assays rather than indiscriminate component testing. [10][11][14]

Interpret a low CH50 with normal AH50 as a classical-pathway pattern. Quantify and, where indicated, assess function of C1, C2, C4, and C1 inhibitor. A low AH50 with normal CH50 instead directs evaluation of factor B, factor D, factor H, and factor I. [10][11]

If both CH50 and AH50 are low, prioritize evaluation of components shared by both pathways: C3 and terminal components C5, C6, C7, C8, and C9. If both assays are normal but recurrent infection still raises concern for a complement disorder, obtain an MBL functional assay or MBL measurement. [10][11][14]
- Low CH50, normal AH50: classical pathway—assess C1, C2, C4, and C1 inhibitor. [10][11]
- Normal CH50, low AH50: alternative pathway—assess factor B, factor D, factor H, and factor I. [10][11]
- Low CH50, low AH50: shared terminal pathway—assess C3 and C5-C9. [10][11][14]
- Normal CH50, normal AH50 with persistent clinical suspicion: assess MBL. [10][11]

*Functional complement assay patterns and directed follow-up testing. [10][11][14]*

| CH50 | AH50 | Most likely localization | Next laboratory step |
| --- | --- | --- | --- |
| Low [10][11] | Normal [10][11] | Classical pathway: C1, C2, C4, or C1-inhibitor abnormality [10][11] | Measure C1, C2, C4, and C1-inhibitor antigen/function. [10][11] |
| Normal [10][11] | Low [10][11] | Alternative pathway: factor B, D, H, or I abnormality [10][11] | Measure factor B, factor D, factor H, and factor I. [10][11] |
| Low [10][11][14] | Low [10][11][14] | Shared terminal pathway: C3 or C5-C9 abnormality [10][11][14] | Obtain component-specific quantitative and functional testing for C3 and C5-C9. [10][11][14] |
| Normal [10][11] | Normal [10][11] | Consider lectin-pathway abnormality if clinical suspicion persists. [10][11] | Obtain MBL functional assay or MBL measurement. [10][11] |

## Match infection and inflammatory patterns to the pathway result

The clinical phenotype helps distinguish inherited absence from acquired depletion.

Early classical-pathway component deficiencies, including C1, C2, and C4 deficiency, are associated with infection by encapsulated bacteria, particularly Streptococcus pneumoniae and Haemophilus influenzae type b. In this phenotype, a low CH50 with preserved AH50 supports a classical-pathway defect and should trigger component confirmation. [10]

C3 deficiency is associated with severe recurrent pyogenic infections early in life. A pattern of both low CH50 and low AH50 is compatible with C3 or terminal-pathway deficiency, but component-level assays are required before assigning a specific diagnosis. [10][11][14]

Do not label every abnormal functional assay as inherited deficiency. Complement abnormalities may be acquired through complement overconsumption, impaired protein synthesis, protein-loss disorders, autoimmunity, or high catabolic states. Reassess CH50/AH50 and component levels in the clinical context before interpreting a single abnormal result as a constitutional defect. [10]
- Encapsulated bacterial infection pattern plus isolated low CH50: confirm an early classical-pathway abnormality. [10][11]
- Early severe recurrent pyogenic infections plus low CH50 and AH50: prioritize C3 testing while evaluating terminal components. [10][11]
- Systemic autoimmune disease, protein loss, impaired synthesis, or high catabolic illness: consider acquired complement reduction before diagnosing hereditary deficiency. [10]

*Clinical patterns that refine interpretation of complement testing. [10][11]*

| Clinical presentation | Laboratory clue | Interpretation | Action |
| --- | --- | --- | --- |
| Recurrent pneumococcal or Hib infection [10] | Low CH50 with normal AH50 [10][11] | Early classical-pathway defect is favored. [10][11] | Confirm C1, C2, and C4 abnormalities; include C1 inhibitor when clinically indicated. [10][11] |
| Severe recurrent pyogenic infection beginning early in life [10] | Low CH50 and low AH50 [10][11][14] | C3 deficiency or a terminal common-pathway defect is possible. [10][11][14] | Measure C3 and C5-C9 quantitatively and functionally. [10][11][14] |
| Autoimmune disease, protein loss, impaired synthesis, or high catabolic state [10] | Abnormal functional assay and/or low components [10] | Acquired complement reduction is possible. [10] | Evaluate the active underlying disorder and repeat directed complement testing as clinically appropriate. [10] |

## Test C1-inhibitor deficiency with antigen, function, and C4

Nonurticarial recurrent angioedema requires a distinct complement-directed workup.

