The diagnosis of acute and chronic urticaria is usually based on clinical observation and history.

A physical urticaria is diagnosed by careful history taking and challenge testing.

If a complete review of systems is normal, and a physical urticaria is ruled out, it is often futile to perform multiple laboratory tests to determine a cause for chronic urticaria.

Nonetheless, a symptom-directed search for underlying illness (e.g., SLE, thyroid disease, lymphoma, and necrotizing vasculitis) may warrant evaluations such as:

• Complete blood count.

• Erythrocyte sedimentation rate.

• Fluorescent antinuclear antibody test.

• Thyroid function studies.

• Hepatitis-associated antigen test.

• Assessment of the complement system.

• Radioallergosorbent test for IgE antibodies.

• Stool examination for ova and parasites—in the presence of eosinophilia.

• CD203c assay, an in vitro diagnostic test that is used to detect autoimmune urticaria and identifies antibodies that are responsible for many cases of chronic idiopathic urticaria.

Acute urticaria, by definition, lasts less than 6 weeks.

Outbreaks are often IgE-mediated.

There may be an obvious precipitant such as drug ingestion, an acute respiratory illness, a parasitic infection, or a bee sting.

The most common drugs that may cause acute hives are antibiotics (especially penicillin and sulfonamides). Pain medications such as aspirin, nonsteroidal anti-inflammatory drugs (NSAIDs), narcotics, radiocontrast dyes, diuretics, and opiates such as codeine, are also frequently implicated.

The most common foods associated with acute urticaria are milk, wheat, eggs, chocolate, shellfish, nuts, fish, and strawberries. Food additives and preservatives such as salicylates and benzoates may also be responsible.

Systemic diseases such as lymphomas and collagen vascular diseases may have an associated urticaria.

Acute urticaria also may be caused by physical stimuli such as pressure, cold, sunlight, or exercise. Such hives are called physical urticaria (see later discussion).

In children the most common trigger of acute urticarial is a viral illness.

Anaphylaxis or an anaphylactoid reaction can be associated with acute urticaria.

Episodes of acute urticaria lasts for hours to days (generally less than 30 days).

Chronic urticaria is, by definition, urticaria that lasts longer than 6 weeks, although this cutoff point is an arbitrary one.

Occurs in a 2:1 female-to-male ratio.

Seen predominantly in adults.

Lesions generally itch.

The cause is usually unknown or undetermined; however, chronic urticaria may, very infrequently, be a sign or symptom of one of the following systemic diseases: systemic lupus erythematosus (SLE), serum hepatitis, lymphoma, polycythemia, macroglobulinemia, or thyroid disease.

Clinically presents with skin-colored or pale red wheals that are sometimes accompanied by a white halo at the periphery (Fig. 27.1).

Papules and plaques have various shapes: annular, linear, arciform, or polycyclic; frequently they are multiple, with bizarre shapes (Figs. 27.2 and 27.3).

Individual lesions disappear within 24 hours (evanescent wheals).

Lesions may be accompanied by a deep swelling (angioedema) around the eyes, lips, and tongue that often looks frightening (Fig. 27.4). Fortunately, angioedema usually lasts less than 24 hours.

Less often, arthralgia, fever, malaise, and other symptoms may accompany urticaria when it is the result of an underlying disorder such as hepatitis, serum sickness, or a collagen vascular disease.

Emotional stress may trigger recurrences.

In contrast to atopic dermatitis and many other dermatitides, scratching and rubbing of urticarial lesions generally does not produce scabs or crusts.

Dermatographism affects more than 4% of the general population, in whom it is physiologic and asymptomatic.

Linear erythematous wheals occur within 3 to 4 minutes after firmly stroking the skin with the wooden handle of a cotton swab; lesions fade within 30 minutes (Fig. 27.5).

Lesions may arise under constrictive garments, such as belts and bras, or after a person scratches.

In some patients, itching is the primary symptom.

Episodes of dermatographism are generally self-limited but may persist for years.

Cold urticaria occurs mainly in young adults and children.

Itchy hives occur at sites of cold exposure, such as areas exposed to cold winds or immersion in cold water.

In the “ice cube test,” a wheal arises on the skin after application of an ice cube (Fig. 27.6A,B).

Solar urticaria occurs in sun-exposed areas of the skin and is triggered by various wavelengths of light (Fig. 27.7A,B).

Cholinergic urticaria is induced by exercise or a hot shower.

The patient exercises to the point of sweating, which provokes lesions and establishes the diagnosis.

Typical lesions are multiple, small, monomorphic wheals (Fig. 27.8).