Investigations
1st investigations to order
FBC with differential
Test
Recommended initial test for chronic urticaria with or without angio-oedema.
Eosinophil count may be elevated in some drug-induced reactions. An elevated neutrophil count can be associated with urticarial vasculitis.[32]
Establishes a baseline for monitoring necessary with some drug therapies.
Result
may be normal; eosinophil or neutrophil count may be elevated
erythrocyte sedimentation rate (ESR)
Test
Recommended in the initial evaluation of chronic urticaria, and in cases of acute urticaria if urticarial vasculitis is suspected.
Provides non-specific evidence of inflammation and/or a possible vasculitic component.
Result
elevated or normal
CRP
Test
Recommended in the initial evaluation of chronic urticaria, and in cases of acute urticaria if urticarial vasculitis is suspected.
Provides non-specific evidence of inflammation and/or a possible vasculitic component.
Result
elevated or normal
Investigations to consider
thyroid-stimulating hormone (TSH)
Test
Ordered in patients with chronic urticaria (with or without angio-oedema) who have symptoms or signs of thyroid dysfunction.
May reveal evidence of underlying thyroid dysfunction.
If TSH is elevated, antithyroid antibody testing should be considered.
Result
elevated or normal
antinuclear antibodies (ANA)
Test
Only ordered in cases of chronic urticaria if history and/or physical examination suggests concomitant rheumatological disease.
May provide evidence of underlying rheumatological disorder.
Result
positive in many rheumatological diseases
skin prick testing
Test
Considered in cases of chronic urticaria if IgE-mediated food allergy is suspected as a cause.
One study found that nearly 40% of people with chronic urticaria had a positive skin prick test, but in double-blind, placebo-controlled provocation tests, administration of the allergen did not provoke urticaria symptoms in any patients.[9]
Food allergy is an uncommon cause of chronic urticaria, and routine testing for food allergy is not recommended.
Result
may be positive
allergen avoidance diet
Test
Considered in cases of chronic urticaria if the history suggests food pseudoallergy (non-IgE-mediated hypersensitivity to food pseudoallergens). Pseudoallergens include some artificial food additives, naturally occurring salicylic acid, and aromatic volatile compounds in herbs, wine, and tomatoes.[34]
Patients may strongly believe that certain foods are the cause of their urticaria, but foods are uncommon causes of chronic urticaria. In this situation, the best approach is to have the patient maintain a detailed food diary. If any foods are found to correlate with episodes of urticaria, they can be eliminated from the diet and the patient observed for resolution of the urticaria. Foods can then be gradually reintroduced into the diet as tolerated. If angio-oedema has been a feature of the patient's condition, this reintroduction should be performed in a setting equipped to manage allergic reactions, such as a doctor's surgery. However, it should be stressed that foods typically do not cause chronic urticaria.
Result
symptoms may improve
serum tryptase
Test
Ordered if history or clinical examination suggests systemic mastocytosis or anaphylaxis.[28]
Result
may be positive
skin biopsy
Test
Sample is sent for standard staining, but immunofluorescence studies may also be requested.
Usually performed if urticarial vasculitis is suspected or if chronic urticaria does not respond to antihistamine treatment.
Result
may show urticarial vasculitis in setting of atypical urticarial lesions
C1 esterase inhibitor level
Test
Ordered in cases with a high suspicion of hereditary angio-oedema. A low level is highly suggestive of the diagnosis.
Rarely, patients can have normal levels of C1 esterase inhibitor yet low function.
Result
decreased in hereditary angio-oedema
C1 esterase inhibitor function
Test
Given that rare patients can have normal levels of C1 esterase inhibitor yet low function, this test is ordered in cases with a high suspicion of hereditary angio-oedema, with a low level confirming the diagnosis.
Result
decreased in hereditary angio-oedema
C1q levels
Test
Ordered to help differentiate between hereditary and acquired angio-oedema.
Result
normal levels in hereditary angio-oedema; usually low levels in acquired angio-oedema
specific IgE to suspected allergen
Test
Ordered in cases of acute urticaria where an IgE-mediated allergy is suspected.
Serological testing is preferred to skin prick testing because development of hives can interfere with interpretation of skin prick test results.
Result
may be elevated
Use of this content is subject to our disclaimer