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Proposal of a linear definition of the Bath Ankylosing Spondylitis Metrology Index (BASMI) and comparison with the 2-step and 10-step definitions

¹ Department of Rheumatology, Leiden University Medical Center, Leiden, The Netherlands
² Division of Internal Medicine, Department of Rheumatology, University Hospital Maastricht, Maastricht, The Netherlands
³ Ankylosing Spondylitis International Federation, München, Germany

Correspondence to:
D van der Heijde, Department of Rheumatology, Leiden University Medical Center, Leiden, PO Box 9600, 2300 RC Leiden, The Netherlands; d.vanderheijde{at}kpnplanet.nl

Background: The Bath Ankylosing Spondylitis Metrology Index (BASMI) characterises the spinal mobility of patients with ankylosing spondylitis. Two versions have been published using categorical scores with either scores 0–2 for each of the five assessments, or scores 0–10. For metric purposes, we recently defined a BASMI version with linear score definitions.

Aim: to evaluate agreement between three BASMI definitions and to test sensitivity to change.

Patients and methods: The performance of the BASMI₂ (based on the 2-step definition), BASMI₁₀ (based on the 10-step definition), and BASMI_lin (based on the linear definition), are compared in 598 status assessments and 222 follow-up assessments with a 24-week interval after an intervention with either placebo or a tumour necrosis factor (TNF) blocker from various cohorts of patients with ankylosing spondylitis (AS). Descriptive statistics and Bland–Altman plots were applied to compare the pairwise agreement of the three definitions. To assess sensitivity to change, Guyatt effect size using change data from the placebo and actively treated patients were used.

Results: Bland–Altman analysis showed that the differences between BASMI₂ scores and scores obtained by either of the two other definitions were highly dependent on the magnitude of the measurement. Guyatt effect sizes were 0.66 for the BASMI₂, 0.95 for the BASMI₁₀, and 1.04 for the BASMI_lin, respectively, demonstrating best sensitivity to change for the newly-developed BASMI_lin.

Conclusions: The BASMI₁₀ and BASMI_lin have clear metric advantages as compared to BASMI₂, among which are their superior sensitivity to change and feasibility of BASMI_lin in computer evaluations. The BASMI₁₀ and BASMI₂ are not interchangeable.