Complement Testing - Complement Deficiency and Anticomplement Therapeutic Response Monitoring

Last Literature Review: February 2023 Last Update:

Complement deficiency is a category of primary immunodeficiency disease.  The complement system comprises a complex group of proteins that play a role in host defense and inflammation. These proteins act as a cascade with three parallel pathways, each activated by different stimuli: the classical pathway (CP), alternative pathway (AP), and lectin pathway (LP). These three pathways converge on a terminal pathway (TP). Deficiency in any of the proteins involved in these pathways, including complement proteins and regulators (eg, complement component 3 [C3]) and factors (eg, factor B), may lead to recurrent infections (eg, Neisseria) or inappropriate immune responses.  Laboratory testing for complement deficiency includes primary testing to determine which pathway is affected, followed by secondary testing to identify the deficient complement protein and/or its regulators to reach a definitive diagnosis. Laboratory testing may also be used to monitor complement blockage in patients treated with anticomplement therapeutics.

Quick Answers for Clinicians

Which laboratory tests can be used to monitor complement blockage in individuals treated with complement component 5 (C5) inhibitors?

Anti-C5 therapeutics (eg, eculizumab, ravulizumab) will impact complement functional assays that rely on the activation of complement component 5 (C5), such as total complement activity (CH50), alternative complement activity (AH50), and C5 functional activity assays.   These tests may be helpful to monitor therapeutic efficacy by assessing the impact of complement blockage relative to baseline activity. Refer to the Testing to Monitor Anti-C5 Therapeutic Response section for more information.

When is testing for complement function, rather than complement concentration, preferred?

Patients with congenital deficiencies in any components of the terminal pathway (C5-C9) may be susceptible to recurrent infections (eg, Neisseria) and inappropriate immune responses. A small number of individuals with these findings may have normal protein concentrations but defective protein functions. In these cases, functional assays may be useful to detect deficiency. Abnormal functional test results can be used to investigate any defects in upstream components of the complement pathway. Complement functional test results can also be impacted when the patient is being treated with complement inhibitory drugs (eg, eculizumab or ravulizumab).

Which complement tests are useful in the evaluation of systemic lupus erythematosus?

Complement consumption often occurs in active systemic lupus erythematosus (SLE), which leads to low concentrations of classical pathway complement proteins, particularly complement component 3 (C3) and C4.  Complement testing, including total complement testing and testing for the C3 and C4 components, may therefore be useful to support a diagnosis of SLE.  Complement tests are also recommended every 3-6 months to monitor disease activity in patients with SLE.  Decreased C3 and C4 concentrations may also be associated with glomerulonephritis in SLE. For more information, refer to the ARUP Consult Systemic Lupus Erythematosus topic.

What is the role of complement testing in hereditary angioedema?

The complement component 1 (C1) esterase inhibitor protein is deficient in hereditary angioedema, which may lead to complement defects.  Laboratory testing for hereditary angioedema includes testing for C1 esterase and C4 concentrations.

Laboratory Testing

Primary Testing for Complement Deficiency

Initial evaluation for suspected complement deficiency is used to identify the affected pathway and should include testing for CP and TP activity (using the total complement activity [CH50] assay for total hemolytic complement) and AP activity (using the alternative complement activity [AH50] assay for alternative pathway hemolytic activity). 

The initial evaluation may also include testing for LP function (using a mannose-binding lectin [MBL] test), depending on the clinical circumstance. 

Secondary Testing for Complement Deficiency

The results from the initial evaluation guide secondary testing decisions to identify the deficient component or factor. Complement deficiency may present similarly to complement consumption (eg, due to autoimmune disease); complement consumption can be distinguished by the simultaneous reduction in multiple complement component concentrations.  Secondary tests may include tests to directly measure protein concentrations or to assess protein function.  Genetic testing may be useful if a hereditary deficiency of a particular component or regulatory factor is suspected.

Primary and Secondary Testing for Complement Deficiencya
Primary Testing Defect Implicated Secondary Testing

Low or absent



CP defect or complement consumptionb

Complement concentrations or functional testing: C1, C2, C4

Normal Low or absent Normal AP or regulatory factor deficiency

Properdin concentration or functional testing

Factor levels: B, D

Low or absent

Low or absent


TP defect, regulatory factor deficiency, or complement consumptionb

Complement component concentrations or functional testing: C3, C5, C6, C7, C8, C9

Factor levels: H, I




LP defect

LP components

aRefer to the Complement Deficiency Testing Algorithm for visual representation of the suggested testing sequence.

bThe simultaneous reduction in concentrations of multiple components (eg, C3 and C4) suggests a disorder of complement consumption.

Source: Bonilla, 2015 

Testing to Monitor Anti-C5 Therapeutic Response

The activation of C5 results in a proinflammatory response and cell lysis, which plays a role in the pathophysiology of several complement-associated diseases.  Complement-targeted therapy, specifically selective inhibition of C5 activation and the subsequent activation of C5a and C5b, is an ongoing area of research and drug development.  Some C5 inhibitors (eg, eculizumab and ravulizumab) have been approved by the U.S. Food and Drug Administration (FDA) and are prescribed for various conditions (eg, atypical hemolytic uremic syndrome, myasthenia gravis, neuromyelitis optica spectrum disorder, and paroxysmal nocturnal hemoglobinuria) ; other C5 inhibitors are being evaluated in clinical trials. 

Personalized monitoring is strongly recommended for individuals taking anti-C5 therapeutics because the clearance of these drugs is variable based on disease activity and renal state. Treatment with C5 inhibitors is associated with side effects such as increased susceptibility to Neisseria infection  and may be costly. Therefore, complement testing, in conjunction with clinical evaluation, may be useful to monitor and optimize treatment efficacy.   In patients with partial clinical response, assessing complement blockage may be helpful to escalate therapy. Similarly, a select group of patients with less severe pathology may benefit from testing to assess the appropriateness of dose deescalation or drug discontinuation.

C5 inhibitors will impact the complement functional assays that depend on the activation of C5 and the formation of the membrane attack complex. CH50, AH50, and C5 functional assays all rely on this activation and may be useful,   although C5 functional assays provide the most specific measurement of C5 blockage by C5 inhibitors.

ARUP Laboratory Tests

Primary Tests for Complement Deficiency

Initial test for a suspected deficiency in the classical or terminal complement pathway

Initial test for a suspected deficiency in the alternative complement pathway

Initial test for a suspected deficiency in the lectin complement pathway

Secondary Tests for Complement Deficiency

Use to identify a deficiency of specific complement components or factors

Tests to Monitor Anti-C5 Therapeutic Response

Use to monitor complement blockage in patients treated with C5 inhibitor drugs


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