[Patient-centered evaluation of illness outcome in musculoskeletal diseases: adaptation and revision of outcome instruments].

Assessment of disease consequences (outcome) requires reliable, valid, and sensitive instruments. Psychometrically sound generic health-status instruments such as the SF-36 have been validated for many languages and are available in German. Instead, most disease specific instruments have been developed in English and need to be adapted for the German language. To allow for cross-cultural comparisons and use of results across cultures, for instance, in international multicenter studies, instruments need to have both content and metric equivalence. Thus, adaptation of health-status instruments requires a standarized process including translation, backtranslation, review and assessment of metric properties (reliability, internal consistency, factors), and validity. Developments of new instruments from scratch are time and cost intensive and should only be considered if no current instrument is available. Ideally, newly developed instruments have interval-scale properties.