Use of Rasch Measurement Theory–transformed Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) scores allowed investigators to find ways of simplifying administration and scoring while confirming internal construct validity, ensuring linear measurement, and justifying the use of parametric statistical analyses when analyzing datasets, according to a study published in The Journal of Rheumatology. Furthermore, the transformation table created can be used with existing BASDAI datasets for the direct comparison of disease activity scores with those generated from future studies.

This secondary analysis of a BASDAI database tested the BASDAI using Rasch Measurement Theory and investigated whether measurement precision could be improved. Data were collected from participants (N=250; 23.6% women; mean age 52.8 years, SD 14.6) who were starting an ankylosing spondylitis exercise course at the Royal National Hospital for Rheumatic Diseases in Bath, United Kingdom. Initially, the data appeared to fit the Rasch model well, with consistent item thresholds. After local item dependence was identified and addressed, a unidimensional measure was achieved.

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The location of items was well-matched to the respondents with a Person Separation Index (reliability) of 0.83. Investigators generated a transformation table to convert total raw BASDAI scores into linearized Rasch-transformed BASDAI scores, forming an interval scale, and improving the smallest detectable difference (SDD) from 2 to 1.2. This suggests that a change of score on the modified BASDAI greater than 1.2 points is required to achieve meaningful change.

Removal of an item concerning the duration of morning stiffness simplified administration and scoring without loss of information but did lead to differential item functioning (DIF) by gender on the peripheral joint pain item (P <.05 [Bonferroni adjusted]).  However, further investigation with a paired samples t-test showed virtually no difference between split person estimates (median -0.43; SD 0.87) and unsplit person estimates (median -0.43; SD 0.86), [t(248) = -0.09, P=.93, d=0.01], and as the DIF did not cause misfit to the model at scale level, it was decided that the peripheral joint pain item could be maintained.

Study investigators conclude, “Adapting the BASDAI by removing the item on duration of morning stiffness, creating two subtests, and applying Rasch transformed scores simplifies completion and scoring of the measure and enables it to provide interval-level scores, suitable for analysis using parametric statistical analyses. The low SDD value achieved also indicates that it is more likely to be responsive to true changes in disease activity. However, formal testing of responsiveness would require an intervention study.”

Reference

Heaney A, McKenna SP, Hagell P, Sengupta R. Improving scoring precision and internal construct validity of the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) using Rasch Measurement Theory [published online May 15, 2019]. J Rheumatol. doi:10.3899/jrheum.180943