Test Catalog

Test ID: C1Q    
Complement C1q, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Assessment of an undetectable total complement (CH50) level


Diagnosing congenital C1 (first component of complement) deficiency


Diagnosing acquired deficiency of C1 inhibitor

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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, C2, C4) results in an inability to clear immune complexes. Inherited deficiency of C1 is rare.


Like the more common C2 deficiency, C1 deficiency is associated with increased incidence of immune complex disease (systemic lupus erythematosus, polymyositis, glomerulonephritis, and Henoch-Schonlein purpura). Low C1 levels have also been reported in patients with abnormal immunoglobulin levels (Bruton and common variable hypogammaglobulinemia and severe combined immunodeficiency). This is most likely due to increased catabolism.


The measurement of C1q is an indicator of the amount of C1 present.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

12-22 mg/dL

Interpretation Provides information to assist in interpretation of the test results

An undetectable C1q in the presence of an absent total complement (CH50) and normal C2, C3, and C4 suggests a congenital C1 (first component of complement) deficiency.


A low C1q in combination with a low C1 inhibitor and low C4 suggests an acquired C1 inhibitor deficiency.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

This is a different assay than C1q binding, which is an assay for circulating immune complexes.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Frank MM: Complement in the pathophysiology of human disease. N Engl J Med 1987 June 11;316(24):1525-1530

2. Frank MM: Complement deficiencies. Pediatr Clin North Am 2000 December;47(6):1339-1354

3. Frigas E: Angioedema with acquired deficiency of the C1 inhibitor: a constellation of syndromes. Mayo Clin Proc 1989 October;64(10):1269-1275