Anaphylaxis is a condition that cannot be diagnosed ahead of time. It becomes apparent only after an individual has suffered an allergic reaction, and is diagnosed solely on the basis of the symptoms experienced and their severity.

However, a physician may recommend allergy skin testing to determine the risks to an individual of exposure to certain allergens. For example, a physician treating an individual who has experienced anaphylaxis in reaction to penicillin may use a skin test to try out alternative antibiotics.  Such testing must be done with great care, as there is a chance the procedure could provoke anaphylaxis.

An allergy blood test is another option that can help a physician pinpoint allergens that may trigger a reaction. Blood tests are often used for patients who are not good candidates for skin testing, such as infants or individuals with skin disorders (e.g., eczema). They may also be useful to help confirm or rule out diagnoses. 

The most commonly used blood test for allergy-related conditions is the radioallergosorbent test (RAST). This test looks for the presence of allergy-specific immunoglobulin E (IgE) in the bloodstream. During an allergic reaction, the body produces IgE antibodies to protect against what it mistakenly perceives to be a dangerous substance. The RAST test measures the amount of specific IgE produced to an individual allergen in a sample of blood. The level of IgE correlates to the sensitivity a patient has to the allergen. Related tests include MAST (multiple-antigen simultaneous testing) and PRIST (paper radioimmunosorbent test).