The laboratory evaluation of patients with urticaria and angioedema must be tailored to the clinical situation. Acute urticaria and angioedema do not require specific laboratory evaluation except to document the suspected cause. For patients with chronic urticaria and angioedema, laboratory evaluation should be performed to exclude underlying diseases (Table 81-4). Patients with recurrent angioedema without urticaria should be evaluated for HAE (Table 81-5).
Measurement of the mast cell mediators, histamine and tryptase, may be helpful when the diagnosis of anaphylaxis isin question. A tryptase level is a more useful test because histamine is released quickly, has a short half-life, and is often difficult to detect in the serum. Serum tryptase levels peak 1 to 1.5 hours after anaphylaxis. Elevated levels may be helpful in
Figure 81-1 Examples of urticaria. (From Zitelli BJ, Davis HW, editors: Pediatric Physical Diagnosis Electronic Atlas,
Philadelphia, 2004, Mosby.)
Complete blood count with If vasculitis is suspected differential Erythrocyte sedimentation rate Antinuclear antibody Urinalysis Skin biopsy
From Zuraw B: Urticaria and angioedema. In Leung DYM, Sampson HA, Geha
RS, et al, editors: Pediatric Allergy: Principles and Practice, St Louis, 2003,
Mosby, p 580.
establishing the diagnosis, but normal tryptase levels do not ruleout the diagnosis. It is best to measure a serum tryptase level 1to 2 hours after the onset of symptoms. It also can be orderedretrospectively on stored serum that is less than 2 days old.
The diagnosis of urticaria and angioedema is straightforward;finding the etiology may be more difficult. Other dermatologic conditions can mimic urticaria. Erythema multiforme has target-shaped, erythematous, macular or papular lesionsthat may look similar to urticaria, but the lesions are fixedand last for several days. Other dermatologic diseases includedermatitis herpetiformis and bullous pemphigoid, which arequite pruritic, and early on, the lesions may resemble urticaria. Mastocytosis is characterized by mast cell infiltrationof various organs, including the skin. Some patients have skin lesions similar in appearance to urticaria rather than theclassic urticaria pigmentosa. Urticaria pigmentosa appears ashyperpigmented, red-brown macules, which may coalesce.
From Zuraw B: Urticaria and angioedema. In Leung DYM, Sampson HA, Geha RS, et al, editors: Pediatric Allergy: Principles and Practice, St Louis, 2003, Mosby, p 580.
When these lesions are stroked, they urticate, which is calledthe Darier sign. A rare disorder that should be included in the differential diagnosis of urticaria is Muckle-Wells syndrome. It is an autosomal dominant disorder characterized by episodic urticaria presenting in infancy, with sensorineuraldeafness, amyloidosis, arthralgias, and skeletal abnormalities.Another rare syndrome is Schnitzler syndrome, which is characterized by chronic urticaria, macroglobulinemia, bonepain, anemia, fever, fatigue, and weight loss. Urticarial vasculitis is a small vessel vasculitis with histologic features of aleukocytoclastic response. The main distinguishing feature isthat the lesions last longer than 24 hours, may be tender, andleave behind skin pigmentation. Skin biopsy is required fordefinitive diagnosis.