BACKGROUND: Thyroid diseases affect quality of life (QoL). The Thyroid-Related Patient-Reported Outcome (ThyPRO) is an international comprehensive well-validated patient-reported outcome, measuring thyroid-related QoL. The current version is rather long--85 items. The purpose of the present study was to develop an abbreviated version of the ThyPRO, with conserved good measurement properties.
METHODS: A cross-sectional (N = 907) and a longitudinal sample (N = 435) of thyroid patients were analyzed. A graded item response theory (IRT) model was fitted to the cross-sectional data. Short-form scales with three items were aimed for, by selecting items with best fit according to the IRT model, avoiding cross-culturally noninvariant items. Seven scales measuring mental and social well-being and function as well as one overall QoL impact item were analyzed in a bifactor model, to develop a supplementary composite score. Short-form scales were linked to original scales with IRT-based summed-score-linking. Agreement between the short and long form was estimated by agreement plots, intraclass correlations, and mean score levels. Responsiveness was compared by relative validity indices, clinical validity by ability to detect clinically relevant differences, and test-retest reliability by intra-class correlation.
RESULTS: One four-item scale was not abbreviated and one two-item scale was omitted from the short-form. For the 11 scales undergoing abbreviation, 10 with three and one with four items were developed. A bifactor model with good overall fit was fitted to the composite score, including the single QoL item. Responsiveness and clinical validity of the short-form scales were preserved, as were test-retest reliability (0.75-0.89). Short- versus long-form intraclass correlations were high (0.89-0.98), and the mean scale levels were similar.
CONCLUSIONS: A 39-item version of the ThyPRO, with good measurement properties, was developed and is recommended for clinical use.
Available at: http://works.bepress.com/john_ware/221/