There is a longstanding recognition that asthma is a variable disease and attempts to elucidate the causes of asthma have been hampered by its genetic and phenotypic heterogeneity. The recognition that asthma constitutes several distinct phenotypes, the development of novel non-invasive biomarkers of airway inflammation and the desire to maximise information from large-scale genotyping studies have prompted new approaches to defining asthma phenotypes in adults and children. These have included statistical modelling of longitudinal symptom data in epidemiological studies and a re-examination of combinations of clinical, physiological and pathological markers that signify discrete disease entities. It is hoped that better understanding of asthma phenotypes will provide useful new insights into asthma aetiology but will also be of immense benefit in developing and tailoring asthma therapy to individual patients. This review aims to summarise traditional approaches to categorising asthma into different types and to consider novel and emerging approaches to this problem and their likely impact on understanding the causes and natural history of asthma.

PHENOTYPE: WHAT’S IN A NAME?

A phenotype refers to a set of characteristics that can be used to classify organisms into discrete groups. The Danish botanist and geneticist, Wilhelm Johannsen coined the terms “genotype” and “phenotype” in an address to the American Society of Naturalists in 1910.1 Phenotype was interpreted to designate “a group of organisms, which in outward appearance seemed to belong to one type”.2 This was clarified in correspondence from Shull, who noted that the term phenotype referred to “the constitution or assemblage of characteristics” with respect to which a group of individual organisms is apparently homogeneous and not to the group of individuals themselves.3 Therefore, phenotype can refer to any observable characteristic of an organism, including morphology, development, biochemical or physiological properties, and behaviour. Each phenotypic characteristic will depend to a greater or lesser degree …