Defining cut-off values for disease activity states and improvement scores for patient-reported outcomes: the example of the Rheumatoid Arthritis Impact of Disease (RAID)

Introduction

The Rheumatoid Arthritis Impact of Disease (RAID) is a patient-reported outcome measure evaluating the impact of rheumatoid arthritis (RA) on patient quality of life. It comprises 7 domains that are evaluated as continuous variables from 0 (best) to 10 (worst). The objective was to define and identify cut-off values for disease activity states as well as improvement scores in order to present results at the individual level (for example, patient in acceptable state, improved patient).

Methods

Patients with definite active RA requiring anti-tumour necrosis factor (anti-TNF) therapy were seen at screening, baseline and after 4 and 12 weeks of etanercept therapy. Answers to "Gold standard" questions on improvement (MCII: Minimum Clinically Important Improvement) and an acceptable status (PASS: Patient Acceptable Symptom State) were collected as well as the RAID score and Disease Activity Score 28- erythrocyte sedimentation rate (DAS28-ESR). Cut-offs were defined by different techniques including empirical, measurement error and gold standard anchors. The external validity of these cut-offs was evaluated using the positive likelihood ratio (LR) based on the patient's perspective (for example, patient's global) and on low disease activity status (such as DAS28-ESR).

Results

Ninety-seven (97) of the 108 recruited patients (age: 54 ± 13 years old, female gender: 75%, rheumatoid factor positive: 81%, disease duration: 8 ± 7 years, CRP: 18 ± 30 mg/l, DAS28-ESR: 5.4 ± 0.8) completed the 12 weeks of the study. The different techniques suggested thresholds ranging from 0.2 to 3 (absolute change) and from 6 to 50% (relative change) for defining MCII and thresholds from less than 1 to less than 4.2 for defining PASS. The evaluation of external validity (LR+) showed the highest LR+ was obtained with thresholds of 3 for absolute change; 50% for relative change and less than 2 for an acceptable status.

Conclusions

This study showed that thresholds defined for continuous variables are closely related to the methodological technique, justifying a systematic evaluation of their validity. Our results suggested that a change of at least 3 points (absolute) or 50% (relative) in the RAID score should be used to define a MCII and that a maximal value of 2 defines an acceptable status.

Trial Registration

Clinicaltrial.gov: NCT004768053