The lack of HS phenotype information is particularly relevant, as PSD is thought to be more common in follicular phenotypes of HS.[8] The use of International Classification of Disease codes in diagnosis of HS and PSD has limitations, including lack of information on how each diagnosis of PSD was made (clinically, histologically and/or radiographically). However, HS diagnosis is often delayed, and exclusion of these cases may result in exclusion of undiagnosed HS cases. The occurrence of pilonidal sinus disease in patients with HS was higher than in control subjects in all five included studies (see Table 1). [Extracted from the article]