Complement System Assessment

Complement system assessment is the laboratory measurement of complement proteins and their functional activity. The complement system is a cascade of plasma proteins that participates in innate immune defence, inflammation, and clearance of immune complexes; its testing supports the evaluation of autoimmune disease activity, certain infections, and inherited or acquired complement deficiencies.

Definition

Complement assessment comprises laboratory tests that quantify complement proteins or measure the functional integrity of the complement activation pathways to evaluate immune and inflammatory disorders.

Scope

The topic covers measurement of individual complement components (such as C3 and C4), functional pathway assays (such as the total haemolytic complement test), and the interpretation of complement consumption and deficiency. It is a methodological and reference topic within clinical immunology.

Core questions

• When does measuring individual components (C3, C4) add to a functional pathway assay, and vice versa?
• How are low complement levels interpreted as consumption versus deficiency?
• Which pathway (classical, alternative, lectin) does a given functional assay interrogate?

Key concepts

• Classical, alternative, and lectin activation pathways
• Complement components C3 and C4
• Total haemolytic complement (CH50) and alternative-pathway (AH50) assays
• Complement consumption versus inherited deficiency
• Complement regulators and their loss of function
• Immune-complex clearance

Mechanisms

Quantitative immunoassays (often nephelometry or turbidimetry) measure the concentration of individual complement proteins such as C3 and C4. Functional assays test whether a serum sample can complete a pathway: the total haemolytic complement (CH50) assay measures classical-pathway integrity by the ability of serum to lyse antibody-coated red cells, while alternative-pathway assays interrogate that route. Reduced levels may reflect consumption during active immune-complex disease or an inherited deficiency of a component or regulator; combining quantitative and functional results helps distinguish these possibilities.

Clinical relevance

Complement results contribute to the assessment of conditions such as systemic lupus erythematosus, where consumption can accompany disease activity, and to the recognition of complement-deficiency states associated with recurrent infection or complement-mediated disease. The entry explains how these tests work and what their patterns indicate at a population level; it is not a basis for individual management decisions.

Epidemiology

Inherited deficiencies of individual complement components are individually uncommon but collectively contribute to susceptibility to certain encapsulated-organism infections and to immune-complex disease. Acquired low complement is more frequently seen in the context of active immune-complex-mediated disorders.

Evidence & guidelines

Complement testing is guided by laboratory-immunology reference texts and standardisation efforts that define reference methods and units for component and functional assays. Reviews of complement biology frame how the pathways and their regulators map onto the available laboratory measurements.

History

The haemolytic activity of serum was characterised at the turn of the twentieth century, and the total haemolytic complement assay grew from that early work. Later identification of the individual components, the alternative and lectin pathways, and the regulatory proteins expanded the testing repertoire from a single functional measure to a panel of component and pathway-specific assays.

Seminal works

• ricklin-2010
• zipfel-2009

Frequently asked questions

What does the CH50 test measure?

It measures the functional integrity of the classical complement pathway by testing whether a serum sample can lyse antibody-coated red blood cells; a very low result suggests a missing or consumed component.

Why might complement levels be low?

Low complement can reflect consumption during active immune-complex disease or an inherited deficiency of a complement component or regulator; quantitative and functional tests together help distinguish these.