Evaluation of the smallest detectable difference in outcome or process variables in ankylosing spondylitis

Objective. To evaluate the smallest detectable difference (SDD) of symptomatic outcome or process variables in ankylosing spondylitis (AS) patients from various countries. Methods. Thirty consecutive AS patients with axial involvement were recruited from 1 center in each of 4 countries (Spain, Morocco, France, The Netherlands), for a total of 120 patients. Fourteen variables were studied in 6 domains: pain (3 variables), stiffness (1 variable), function (2 variables), spinal mobility (3 variables), patient global assessment (4 variables), and the domain of enthesiopathy (1 variable). All patients were evaluated twice within a 1-week period during which no clinical or therapeutic change occurred. Intracenter reliability was evaluated using the intraclass correlation coefficient (ICC). The SDD was determined using the Bland-Altman method. Results. Of the 14 variables evaluated in the 120 patients (82% males, 42 12 years old, with a mean disease duration of 17 13 years), only the SDD for the variable occiput-to-wall distance showed statistically significant difference among centers. For the entire group, the SDD, expressed as percentage of the range of the variable, varied from 10% (Mander enthesis index) to 39% (spinal pain at night last week). The intraobserver reliability was good (ICC > 0.80) except for the variables morning stiffness and modified Schober test (ICCs of 0.76 and 0.60, respectively). Conclusion. This study suggests that the evaluation of AS patients is homogenous and reliable in different centers of different European and North African countries. Evaluation of the SDD of the symptomatic outcome or process variables is a starting point to determine the minimum clinically important difference, permitting the presentation of results of clinical studies on an individual basis.