The role of hair follicle counts and ratios in the histopathological assessment of androgenic alopecia, alopecia areata and telogen effluvium: does counting 'count'?

The histopathological assessment of scalp biopsies for the diagnosis of alopecia is most commonly undertaken in specialist centers. Occasionally, pathologists encounter such specimens outside a specialist setting or at a low frequency, which makes a confident diagnosis difficult. A methodical approach is essential to identify and interpret the histopathological findings, and one of the diagnostic tools is the use of follicular counts and ratios. This method is particularly emphasized in the context of non-scarring alopecia and, furthermore, for the identification of alopecias with overlapping features. We raised the question as to what is the role of follicular hair counts and ratios in the differential diagnosis of non-scarring alopecia with overlapping features, and sought the answer in our literature review. The English literature on the histopathological assessment of horizontal scalp biopsies taken for the evaluation of non-scarring alopecia, focusing on hair follicle counting as a diagnostic tool, with particular emphasis on androgenetic alopecia, alopecia areata, and telogen effluvium, was reviewed. Follicular counts and ratios are a helpful diagnostic tool. However, these need to be coupled with the morphological features specific for each alopecia subtype to render a confident diagnosis.