Diagnosis of hives or urticaria is often made on clinical examination of the lesions which are caused by exposure to an allergen.
Outline of hives diagnosis
Diagnosis of hives usually involves
Obtaining a detailed clinical history of the patient's experience of the condition including information about possible triggers, and the location, severity and duration of lesions. The patient is asked about any new medications or foods they may have introduced as well as about any known triggers of the condition such as nickel, latex, pets, chemicals, dyes and insect stings or bites.
Obtaining details of any allergies such as contact dermatitis, eczema or asthma among family members.
Routine blood tests to check for anemia and also liver, kidney and thyroid function. White blood cell counts may also be assessed, particularly the eosinophil count, which is often elevated when allergy is present. Similarly, the erythrocyte sedimentation rate (ESR) is a good flag for allergic or inflammatory conditions.
A stool examination as some intestinal parasites can trigger hives.
Acute urticaria often manifests within a few hours of contact with an allergen, which is most often a food item. Peanuts, eggs, wheat, soy and shellfish are some of the foods most commonly known to trigger hives. In addition, certain medications can cause a reaction, such as glimepiride, a medication prescribed for diabetes, the antibacterials penicillin and sulphonamide, the antifungal clotrimazole and some anticonvulsants.
The clinical examination of acute hives is similar to that outlined for chronic hives. However, chronic hives usually lasts for over 6 weeks while acute hives have usually resolved in this time.
Types of hives
Depending on the reaction trigger, hives may differ in type and include:
Dermatographic hives: One of the most common forms of hives that occurs as a result of a skin scratch.
Cholinergic hives: These occur in response to heat, such as after a hot bath or after vigorous exercise.
Solar hives: Hives that may develop on exposure to sunlight.
Pressure induced hives: Those caused by tight straps or elastic bands, for example.
Aquagenic hives arise after contact with water
A skin prick test is commonly used to tests for or confirm an allergy. Usually the skin us on the inner side of the forearm or on the back is cleaned and pricked with a tiny amount of the suspected allergen. If a person is allergic, the skin reacts and turns itchy, red and swollen. Since the amount of the allergen is so small, there is very little risk of the test triggering a more severe allergic reaction.
A similar test is the patch test, where a small amount of the suspected allergen is put on metal discs which are then taped to the skin and left for 48 hours to see if an allergic reaction occurs.
Blood tests include the radio-allergosorbent tests or RAST test, which measures the amount of any allergen-related antibodies in order to identify a trigger.
Reviewed by Sally Robertson, BSc