Reliability of patient-reported outcomes for pain, fatigue, and other disease activity measurements in rheumatoid arthritis patients decreased as time intervals between assessments increased, according to research published in Rheumatology.
It is unclear to what degree variability between the measurements is explainable by within-patient variability and measurement error, reported Dr. Paul Studenic of the Medical University of Vienna (Austria) and his associates.
They assessed 40 rheumatoid arthritis patients and gathered self-assessed pain, fatigue, and patient global assessment (PGA) scores three times per day for 2 months. Twenty patients recorded their assessments over the phone, and 20 kept a diary. PGA was assessed on a 100-mm visual analog scale (VAS), using no disease activity and highly active disease as anchors. The investigators calculated intraclass correlation coefficients (ICCs) and the smallest detectable differences (SDDs) for 1-, 7-, 14- and 28-day testing intervals.
Overall, the SDD and ICC were 25 mm and 0.67 for pain, 25 mm and 0.71 for PGA, and 30 mm and 0.66 for fatigue, respectively. SDD was higher with longer time periods between assessments, ranging from 19 mm (1-day intervals) to 30 mm (28-day intervals) for pain, 19-33 mm for PGA, and 26-34 mm for fatigue. Meanwhile, ICC was smaller with longer intervals, and ranged between the 1- and the 28-day interval from 0.80 to 0.50 for pain, 0.83 to 0.57 for PGA, and 0.76 to 0.58 for fatigue. In general, higher reliability according to ICC coincided with smaller SDD. The researchers noted that pain scores had a significant effect on the patient’s estimation of disease activity.
“The results of this study suggest that, in stable RA patients, a 25-mm change on the VAS for pain or PGA and a 30-mm change for fatigue may identify true change; however, this is clearly dependent on (and can be refined based on) the starting measurement level. It is also apparent that in patients who are assessed less frequently, the evaluation of measurement differences as indicating changes is more difficult,” the investigators concluded.
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