Lab Tests Reference Library

ANA (Antinuclear Antibodies) is a screening test for autoimmune diseases. A positive ANA result means the immune system is producing antibodies that attack the body's own cell nuclei. It is the first-line screening test for Systemic Lupus Erythematosus (SLE), Sjögren's syndrome, scleroderma, myositis, and mixed connective tissue disease.

Anti-double-stranded DNA (anti-dsDNA) antibodies are highly specific for Systemic Lupus Erythematosus (SLE/Lupus). Unlike the broad ANA screen, anti-dsDNA is one of the most specific markers for SLE available. Rising anti-dsDNA levels often correlate with lupus disease activity, especially lupus nephritis (kidney inflammation).

Rheumatoid Factor (RF) is an autoantibody — typically IgM — directed against the Fc portion of IgG immunoglobulin. It is detected in rheumatoid arthritis and several other autoimmune, chronic inflammatory, and infectious conditions. RF is a screening test for rheumatoid arthritis, but it is neither sensitive nor specific enough to diagnose RA alone.

Anti-CCP (anti-cyclic citrullinated peptide) antibodies are highly specific autoantibodies for Rheumatoid Arthritis (RA). They target citrullinated proteins produced during inflammation. Anti-CCP is more specific for RA than Rheumatoid Factor and can be detected years before clinical symptoms appear — making it valuable for early diagnosis and prognosis.

C3 (Complement component 3) is the most abundant protein in the complement system — the arm of innate immunity responsible for clearing pathogens and immune complexes. C3 acts as a central hub connecting both classical and alternative complement pathways. Low C3 indicates complement consumption (being used up) by immune complex disease, while high C3 is a non-specific acute phase response.

C4 (Complement component 4) is an early-pathway complement protein involved in immune complex clearance. It is part of the classical complement pathway. Low C4 with low C3 is the hallmark of active lupus (SLE). C4 can also be low in hereditary angioedema (HAE) — a rare but life-threatening condition of recurrent swelling.

IgG is the most abundant immunoglobulin in blood, comprising approximately 75% of total immunoglobulins. It provides long-term humoral immunity, crosses the placenta to protect newborns, and opsonizes pathogens for phagocytosis. IgG is produced by plasma cells as the predominant antibody in secondary (memory) immune responses.

Total IgE measures the total amount of IgE immunoglobulin in the blood. IgE is the antibody class responsible for immediate hypersensitivity reactions (Type I allergy). It binds to mast cells and basophils; when allergen cross-links IgE on these cells, histamine and other mediators are released — causing allergic symptoms. Elevated IgE is seen in atopic conditions and parasitic infections.