The differential diagnosis of AD is broad and includes other chronic dermatoses, infections, infestations and malignancies as well as metabolic, genetic and autoimmune disorders (Table 13.7). In addition, a number of primary immunodeficiencies are associated with eczematous eruptions. Depending upon the age of the patient and the clinical presentation, such entities should be considered prior to diagnosing AD, especially when the history or morphology and distribution of skin lesions is atypical.

During infancy, it is particularly important to exclude two potentially life-threatening primary immunodeficiencies associated with pruritic dermatitis. Wiskott–Aldrich syndrome is an X-linked recessive disorder associated with thrombocytopenia, abnormalities in humoral and cell-mediated immunity, recurrent bacterial infections, and cutaneous findings nearly identical to AD; the former may have a more exfoliative eruption with prominent hemorrhagic crusting and petechiae …