Complement analysis

Indication

Suspicion of congenital or acquired complement deficiency or complement activation.

Method

Nephelometry (C3 and C4), immuno electrophoresis (C1q and C3d) and complement function by ELISA.

Answer

The result of C3 and C4 is given in g/L, C3d in mg/L and C1q as a percentage of normal sera. Results of complement function is also given as percentages of normal sera.

Reference range

C3: 0.77-1.38 g/L. C4: 0.12-0.33 g/L. C1q: 78-131% of normal sera. C3d: 5 mg/L.

Complement function: classical pathway: 69-129%, alternative pathway: 30-113%, MBL pathway: 0-125%.

Interpretation

The complement system is a part of the immune defence. It strengthens the function of the antibodies and contributes to the killing of bacteria. Three pathways of activating the complement system are known: the classical, the alternative and the MBL pathway. (MBL=Mannan binding lectin). The classical pathway is activated by immune complexes, which is the case in SLE. C1q then binds to C1r and C1s and this triggers a cascade where C2 and C4 form a complex which cleaves C3. The alternative pathway is activated by formation of a complex between C3 and factor B through a reaction that especially occurs on the surface of bacteria. The MBL pathway is activated by a reaction between MBL and certain carbohydrates, which are found only in certain bacteria.

When measuring complement activation in vitro, a measure is obtained of the ability of the serum to activate the complement system through a certain pathway. The activation rate is a measure of the serum concentration of the components that are contained in the activating pathway. If the patient has a total lack of a component, the activation of that pathway becomes zero. In a patient with an active SLE disease, an activation of the classical pathway often occurs. This leads to a depletion of the components (C1q, C1s, C1r, C2, C4, C3 and C5-9). Serum from this patient is likely to have a reduced ability to activate the classical pathway in vitro. As some components are shared with the other pathways, their ability to be activated may be reduced because of the depletion of the components of the classical pathway.

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Packages and other tests

View other packages Complement deficiency (572), Hereditary angioedema (HAE) (571). View other tests C1- inhibitor, C1q, C3, C4, C3d, C4d

Acute test