Diagnostic Criteria for Contact Dermatitis

Essential Clinical Features

The British Association of Dermatologists recommends that contact dermatitis be diagnosed clinically based on a detailed exposure history, characteristic morphology and distribution of eczematous lesions, and patch testing to distinguish allergic from irritant forms 1, 2

The British Association of Dermatologists emphasizes the importance of a detailed history, including personal and family background, such as a personal history of atopic dermatitis in infancy/childhood and presence of asthma or hay fever, in determining the etiology of contact dermatitis 1
A family history of atopy is also a relevant factor in the diagnosis of contact dermatitis 1
The location where initial symptoms began and subsequent spread pattern, as well as whether symptoms improve with environmental changes, are crucial in determining the cause of contact dermatitis 1, 2

Allergic contact dermatitis is a cell-mediated delayed hypersensitivity reaction occurring only in sensitized individuals with predisposition to specific allergens, and requires prior sensitization through skin contact, followed by elicitation upon re-exposure 3
The diagnosis of allergic contact dermatitis is confirmed by positive patch testing with eczematous reactions assessed at 48-72 hours 3, 1
Irritant contact dermatitis, on the other hand, is a non-immune-modulated irritation caused by toxic reaction to substances, and is diagnosed primarily by exclusion after ruling out allergic contact dermatitis through negative patch testing 2

Patch testing is the gold-standard investigation when allergic contact dermatitis is considered, and must be performed because clinical features alone cannot reliably distinguish allergic from irritant or endogenous dermatitis 1, 2
The sensitivity of patch testing for detecting allergic contact dermatitis is 60-80% 4
Patch testing methodology involves the application of standardized contact allergens or suspected allergens to healthy skin, with eczematous reactions assessed at 48-72 hours 3, 1

Food-related contact dermatitis can occur through immediate reactions initiated by chemical moieties in foods, and can be diagnosed through patch testing with suspected food allergens 3, 4
Systemic contact dermatitis is a rare disorder with generalized eczematous dermatitis plus systemic symptoms, and develops after oral/parenteral exposure to allergen with prior skin sensitization 3

Failing to consider contact dermatitis in patients with known atopic dermatitis can lead to misdiagnosis, as allergic contact dermatitis occurs in 6-60% of atopic dermatitis patients 4
Relying solely on clinical appearance can also lead to misdiagnosis, as irritant contact dermatitis is indistinguishable from endogenous, dyshidrotic, nummular, and atopic dermatitis 2

A diagnostic algorithm for contact dermatitis involves obtaining a detailed exposure history, performing a clinical examination, assessing response to avoidance of suspected causative substance, conducting patch testing, correlating patch test results with clinical history and examination findings, and providing clear communication of relevant allergen information 1, 2, 3