The Bowel Function Index for evaluating constipation in pain patients: definition of a reference range for a non-constipated population of pain patients.

Opioid-induced constipation (OIC) is a severe, persisting side-effect of opioid therapy. The Bowel Function Index (BFI(a), numerical analogue scale 0 - 100), calculated as the mean of three variables (ease of defaecation, feeling of incomplete bowel evacuation, and personal judgement of constipation) was developed to evaluate bowel function in opioid-treated patients with pain. This clinician-administered tool allows easy measurement of OIC from the patient's perspective. The purpose of this investigation was to define a reference range reflecting BFI values in non-constipated chronic pain patients who were recruited into a cross-sectional survey and asked for their perceptions of constipation. The BFI scores were assessed and compared with those of patients with confirmed OIC obtained from two previously published trials. Results were analysed and a reference range of BFI values of 0 - 28.8, into which 95% of non-constipated chronic pain patients fell, was defined. This permits discrimination between chronic pain patients with, or without, constipation.