For recurrent angioedema without a clear histaminergic pattern, obtain C4, C1-inhibitor antigen concentration, and C1-inhibitor functional activity. C1 inhibitor regulates both the classical complement pathway and bradykinin generation through kallikrein and activated factor XII; deficiency permits bradykinin accumulation and increased vascular permeability. [11][13][20]

C1-inhibitor antigen and function must both be measured. In type 1 hereditary angioedema, C1-inhibitor quantity and/or function is reduced; in type 2 disease, approximately 15% of patients have normal or elevated antigen concentration despite reduced functional activity. C4 is usually low even between attacks, whereas C3 is usually normal. [13]

Acquired anti–C1-inhibitor antibodies can produce acquired angioedema with a clinical presentation similar to hereditary C1-inhibitor deficiency; anti–C1-inhibitor antibodies are also reported in systemic lupus erythematosus with angioedema. When onset and context suggest acquired disease, test for anti–C1-inhibitor antibodies and evaluate for an associated condition. [11][5]

Treat upper-airway attacks as emergencies because laryngeal edema can be life-threatening. Specific acute therapies described for C1-inhibitor–deficient hereditary angioedema include plasma-derived C1-inhibitor, recombinant C1 inhibitor, icatibant 30 mg subcutaneously, and ecallantide 30 mg subcutaneously; acute laryngeal-attack studies report median onset of symptom relief from 15 minutes to approximately 2 hours across regimens. [4][23][5]
- Low C4 plus low C1-inhibitor antigen and function supports type 1 hereditary angioedema. [13]
- Low C4 plus reduced C1-inhibitor function with normal or elevated antigen supports type 2 hereditary angioedema. [13]
- C1-inhibitor autoantibodies support acquired C1-inhibitor deficiency and should prompt evaluation for an associated disease process. [11][5]
- Laryngeal edema requires immediate airway-focused emergency management and prompt administration of a specific on-demand therapy. [23][4]

### Acute and preventive treatment decisions in C1-inhibitor deficiency

For acute hereditary angioedema, a body-weight–adjusted plasma-derived C1-inhibitor dose of 20 IU/kg was effective across body weights and rarely required redosing in one study. In an indirect analysis of 881 laryngeal attacks, no redosing was reported after a single body-weight–adjusted plasma-derived C1-inhibitor dose in 48 attacks; this was not a head-to-head comparison. [3][23]

Fresh frozen plasma contains C1 inhibitor but also prekallikrein, kininogen, and factor XII and may worsen attacks; it is not equivalent to targeted C1-inhibitor replacement. For long-term prevention in recurrent hereditary or acquired C1-inhibitor deficiency, reported options include C1-inhibitor concentrate, tranexamic acid, and androgens, while management of acquired disease also requires treatment of the underlying condition when identified. [4][5][1]

ACE inhibitor–induced angioedema is a bradykinin-mediated alternative diagnosis. Stop the implicated drug and prioritize airway management; C1-inhibitor concentrate, icatibant, ecallantide, and fresh frozen plasma have been described off-label, but severe cases require rapid individualized treatment because consensus is limited. [20]
- Plasma-derived C1 inhibitor: 20 IU/kg for acute HAE attacks in the cited weight-stratified study. [3]
- Icatibant: 30 mg subcutaneously for acute C1-inhibitor–deficient angioedema in cited clinical reports. [5][23]
- Ecallantide: 30 mg subcutaneously in acute laryngeal-attack analyses. [23]
- Avoid relying on fresh frozen plasma as a targeted therapy because its contact-system substrates may worsen an attack. [4]

*C1-inhibitor–directed evaluation and acute treatment options for bradykinin-mediated angioedema. [11][13][20][23]*

| Finding or scenario | Interpretation | Next action |
| --- | --- | --- |
| Low C4; low C1-inhibitor antigen and low function [13] | Type 1 hereditary C1-inhibitor deficiency pattern. [13] | Plan access to on-demand HAE treatment and assess need for prevention based on attack burden. [3][4][5] |
| Low C4; normal or elevated C1-inhibitor antigen with low function [13] | Type 2 hereditary C1-inhibitor deficiency pattern. [13] | Treat and prevent attacks using a C1-inhibitor-deficiency management plan. [4][5] |
| Angioedema with anti–C1-inhibitor antibodies [11] | Acquired C1-inhibitor deficiency is favored. [11] | Evaluate and treat an associated underlying disease; use acute therapy as for hereditary disease. [1][5] |
| Laryngeal attack [23] | Immediate risk of airway compromise. [23] | Provide airway-focused emergency management and administer specific acute therapy without delay. [23][4] |
| Angioedema during ACE inhibitor exposure [20] | Bradykinin-mediated drug-induced angioedema is possible. [20] | Discontinue the ACE inhibitor; manage airway and consider off-label bradykinin-directed therapy in severe disease. [20] |

## Screen for encapsulated-bacterial risk before complement inhibition

Therapeutic complement blockade creates a preventable infection hazard.

