Contact Urticaria Syndrome Workup

Updated: Jan 30, 2020
  • Author: Shweta Shukla, MD; Chief Editor: Dirk M Elston, MD  more...
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Workup

Approach Considerations

General principles to laboratory testing

Commonly used topical application techniques in immunologic and nonimmunologic contact urticaria are the prick test, the chamber prick test, the scratch test, the open test, and the chamber test. In any of the above in vivo tests, performing positive (histamine, 1mg/mL) and negative (normal saline) control tests is important.

Prick testing theoretically has the lowest risk of anaphylaxis, because only minute amounts of allergen are introduced into the skin. However, anaphylaxis is a risk in all of the above test methods if the patient has immunologic contact urticaria. 

A difficulty with these tests is that not all possible allergens are available commercially. In these patients, nonstandardized products may have to be tested. This needs to be performed very carefully as it poses a much higher risk for anaphylaxis.

Laboratory studies

The total serum IgE level does not provide insight into the clinical contribution that an allergen is making to the patient's symptoms.

If the etiology is immunologic contact urticaria, the radioallergosorbent test (RAST) result (for allergen-specific IgE) may be positive for the offending substance. Nonimmunologic contact urticaria cannot be diagnosed by RAST.

Imaging studies

Radiologic imaging is not necessary in the dermatologic workup of contact urticaria syndrome. As a research tool, ultrasonography can be used to document the extent of edema.

Histologic findings

Contact urticaria does not have any specific histology as compared with urticaria. See the Histologic Findings section of the Medscape Drugs & Diseases article Acute Urticaria.

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In Vivo Tests

The prick test, the scratch test, and the chamber prick test are the most commonly used in vivo techniques for detecting immunologic contact urticaria. However, if these results are negative and immunologic contact urticaria remains in the differential diagnosis, the chamber test, the open test, or the use test may be necessary. An in vitro RAST may also be beneficial for establishing the diagnosis and determining the cross-reactivity (eg, latex and banana).

In all of the above referenced in vitro test methods, contact urticaria can be graded visually by marking the degree of erythema and edema on an ordinal scale (see Physical Examination).

NSAIDs, antihistamines, and exposure to ultraviolet light can cause false-negative results, as can tachyphylaxis. Patients must be counseled on stopping any medications that may lead to false-negative results 1 week prior to examination.

In testing for immunologic contact urticaria in patients with a history of extracutaneous involvement, particular care must be taken to use low concentrations of test substances and to have resuscitation equipment immediately available in case of anaphylaxis.

Prick test

This test is considered the criterion standard for contact urticaria and is the most commonly used. The preferred site is the inner forearm. Multiple substances can be tested simultaneously. In this test, a small lancet is placed through the material on the skin and a small prick is obtained. These results are then read at 15 minutes, and a greater than 3-mm reading is considered a positive result.

Scratch test

This test is used for more solid/nonstandardized allergens. A scratch is made on the skin and a small amount of test material is applied to the scratch. At 15 minutes, the results are read and a greater than 3-mm reading is a positive result.

Use test

In the use test, a research subject known to be affected uses the causative substance in the same way as when the symptoms first appeared; for example, wearing surgical gloves on wet hands to provoke latex immunologic contact urticaria.

Because a use test can provoke anaphylaxis in patients with immunologic contact urticaria, clinicians should proceed cautiously with such testing. However, use testing can be especially helpful in patients with nonimmunologic contact urticaria. A positive reaction appears as a wheal and flare and sometimes an eruption of vesicles.

Serial dilutions are useful in determining the test dose. Examples of concentrations that have been used in dilution series in alcohol vehicles are 250, 125, 62, and 31 mmol/L for benzoic acid and 50, 10, 2, and 0.5 mmol/L for methyl nicotinate.

Initially, the upper back, the flexor aspect of the upper arm, or the forearm is the site used. However, if the reaction is negative, previously or currently affected skin should be tested because site variability exists in the nonimmunologic and immunologic forms of contact urticaria. Repeated use of the same site may result in tachyphylaxis and can cause false-negative results.

Open test

In the open test, 0.1 mL of the test substance is spread over a 3 x 3-cm area on the desired site. Lahti suggests using alcoholic vehicles. [39] The addition of propylene glycol to a vehicle enhances the sensitivity of the test compared with previously used petrolatum and water vehicles.

The test sites are usually read at 20, 40, and 60 minutes to see the maximal response. Immunologic contact urticaria reactions typically appear within 15-20 minutes, whereas nonimmunologic contact urticaria reactions can be delayed up to 45-60 minutes following application.

Chamber test

The chamber test is an occlusive method of applying the substance to be tested. The substances to be tested are applied in small aluminum containers (Finn Chamber, Epitest Ltd; Hyryl, Finland) and attached to the skin via a porous tape. The chambers are applied for 15 minutes, and the results are read at 20, 40, and 60 minutes.

The advantages of this method are that occlusion enhances percutaneous penetration; therefore, the sensitivity of the test is probably higher. Additionally, a smaller area of the skin is required than in an open test. For unexplained reasons, however, the chamber method may provide less responsiveness than the open test. Patients may experience a delayed-type response and thus need to be tested at 48-96 hours after application.

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