The Nagler test is a biochemical test that is used to identify organisms which liberate phospholipases (lecithinases) e.g. Clostridium perfringens. The alpha toxin of C. perfringens has phospholipase activity and hence, when grown on a medium containing egg yolk phospholipid, the organism can break down this insoluble triglyceride.
Phosphoryl choline release is seen as an area of opacity around the bacterial colonies. This is described as being Nagler positive.
Antitoxin can inhibit a positive response.