On the definition of dermatological disease. Part 2: approaches for defining dermatological diseases

In Part 1 of this two-part review, conceptual frameworks for defining skin diseases were articulated. In this review, the main approaches that can be used to develop diagnostic criteria for skin disease are summarized, using atopic dermatitis (AD) as an example. Different frameworks for defining skin disease for research purposes are articulated, including statistical, prognostic, operational, clinical and epidemiological approaches. All share the common aim of attempting to develop criteria that enable meaningful comparisons between groups of people. The desirable attributes of a good definition are described: diagnostic criteria should measure what they are meant to measure; the results should be the same for different assessors; the criteria should be coherent with what is known about that disease; they should reflect some degree of morbidity and not pick up subclinical disease; they should be easy to administer; and they should be applicable to a range of people of different ages, sexes/genders and ethnicities. Consensus-based criteria are contrasted with epidemiological derivation methods that assess the performance of diagnostic criteria in relation to a reference standard. The sensitivity and specificity of a disease definition is explained, along with how the trade-off between these two properties can vary, depending on the purpose of the study and the study setting. The review closes with some reflections on when it is appropriate to consider splitting a disease into more than one category and how diagnostic criteria can be interpreted in the clinical setting.