Study design

This study was an analysis of cross-sectional survey data assessing HRQoL and work productivity and daily activity impairment among respondents with IBS-D relative to controls. HRQoL was evaluated based on the Medical Outcomes SF-36 version 2 (SF-36v2) [11], which includes a Physical Component Summary (PCS) score and a Mental Component Summary (MCS) score (range: 0–100). Higher scores indicate better quality of life. Health utility scores were derived from the SF-36v2 by calculating the Short Form-6 dimension (SF-6D) score, a health state classification measure ranging from 0.0 (death) to 1.0 (best health state).

Work productivity and activity impairment were assessed based on the Work Productivity and Activity Impairment Questionnaire: General Health version (WPAI:GH) [12]. The WPAI:GH consists of six items measuring four domains: 1) absenteeism (percentage of work time missed due to health problems in the past 7 days, calculated as: [hours missed due to health problems/(hours missed due to health problems + hours worked)] x 100); 2) presenteeism (percentage of impairment experienced at work due to health problems in the past 7 days, calculated as: [degree health affected productivity while working/10] x 100); 3) overall work productivity loss (absenteeism plus presenteeism, calculated as: [absenteeism + (hours worked x presenteeism)] x 100); and 4) daily activity impairment (percentage of impairment in daily activities due to health problems in the past 7 days, calculated as: [degree health affected daily activities/10] x 100). Only respondents who reported being employed (full or part time) provided data for absenteeism, presenteeism, and overall work productivity loss; all respondents provided data for daily activity impairment. All domain scores are expressed as percentages, with higher percentages indicating greater work productivity loss and activity impairment. Days of work missed annually (for employed participants only) were calculated using reported work hours missed based on the WPAI:GH, assuming a 40-hour work week and 50 weeks worked annually.

Estimated annual indirect costs were calculated by integrating data from the Bureau of Labor Statistics and the WPAI:GH, with 2012 median weekly wages by age and sex obtained from the Bureau of Labor Statistics applied as unit costs to work productivity variables (absenteeism and presenteeism) from the WPAI:GH. The number of hours missed due to absenteeism and presenteeism were calculated for each respondent and multiplied by their associated estimated hourly wage, based on age and gender. These figures were annualized by multiplying by 50 work weeks in a year, and combined to estimate the total indirect costs. Costs are presented in 2012 US dollars.