C1q Complement, Functional, Serum

CPT CODE:

USEFUL FOR:

Diagnosis of C1 deficiency
Investigation of a patient with an absent total complement (CH[50]) level

SPECIMEN REQUIRED:

Draw blood in a plain, red-top tube(s) or a serum gel tube(s) froma fasting patient. Spin down, separate from clot, and send 1 mLof serum frozen in plastic vial.

TRANSPORT TEMPERATURE:

Frozen\Refrig <3 days OK\Ambient NO

CLINICAL INFORMATION:

Complement proteins are components of the innate immune system.  There are 3 pathways to complement activation: 1) the classic pathway, 2) the alternative (or properdin) pathway, and 3) the lectinactivation (or mannan binding protein, [MBP]) pathway. The classicpathway of the complement system is composed of a series ofproteins that are activated in response to the presence of immune complexes. The activation process results in the generation ofpeptides that are chemotactic for neutrophils and that bind to immunecomplexes and complement receptors. The end result of thecomplement activation cascade is the formation of the lyticmembrane attack complex (MAC). 
The first component of complement (C1) is composed of 3 subunits designated as C1q, C1r, and C1s. C1q recognizes and binds to immunoglobulin complexed to antigen and initiates the complement cascade. Congenital deficiencies of any of the early complement components (C1-C4) result in an inability to generate the peptidesthat are necessary to clear immune complexes and to attract neutrophils or generate lytic activity. These patients have increased susceptibility to infections with encapsulated microorganisms. Theymay also have symptoms that suggest autoimmune disease andcomplement deficiency may be an etiologic factor in the developmentof autoimmune disease.
Inherited deficiency of C1 is rare. C1 deficiency is associated with increased incidence of immune complex disease (systemic lupus erythematosus [SLE], polymyositis, glomerulonephritis, and Henoch-Schonlein purpura), and SLE is the most common manifestation of C1deficiency. The SLE associated with C1 deficiency is similar to SLEwithout complement deficiency, but the age of onset is often prior topuberty.
Low C1 levels have also been reported in patients with abnormal immunoglobulin levels (Bruton's and common variable hypogammaglobulinemia and severe combined immunodeficiency),and this is most likely due to increased catabolism.
Complement levels can be detected by antigen assays that quantitatethe amount of the protein. For most of the complement proteins a smallnumber of cases have been described in which the protein is presentbut is non functional. These rare cases require a functional assay to detect the deficiency.

CLINICAL INTERPRETATION:

Low levels of complement may be due to inherited deficiencies, acquired deficiencies, or due to complement consumption (e.g., as a consequence of infectious or autoimmune processes).
The measurement of C1q activity is an indicator of the amount of C1 present. Absent C1q levels in the presence of normal C3 and C4 values are consistent with a C1 deficiency. Low C1q levels in the presence of low C4 but normal C3 may indicate the presence of an acquired inhibitor (autoantibody) to C1 esterase inhibitor.

REFERENCE VALUES:

34-63 unit/mL