Differentials
Anaphylaxis
SIGNS / SYMPTOMS
History of anaphylaxis.
Presence of respiratory symptoms (e.g., wheeze), hypotension, diaphoresis, nausea, vomiting, diarrhoea, abdominal pain, flushing.
INVESTIGATIONS
No tests can differentiate between anaphylaxis and urticaria/angio-oedema acutely. If there is a suspicion of anaphylaxis, intramuscular adrenaline (epinephrine) must be given immediately. Serum tryptase is raised in anaphylaxis.
Dermatographism
SIGNS / SYMPTOMS
Dermatographism will appear within minutes of scratching or stroking the patient's skin.
Unlike urticaria, it will be limited only to sites of contact.
INVESTIGATIONS
There are no specific laboratory tests to differentiate the two conditions.
Atopic dermatitis
SIGNS / SYMPTOMS
The lesions of atopic dermatitis are normally accompanied by a greater degree of surrounding xerosis and erythema than is seen in urticaria.
The rash of atopic dermatitis typically lasts beyond 24 hours, which is unusual in simple urticaria.
INVESTIGATIONS
There are no specific laboratory tests to differentiate the two conditions.
Urticarial vasculitis
SIGNS / SYMPTOMS
Lesions in urticarial vasculitis are typically painful, last longer than 24 hours, and often leave residual markings upon their resolution. These characteristics are all unusual in urticaria.
Urticarial vasculitis can also be found on biopsy in patients with evanescent hives lasting less than 24 hours.[6]
INVESTIGATIONS
An elevated erythrocyte sedimentation rate or C-reactive protein may help to distinguish urticarial vasculitis from urticaria.
If there is high suspicion of a vasculitic component, a skin biopsy should be considered.
Urticaria pigmentosa
SIGNS / SYMPTOMS
Lesions are typically small macules or slightly raised papules, as opposed to the large wheals seen with simple urticaria.
Can be associated with Darier's sign, which is characterised by urticaria and erythema upon rubbing, scratching, or stroking affected skin.
INVESTIGATIONS
An elevated tryptase may help with the diagnosis of urticaria pigmentosa, which is often a feature of systemic mastocytosis, although tryptase may also be normal. Skin biopsy can also aid in diagnosis.
Systemic mastocytosis
SIGNS / SYMPTOMS
Patients with systemic mastocytosis often have skin findings consistent with urticaria pigmentosa.
Symptoms often include profound fatigue, and other manifestations can depend on the organ symptoms affected. Gastrointestinal complaints are most common.
INVESTIGATIONS
An elevated tryptase raises the suspicion, but does not confirm the diagnosis. A bone marrow biopsy can confirm the diagnosis.
Carcinoid
SIGNS / SYMPTOMS
Patients typically complain of cutaneous flushing. Diarrhoea and respiratory difficulties are also often reported.
INVESTIGATIONS
The most useful initial test for carcinoid is to measure 24-hour urinary levels of 5-hydroxyindoleacetic acid (5-HIAA), which is the end product of serotonin metabolism.
Contact dermatitis
SIGNS / SYMPTOMS
Lesions tend to be more confluent and irritated in nature as compared with urticaria. The rash of contact dermatitis typically lasts beyond 24 hours, which is unusual in simple urticaria. Peri-ocular lesions often have modest associated swelling that can commonly be confused for angio-oedema.
INVESTIGATIONS
Although there are no specific laboratory tests to differentiate the two conditions, patch testing, which may be helpful in identifying the causative agent, may be considered.
Papular urticaria (insect bites)
SIGNS / SYMPTOMS
These tend to be smaller papules and typically last beyond 24 hours. Enquiring if other close contacts are experiencing similar symptoms may suggest an environmental cause such as insects.
INVESTIGATIONS
Although there are no specific laboratory tests to differentiate the two conditions, scraping for specific organisms or a skin biopsy may be considered.
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