Classification and stratification in pilonidal sinus disease: findings from the PITSTOP cohort

Research in pilonidal disease faces several challenges, one of which is consistent and useful disease classification. The International Pilonidal Society (IPS) proposed a four-part classification in 2017. The aim of this work was to assess the validity and reliability of this tool using data from the PITSTOP cohort study. Face validity was assessed by mapping the items/domains in the IPS tool against tools identified through a systematic review. Key concepts were defined as those appearing in more than two-thirds of published tools. Concurrent and predictive validity were assessed by comparing key patient-reported outcome measures between groups at baseline and at clinic visit. The outcomes of interest were health utility, Cardiff Wound Impact Questionnaire (CWIQ) and pain score between groups. Significance was set at p = 0.05 a priori. Interrater reliability was assessed using images captured during the PITSTOP cohort. Ninety images were assessed by six raters (two experts, two general surgeons and two trainees), and classified into IPS type. Interrater reliability was assessed using the unweighted kappa and unweighted Gwet's AC1 statistics. For face validity items represented in the IPS were common to other classification systems. Concurrent and predictive validity assessment showed differences in health utility and pain between groups at baseline, and for some treatment groups at follow-up. Assessors agreed the same classification in 38% of participants [chance-corrected kappa 0.52 (95% CI 0.42-0.61), Gwet's AC1 0.63 (95% CI 0.56-0.69)]. The IPS classification demonstrates key aspects of reliability and validity that would support its implementation.