Lending a hand: can DASH items help ABILHAND improve manual ability measurement in multiple sclerosis?

BACKGROUND: Our examination in multiple sclerosis (MS) of the ABILHAND, a patient-reported outcome (PRO) instrument measuring manual ability, identified limited measurement range and precision. These deficiencies could lead to type II errors in clinical trials. OBJECTIVES: This paper aims to determine if ABILHAND's measurement performance in MS can be improved by adding relevant items from the Disabilities of the Arm, Shoulder and Hand scale (DASH). METHODS: The 23-item ABILHAND and 30-item DASH were administered to 461 people with MS. Data from the ABILHAND were combined with 16 DASH items to create a 39-item scale (AD-39). Using Rasch Measurement Theory methods, we compared the psychometric properties of AD-39 with ABILHAND. RESULTS: Data were analysed from 300 people. AD-39 performed robustly as a measure and had greater measurement range, lower floor and ceiling effects, and higher reliability (person separation index 0.97) than ABILHAND. Surprisingly, AD-39 appeared no better than ABILHAND at detecting group differences in self-reported hand function. CONCLUSION: Despite improving some psychometric properties, adding 16 DASH items to the ABILHAND did not improve its measurement performance to the degree expected. Our explanations for this anomaly emphasise the importance of evidence-based, conceptually driven scale modifications guided by hypothesis testing psychometric methods.