Objective: To compare different generic instruments in measuring quality of life and to demonstrate dimensions of quality of life (QL) in patients with cystic fibrosis (CF).
Methods: The short-form-36 health survey (SF-36), the quality of life profile for chronic diseases (PLC), and the questions on life satisfaction (FLZ(M)) were simultaneously employed in a cross-sectional study with 70 adolescents and adults with CF. The different concepts of the measures were compared. Internal consistency (Cronbach's alpha), convergent and construct validity (correlation patterns, common factor analysis), and external validity (correlations with symptom and pulmonary function scores, with intensity of therapy; comparisons with healthy peers) of the three instruments were investigated.
Results: Similar reliability, but different validity of the questionnaires are demonstrated. Seventy-three percent of the total variance across the three measures could be explained with a seven-factor-solution: (1) physical functioning (19.3% of total variance), (2) mental health (19.3%), (3) social integration (7.5%), (4) role function/pain (7.5%), (5) economic/material living conditions (7.5%), (6) partnership/family (6.7%) and (7) anxiety (5.2%).
Discussion: The different validity of the instruments has to be considered in chosing a questionnaire appropriate to the purpose of measuring. Shortcomings of each instrument can be overcome by multimethod designs and by developing disease-specific scales.