DOI: 10.25934/pr00008576
title: Validating a Personalised Ankylosing Spondylitis Metrology Index

description: Radiographic axial spondylarthritis (r-axSpA), also known as Ankylosing Spondylitis, is a disease in which inflammation in the spine can lead to bony fusion between vertebrae and arthritis in the hip joints, leading to a temporary or permanent loss of mobility in these joints. With the aid of a tape measure and a simple goniometer (an instrument for the precise measurement of angles), clinicians started tracking these changes in mobility over the course of the disease by measuring in the neck, spine, and hips. 
Since 1944, an index of these measurements was developed, The Bath Ankylosing Spondylitis Metrology Index (BASMI), and its formula has undergone minor modifications (from nominal scales to a continuous scale) being the BASMI Linear the last the version most commonly used in recent clinical trials due to its higher practicability in computer evaluations.
Over the past 15 years, the use of MRI rather than x-ray to achieve an earlier diagnosis has led to the introduction of a new and more inclusive term for the disease ‘axial spondylarthritis’ (axSpA), including patients diagnosed in an earlier stage. Although the BASMI is still widely used, some clinical trials that have focused on patients with early axSpA have often reported a lack of responsiveness of BASMI, suggesting that this outcome tool may no longer be appropriate in this group of patients. Indeed, the Assessment of SpondyloArthritis international Society (ASAS) has recently reviewed the core set for axSpA and determined that the BASMI should no longer be recommended as a core outcome domain for inclusion in all axSpA therapeutic studies. Among the reasons for this change are the floor effect, where patients score at the bottom of the scale, seen in patients with short disease duration, and poor sensitivity to change and a ceiling effect, where patients scope at the top of the scale. 
At the time the BASMI was developed, it had not been tested on a group of normal individuals. When this was systematically studied in the MOBILITY study of healthy individuals, it was found that the BASMI was strongly influenced by factors such as height, age and gender. Another important observation in the MOBILITY study was that whilst the occiput to wall distance was zero in 95% of healthy individuals (as expected), not a single one had a normal Tragus to wall distance (TTW) score which had been a reliable measurement used prior to this. The MOBILITY study in the healthy population clearly showed that the BASMI Linear assigns inappropriately high/low readings for spinal mobility to a substantial proportion of people, particularly those who are shorter/taller than average and those patients who are older.
This means that the clinician’s assessment of the severity of mobility restriction can be quite inaccurate as they are unaware of the adjustments that need to be made. In addition, the formula for calculating the Tragus to wall scale in the BASMI Linear is clearly misleading and needs to be corrected. We therefore believe that an adjusted version of the BASMI Linear, taking age, height and gender into account would have better psychometric properties than the ones from the original BASMI Linear. 
As a first step of this project, the Personalised Axial Spondylarthritis Metrology Index (PASMI) was developed. The PASMI allows, by adjusting each individual spinal mobility measure for age, height and gender, as appropriate, to deliver a more ‘truthful’ representation of spinal mobility for each individual. The PASMI also redefines the normal limit of the scale using the data from the healthy population and corrects the ‘false floor’ in the TTW score. The PASMI score has been designed to be more ‘truthful’ than BASMI for the individual, and hopefully this will also improve its psychometric properties in comparison to those of the BASMI and the individual spinal mobility measures. We further hypothesized that the TTW measurement will have a large floor effect in cohorts with early axSpA, so we will compare the psychometric properties in both instruments (PASMI and BASMI) stratified by duration of disease/symptoms.