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

Désirée van der Heijde ^1*, Robert Landewé ² and Ernst Feldtkeller ³

¹ Leiden University Medical Center, Netherlands
² University of Maastricht, Netherlands
³ Ankylosing Spondylitis International Federation, Germany

^* To whom correspondence should be addressed. E-mail: d.vanderheijde{at}kpnplanet.nl.

Accepted 1 August 2007

Abstract

Background: The Bath Ankylosing Spondylitis Metrology Index (BASMI), characterizes 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 function), BASMI₁₀ (based on the 10-step function), and BASMI_lin (based on the linear function), are compared in 598 status assessments and 222 follow-up assessments with a 24 week interval after an intervention with either placebo or a TNF-blocker from various cohorts of patients with AS. Descriptive statistics and Bland & Altman plots were applied to compare the pair wise agreement of the three definitions. To assess sensitivity to change, Guyatt’s 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’s 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 BAMSI₁₀ and BASMI_lin have clear metric advantages as compared to BASMI₂ among which its feasibility in computer evaluations and superior sensitivity to change. The BASMI₁₀ and BASMI₂ are not interchangeable.

Keywords: Bath Ankylosing Spondylitis Metrology Index, ankylosing spondylitis, outcome assessment, spinal mobility, validation

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