Validity and reliability of the Bristol Stool Form Scale in healthy adults and patients with diarrhoea‐predominant irritable bowel syndrome

Summary Background The Bristol Stool Form Scale (BSFS) is a 7‐point scale used extensively in clinical practice and research for stool form measurement, which has undergone limited validity and reliability testing. Aim To determine the validity and reliability of the BSFS in measuring stool form in healthy adults and patients with diarrhoea‐predominant irritable bowel syndrome (IBS‐D). Methods One hundred and sixty‐nine healthy volunteers provided a stool sample and used the BSFS to classify stool form, which was compared with measured stool water content and with values from 19 patients with IBS‐D. Eighty‐six volunteers used the BSFS to classify 26 stool models to determine accuracy and reliability. Results Volunteers’ classifications of stool type correlated with stool water (Spearman's rho = 0.491, P < 0.001), which increased in hard (Types 1–2), normal (Types 3–5) and loose stools (Types 6–7) (P < 0.001). The BSFS detected differences in stool form between healthy volunteers (mean 3.7, s.d. 1.5) and IBS‐D patients (mean 5.0, s.d. 1.2) (P < 0.001). Overall, 977/1204 (81%) stool models were correctly classified (substantial accuracy, κ = 0.78), although