Development and validation analysis of a corrected axial spondyloarthritis metrology index (CASMI)

To adjust the BASMI for measurement bias related to age, sex and height, and to compare scores and construct validity of a corrected metrology index (CASMI) against BASMI.

Spinal mobility data from non-axSpA individuals and severe axSpA patients were used to develop CASMI. For each BASMI component, the anchors for normal (score=0) and severe (score=10) mobility were redefined based on the 50th percentiles of non-axSpA individuals and the 95th percentile of patients with severe axSpA respectively. These anchors were individually adjusted for age, sex, and height using the regression coefficients from the MOBILITY study of non-axSpA individuals. Score ranges, floor/ceiling and construct validity (correlations and know-group discrimination) of BASMI and CASMI were assessed in an r-axSpA-population (OASIS cohort). It was hypothesized that CASMI would reduce correlations with age/height, while maintaining construct validity.

Applying these corrections resulted in lower CASMI scores compared with BASMI (3.2 (2.0) vs 3.8 (1.6)), particularly in older and shorter individuals. Floor effects increased as more subjects were correctly classified within the normal mobility range, while ceiling effects remained unchanged. CASMI showed markedly reduced correlations with age and height, confirming a lower influence of these non-disease factors. Construct validity was maintained, as correlations with functional, disease activity, and structural outcomes were comparable to BASMI. Known-group discrimination also remained good, with standardized mean differences consistently above 0.80.

An age-, sex- and height-corrected mobility score, the CASMI, has been developed as a more personalised and valid measure of mobility in axSpA, compared to BASMI.