In cases where angiodema is suspected to be due to hereditary or acquired C1 inhibitor deficiency, the most effective screening test is a C4 - this is invariably greatly reduced in untreated patients. Only then is it necessary to go on to measure C1 inhibitor.
It is not necessary to test for C1 inhibitor defeciency in patients who only have urticaria.
It is not necessary to test for C1 inhibitor defeciency in patients who develop angiodema while taking ACE inhibitor, unless symptoms persist after discontinuing the drug (most of these patients can tolerate an angiotensin II receptor antagonist).
