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Chapter 5126feasible and desirable to use condition-specific PROMs for more than 7000 rare disorders.58 It may also not be necessary, as research has shown that PRO domains that patients consider important are very similar among patient populations.37Generic instruments have the advantage of allowing comparison of outcomes between different disease (sub)groups. Generally, all individuals want to feel and function well, such as living without symptoms and being able to carry out daily activities. Feelings and functions can be affected by different health conditions, and these can result to similar problems with considerable overlap in relevant PROs across conditions, which could be measured with one set of generic outcome measures across conditions.36,37Methodological innovations, such as item response theory (IRT), have been used to develop PROMs with good measurement properties that are applicable across different health conditions, such as Patient-Reported Outcomes Measurement Information System (PROMIS®).59–61 IRT-based item banks are large sets of calibrated questions measuring the same construct, enabling efficient measurement through short forms or computerized adaptive testing (CAT).41,62 This provides a valuable solution, since redundant items for specific individuals will be minimized, increasing relevance and efficiency.63To ensure relevance, personalized outcome measurement instruments have gained emerging interest, especially for rare and heterogeneous patient populations since health manifestations are often specific, variable and complex.64 Instruments such as Goal Attainment Scaling enable focusing on personal goals and abilities.65 Additionally, by including outcomes that are specifically relevant to the affected individual, treatment adherence might be enhanced as well.66 Also regulatory agencies have increasing interest in the relevance of what is being measured,67 as treatment effects might be statistically significant, but not clinically or socially relevant, or vice versa.68Recommendations for selecting outcomes and instrumentsIn order to measure what matters to patients, several important factors should be taken into account when selecting outcome measurement instruments in clinical trials (Table 3).69 First, relevance to the patient should be ensured, which contributes to recruitment as well as treatment Annelieke Muller sHL.indd 126 14-11-2023 09:07