C1 Esterase Inh. Ag (Antigen) – What Your Lab Test Means What is C1 Esterase Inhibitor Antigen (C1-INH Ag)? C1 Esterase Inhibitor Antigen, often abbreviated as C1-INH Ag, is a blood test that measures the amount of C1 esterase inhibitor protein in your blood. This protein plays a crucial role in regulating parts of your immune system and the complement system, which helps fight infections and control inflammation. Why is this marker important? C1 esterase inhibitor helps keep your immune system in balance by preventing excessive activation of the complement pathway, a part of your immune defense. If you don’t have enough of this protein, or if it’s not working properly, it can lead to uncontrolled inflammation and swelling in the body. What does this test diagnose? This test is most commonly used to help diagnose Hereditary Angioedema (HAE) Type I — a rare but serious genetic condition that causes sudden, recurrent episodes of swelling in the face, throat, abdomen, limbs, or airway. It may also be used to evaluate: Acquired angioedema (AAE), which can occur in adults due to other conditions like lymphomas or autoimmune disorders. Unexplained recurrent swelling or abdominal pain without allergic triggers. What do the results mean? C1-INH Antigen Level Possible Interpretation Low Suggests Type I Hereditary Angioedema (HAE) or Acquired Angioedema (AAE) Normal or High If symptoms persist, a functional C1-INH test may be needed to check whether the protein is working properly (to rule out Type II HAE) It’s important to interpret this result alongside C4 levels and possibly a C1-INH functional assay for a full diagnostic picture. Symptoms that may prompt this test Swelling of the face, lips, tongue, throat, or extremities Abdominal pain, cramping, or bloating Difficulty breathing during swelling episodes No response to standard allergy medications (like antihistamines or steroids) How is the test performed? A blood sample is drawn, usually from a vein in your arm. No special preparation is needed in most cases. Related Tests Doctors often order additional tests alongside C1-INH Ag for a more complete diagnosis: C1 Esterase Inhibitor, Functional – Measures whether the protein works properly Complement C4 levels – Often low during HAE attacks C1q level – May help differentiate hereditary vs. acquired angioedema Bottom Line The C1 Esterase Inh. Ag (Antigen) test measures how much of this critical immune-regulating protein is in your blood. A low result can point to Type I Hereditary Angioedema, a condition that causes episodes of swelling that can be life-threatening if untreated. If your antigen level is normal but symptoms continue, your doctor may order further tests to check how well the protein is functioning.
Diagnosing hereditary angioedema, monitoring response to C1 esterase inhibitor replacement therapy
Measurement of the C1 esterase inhibitor (the first component of the complement) is used to diagnose hereditary angioedema and to monitor levels of the inhibitor during treatment. What is hereditary angioedema? Hereditary angioedema is a disorder characterized by recurrent episodes of severe swelling (= angioedema). The most common areas of the body to develop swelling are the limbs, face, intestinal tract, and airway. Minor trauma or stress may trigger an attack, but swelling often occurs without a known trigger. Episodes involving the intestinal tract cause severe abdominal pain, nausea, and vomiting. Swelling in the airway can restrict breathing and lead to life-threatening obstruction of the airway. About one-third of people with this condition develop a non-itchy rash called erythema marginatum during an attack. Symptoms of hereditary angioedema typically begin in childhood and worsen during puberty. On average, untreated individuals have an attack every 1 to 2 weeks, and most episodes last for about 3 to 4 days. The frequency and duration of attacks vary greatly among people with hereditary angioedema, even among people in the same family. There are three types of hereditary angioedema, called types I, II, and III, which can be distinguished by their underlying causes and levels of a protein called C1 inhibitor in the blood. The different types have similar signs and symptoms. Type III was originally thought to occur only in women, but families with affected males have been identified. The most common form (85% of cases) of hereditary angioedema is due to an absolute decrease in the amount of C1 esterase inhibitor. A less common form (15% of cases) is due to the functional abnormality of the inhibitor where levels may be normal. Both abnormalities need to be tested because of the potentially life-threatening nature of the disease. In addition to the decreased serum esterase C1 inhibitor in patients with hereditary angioedema, there is still a unique