<P> Skin involvement may include: hives, itchiness or a swollen tongue among others . Respiratory difficulties may include: shortness of breath, stridor, or low oxygen levels among others . Low blood pressure is defined as a greater than 30% decrease from a person's usual blood pressure . In adults a systolic blood pressure of less than 90 mmHg is often used . </P> <P> During an attack, blood tests for tryptase or histamine (released from mast cells) might be useful in diagnosing anaphylaxis due to insect stings or medications . However these tests are of limited use if the cause is food or if the person has a normal blood pressure, and they are not specific for the diagnosis . </P> <P> There are three main classifications of anaphylaxis . Anaphylactic shock is associated with systemic vasodilation that causes low blood pressure which is by definition 30% lower than the person's baseline or below standard values . Biphasic anaphylaxis is the recurrence of symptoms within 1--72 hours with no further exposure to the allergen . Reports of incidence vary, with some studies claiming as many as 20% of cases . The recurrence typically occurs within 8 hours . It is managed in the same manner as anaphylaxis . Pseudoanaphylaxis or anaphylactoid reactions are a type of anaphylaxis that does not involve an allergic reaction but is due to direct mast cell degranulation . Non-immune anaphylaxis is the current term used by the World Allergy Organization with some recommending that the old terminology no longer be used . </P> <P> Allergy testing may help in determining the trigger . Skin allergy testing is available for certain foods and venoms . Blood testing for specific IgE can be useful to confirm milk, egg, peanut, tree nut and fish allergies . </P>

What happens to blood pressure in anaphylactic shock