A method for determining a patient's susceptibility to anaphylaxis comprises assaying the activity of platelet activating factor acetylhydrolase (PAF-AH) activity and comparing the measured activity to a reference value, wherein the level of PAF-AH activity inversely correlates with the susceptibility to anaphylaxis. The method further comprises determining the severity of anaphylaxis by correlating the degree of reduced PAF-AH activity to the reference value. The method further comprises measuring IgE concentration and comparing the measured concentration to a reference concentration. The invention further provides a method for diagnosing anaphylaxis in a patient comprising measuring the serum concentration of platelet activating factor (PAF) and comparing the measured concentration to a reference value, wherein the concentration of PAF correlates directly with severity of anaphylaxis. The invention also provides a kit for performing the required assay or assays.