quinine polypeptide at elevated concentration during the edema crisis. Patients with episodes of hereditary angioedema also have a low total complement and C4 and C2 components of the complement. Hereditary angioedema is transmitted in an autosomal dominant manner. Heterozygotes also have reduced levels of the C1 esterase inhibitor. During acute attacks of the disease, complement components of C4 and C2 may be significantly reduced, but C1 and C3 components are normal. The factors that promote the onset of angioedema attacks are often unknown. Notes: - Angioedema can also occur as an acquired disease. The acquired form includes non-hereditary C1 esterase deficiency and may be caused by medications, allergies, idiopathic forms, angioedema associated with autoimmune disease especially with systemic lupus erythematosus and hypereosinophilia, angioedema occasionally associated with malignancy, and angioedema caused by natural stimuli. - Abnormally low results are consistent with a heterozygous C1 esterase inhibitor deficiency and hereditary angioedema. - Fifteen percent of hereditary angioedema patients have a normal or elevated level but nonfunctional C1 esterase inhibitor protein. Detection of these patients requires a functional measurement of C1 esterase inhibitor; FC1EQ / C1 Esterase Inhibitor, Functional Assay, Serum. - Measurement of C1q antigen levels; C1Q / Complement C1q, Serum, is key to the differential diagnoses of acquired or hereditary angioedema. Those patients with the hereditary form of the disease will have normal levels of C1q, while those with the acquired form of the disease will have low levels. - Studies in children show that adult levels of C1 inhibitor are reached by 6 months of age. References: Atkinson JP, Du Clos TW, Mold C, Kulkarni H, Hourcade D, Wu X. The human complement system: basic concepts and clinical relevance. In: Rich RR, Fleisher TA, Shearer WT, Schroeder HW, Frew AJ, Weyand CM, eds. Clinical Immunology: Principles and Practice. 5th ed. Philadelphia, PA: Elsevier; 2019:chap 21. Cicardi M, Aberer W, Banerji A, et al. Classification, diagnosis, and approach to treatment for angioedema: consensus report from the Hereditary Angioedema International Working Group. Allergy. 2014;69(5):602-616. PMID: 24673465 www.ncbi.nlm.nih.gov/pubmed/24673465/. Leslie TA, Greaves MW. Hereditary angioedema. In: Lebwohl MG, Heymann WR, Berth-Jones J, Coulson IH, eds. Treatment of Skin Disease: Comprehensive Therapeutic Strategies. 5th ed. Philadelphia, PA: Elsevier; 2018:chap 101. Zanichelli A, Azin GM, Wu MA, et al. Diagnosis, course, and management of angioedema in patients with acquired C1-inhibitor deficiency. J Allergy Clin Immunol Pract. 2017;5(5):1307-1313. PMID: 28284781 www.ncbi.nlm.nih.gov/pubmed/28284781/. Frank MM: Complement deficiencies. Pediatr Clin North Am. 2000;47(6):1339-1354 Gelfand JA, Boss GR, Conley CL, et al: Acquired C1 esterase inhibitor deficiency and angioedema: a review. Medicine. 1979;58(4):321-328 Rosen FS, Alper CA, Pensky J, et al: Genetically determined heterogeneity of the C1 esterase inhibitor in patients with hereditary angioneurotic edema. J Clin Invest. 1971;50(10):2143-2149 Frigas E: Angioedema with acquired deficiency of the C1 inhibitor: a constellation of syndromes. Mayo Clin Proc. 1989;64:1269-1275 Soldin SJ, Hicks JM, Bailey J, et al: Pediatric reference ranges for estradiol and C1 esterase inhibitor. Clin Chem. 1998;44(6s):A17
C3 Complement, often simply referred to as C3, is a crucial protein component of the immune system's complement system. The complement system is a part of the innate immune response and plays a significant role in inflammation and defending against infections. C3, in particular, is central to the activation of the complement system. It gets activated through one of the three complement pathways (classical, lectin, or alternative) and subsequently participates in opsonization (marking pathogens for destruction), chemotaxis (attracting immune cells to the site of infection), and cell lysis (breaking down of cells). The level of C3 in the blood is an important diagnostic marker and is commonly measured to assess and monitor various immune and inflammatory conditions. Low levels of C3 are often seen in conditions like systemic lupus erythematosus (SLE) and other autoimmune disorders, where the immune system is overactive and depletes complement proteins. Conversely, elevated levels can be indicative of acute inflammation or infection. The "RDL" here likely denotes a specific laboratory or testing methodology for measuring C3 levels. Regular monitoring of C3 levels can be crucial in managing autoimmune diseases, as changes in its levels can indicate fluctuations in disease activity or the effectiveness of treatment.
desArg = without arginine
Immune complex activation marker