Patient reported outcome measures require scale metrification and quantified precision: evidence from the assessment of breathlessness in people with ALS/MND

Introduction: Precision (how closely repeated measures match) and responsiveness (ability to detect change over time) are critical properties of patient reported outcome measures (PROMs). Smallest Detectable Difference (SDD) is a useful statistic regarding precision; Minimal Detectable Change (MDC) and Minimal Important Change (MIC) assess responsiveness.

Methods: We examined measurement properties of Numeric Rating Scale for Breathlessness, ALSFRS-R respiratory subscale and Dyspnea-12, contributed by participants in the Trajectories of Outcome in Neurological Conditions-ALS study. Rasch analysis converts ordinal scale data to interval equivalents.

Results: Data from 1120 people with ALS showed ALSFRS-R Respiratory is only valid as ordinal data. The NRS Breathlessness requires computation from a wider NRS set for Rasch analysis; its SDD is 3.2, MDC 2.59, MIC 2.39, with score range of 0–10. The Dyspnea-12 has SDD 7.0, MDC 6.14, MIC 4.5, with score range of 0–36. The %MDC, indicating smallest change detectable above measurement error as % of scale range, is superior for the Dyspnea-12 (17.1%) compared to the NRS Breathlessness (25.9%). Another advantage of Dyspnea-12 is transformation of raw ordinal to interval equivalent data using published conversion tables. Both NRS and Dyspnea-12 have disadvantages of MIC < MDC.

Conclusions: Accurate measurement underpins optimal clinical decision making and high-quality research. Informed choice of PROMs reduces risk of misinterpreting clinical and research data. Patients want PROMs which they feel give an accurate account of their progression when participating in research and communicating with their clinical team. The Dyspnea-12 is preferrable for clinical and research use based on its psychometric properties.