Complement component 3, often simply called C3, is a protein of the immune system that is found primarily in the blood. It plays a central role in the complement system of vertebrate animals and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3.

High C3 complement levels indicate inflammation in the body. Lower-than-normal levels of C3 proteins can be a sign of: An autoimmune disease such as psoriatic arthritis, Crohn's disease, lupus or rheumatoid arthritis, or that you're at a higher risk of developing an autoimmune disorder. C3 deficiency, which leads to recurring bacterial infections. Hemolytic anemia (AIHA).

The normal range is 88 to 201 milligrams per deciliter (mg/dL) (0.88 to 2.01 g/L)

The C3 (complement 3) test measures the level of complement component C3 in the blood. Complement proteins are part of the immune system and play a crucial role in inflammation, immune response, and defense against pathogens. Here's how to interpret the results of a C3 test: Normal Range: The normal range for C3 levels in the blood can vary depending on the laboratory and the specific assay used. Typically, normal values fall within the range of 75 to 135 milligrams per deciliter (mg/dL) or 0.75 to 1.35 grams per liter (g/L). Interpretation: Low C3 Levels: Low C3 levels may indicate complement deficiency, which can be congenital (present at birth) or acquired. Congenital complement deficiencies are rare genetic disorders that can predispose individuals to recurrent infections, autoimmune diseases, and inflammatory conditions. Acquired complement deficiencies may occur due to conditions such as autoimmune diseases (e.g., systemic lupus erythematosus), kidney diseases (e.g., lupus nephritis), liver diseases, or certain medications (e.g., corticosteroids). Low C3 levels are also seen in active complement consumption, such as during acute-phase reactions or in immune complex diseases. Clinical Correlation: Interpretation of C3 levels should be done in conjunction with the patient's clinical history, symptoms, and other laboratory tests. Additional diagnostic tests, such as complement functional assays or genetic testing, may be necessary to confirm complement deficiency and identify the underlying cause. Treatment and Management: Management of low C3 levels depends on the underlying cause and clinical context. Treatment may involve addressing the underlying condition (e.g., treating autoimmune diseases with immunosuppressive medications), managing infections promptly, and, in some cases, supplementing with intravenous immunoglobulin (IVIG) or other replacement therapies. High C3 Levels: High C3 levels are less common and may be seen in conditions associated with increased complement activation, such as acute inflammation, infections, or autoimmune diseases. However, isolated high C3 levels without other clinical or laboratory abnormalities are uncommon and may require further evaluation to determine the underlying cause. Follow-Up: Patients with abnormal C3 levels may require monitoring over time to assess response to treatment and disease activity. Serial measurement of C3 levels may be necessary to monitor complement activity and guide treatment decisions. Interpretation of C3 test results should be performed by healthcare professionals experienced in immunology and laboratory medicine. Treatment decisions should be individualized based on the patient's specific clinical circumstances, underlying conditions, and response to therapy. Further evaluation and management may be necessary in cases of abnormal C3 levels to optimize patient care and outcomes.

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