Patients receiving complement inhibitors have increased susceptibility to serious, life-threatening, or fatal infection with encapsulated bacteria, including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae type b. Meningococcal infections have occurred with terminal complement inhibitors and may progress rapidly. [16][18][19]

Before initiating a complement inhibitor when feasible, complete or update vaccination against encapsulated bacteria at least 2 weeks before treatment according to current ACIP recommendations; revaccinate according to ACIP schedules during ongoing therapy. Do not initiate iptacopan in a patient with an unresolved serious encapsulated-bacterial infection. [16]

If urgent complement-inhibitor therapy cannot await vaccination, vaccinate as soon as possible and weigh antibacterial prophylaxis individually. The optimal duration, regimen, and effectiveness of antibacterial prophylaxis in vaccinated or unvaccinated recipients of complement inhibitors remain uncertain. [16]
- Vaccinate against encapsulated bacteria at least 2 weeks before planned complement-inhibitor therapy when feasible. [16]
- Maintain vigilance for meningococcal disease even after vaccination because serious infections have occurred in vaccinated recipients. [16]
- For urgent treatment starts, administer vaccines promptly and individualize antibacterial prophylaxis. [16]

*Infection-prevention decisions for complement-inhibitor therapy. [16][18][19]*

| Decision point | Action |
| --- | --- |
| Planned complement-inhibitor initiation [16] | Complete or update vaccination against encapsulated bacteria at least 2 weeks before therapy when feasible. [16] |
| Ongoing complement-inhibitor therapy [16] | Revaccinate according to current ACIP recommendations for the duration of therapy. [16] |
| Urgent treatment before vaccination interval is complete [16] | Vaccinate as soon as possible and individualize consideration of antibacterial prophylaxis. [16] |
| Unresolved serious infection caused by encapsulated bacteria before iptacopan [16] | Do not initiate iptacopan until the infection is resolved. [16] |

## Confirm the specific defect and document its clinical implications

A pathway result is a localization test, not the final diagnosis.

After an abnormal CH50 or AH50 result, use immunochemical assays to measure implicated components; individual complement proteins can be quantified by nephelometry, ELISA, immunoprecipitation methods, or Western blotting. When an immunochemical defect is verified after pathway testing, further functional assays for individual components may not be required. [13]

Distinguish complete from partial deficiency because lower functional activity tracks with infection phenotype in studied C2 and C8 deficiencies. In one cohort, patients with complete complement deficiency and infections consistently had CH50 or AH50 below the lower limit of normal, while partial deficiencies showed higher values and variable infection phenotypes. [12]

For confirmed hereditary abnormalities, document the affected pathway and infection history and coordinate immunology follow-up for family assessment and management planning. For recurrent angioedema, ensure a written acute-attack plan, access to on-demand therapy, and reassessment of preventive treatment when attacks recur or create procedural risk. [5][20]
- Use component quantification to confirm the defect localized by CH50/AH50. [13]
- Interpret markedly depressed functional activity with the clinical infection phenotype; partial and complete deficiencies may not carry the same clinical burden. [12]
- In C1-inhibitor deficiency, reassess acute-treatment access and prevention when recurrent attacks or procedure-related risk persist. [5][20]

*Confirmation steps after an abnormal complement screen. [12][13]*

| Screen result | Confirmation method | Clinical interpretation |
| --- | --- | --- |
| Abnormal CH50 and/or AH50 [13] | Quantify implicated component(s) by immunochemical assay; use targeted functional testing when needed. [13] | Confirms the specific pathway component abnormality rather than relying on global pathway activity alone. [13] |
| Very low pathway activity with recurrent infection [12] | Compare functional activity and component concentration with laboratory reference ranges; pursue component-specific diagnosis. [12] | Complete deficiencies in studied C2 and C8 cohorts were associated with lower functional activity than partial deficiencies. [12] |
| C1-inhibitor antigen/function abnormality [13] | Pair C1-inhibitor results with C4 and clinical angioedema phenotype. [13] | Separates type 1 from type 2 hereditary C1-inhibitor deficiency and identifies need to assess acquired disease. [11][13] |

## References
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## Editorial note

Prepared from cited clinical literature using Astra's research workflow. Verify recommendations against current guidance and patient-specific factors.