OBJECTIVES: The Immune Thrombocytopaenia (ITP) Life Quality Index (ILQI) is 10-item patient reported outcome (PRO) used in clinical practice to measure the impact of ITP. It was administered as part of the ITP World Impact Survey (I-WISh), a global observational study. This abstract summarizes the results of an Item Response Theory (IRT) analysis of the ILQI.

METHODS: IRT is a family of statistical models providing information about the performance of items and the scales they comprise through analysis of item responses. Two IRT models were considered. First the sample was treated as a single-group and a graded response model (≥3 response choices for each item within the PRO), the latter was employed to calibrate item discrimination and impact response thresholds. Secondly, to access the severity of impacts in patients in various countries, multiple-group one-parameter IRT models were assessed. All countries were compared to the USA, the largest cohort in the study.

RESULTS: Single-group IRT results indicated that all items had good discrimination (slopes≥1.96) and response thresholds covered a wide range of the underlying latent scale (between -2.39 and 6.70 logits), showing that the ILQI can measure a range of disease impacts with good precision. In the multi-group IRT model, statistically significant country-level effects were found compared to the USA. Of these, patients in Canada, China, Columbia, and Turkey had lower impacts than the USA, and patients in Japan and the UK had higher impacts.

CONCLUSIONS: Results support that the ILQI can measure a range of disease impacts. The single-group model assessing item discrimination showed that items could differentiate between patients with higher and lower disease impact. The multi-group IRT model detected some country-level differences in patient’s responses. This does not preclude the use of ILQI in non-USA countries, but suggests that patient report of disease impacts may vary between countries.