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The diagnosis of angioedema is primarily clinical but understanding the cause of the angioedema needs laboratory tests to show the specific type of underlying abnormality when kinin causes the swellings. Three types of hereditary angioedema are now recognized and some even less common types of C1 esterase inhibitor deficiency can sometimes be identified in patients with other types of illness that are not hereditary. The simplest and best screening test for all these types of angioedema is the measurement of a blood protein level called C4 complement. The levels are considerably reduced in all except Type III hereditary angioedema whether the disease is active or not. The cause of angioedema in the majority of patients who do not develop weals is usually unknown and their illness is therefore called ‘idiopathic’. Fortunately the majority of patients with idiopathic angioedema respond to treatments for histamine-mediated urticaria.