BASDAI, ASDAS-CRP good for clinical use in Singaporeans with axial spondyloarthritis

Both the Ankylosing Spondylitis Disease Activity Score with C-reactive protein (ASDAS-CRP) and the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) are reliable methods of determining disease activity in Singaporeans with axial spondyloarthritis (axSpA), according to a recent study.

This study aimed to assess whether the two indices were applicable for clinical use among Singaporean patients. “The findings of the study will give healthcare professionals confidence in using ASDAS‐CRP and BASDAI in the assessment of disease activity in patients with axSpA in Singapore,” said the researchers.

A cross-sectional analysis of 280 axSpA patients (median age, 39 years; 78.2 percent male) revealed poor internal consistency of ASDAS-CRP, as demonstrated by a Cronbach’s alpha value of 0.33. Restricting analysis to patients with CRP measurements above the limit of detection yielded a similar result (Cronbach’s alpha, 0.31). [Int J Rheum Dis 2019;doi:10.1111/1756-185X.13735]

In comparison, BASDAI obtained Cronbach’s alpha value of 0.87, indicating good internal consistency of the tool.

“The Cronbach's alpha for ASDAS-CRP was below critical value of 0.70,” said the researchers. “We postulate [that this] was due to the inclusion of CRP as an acute phase reactant in the calculation of the overall score.” This problem may also have been compounded by the large number of participants in the present study with CRP measurements below the limits of detection.

Testing for construct validity was then performed, using 12 a priori hypotheses regarding magnitude and direction, all of which resulted in statistical significance (p<0.0042). Notably, both ASDAS-CRP and BASDAI demonstrated stronger correlations with patient-reported outcome measures (PROMs) of similar domains, such as disease activity. This suggested convergent validity between the two indices.

On the other hand, ASDAS-CRP correlated poorly with PROMs that fell under other domains, such as mental health.

Taken as a singular factor model, the ASDAS-CRP showed reasonable fit upon structural validity analysis. The comparative fit index (CFI), Tucker-Lewis Index (TLI), root mean square error of approximation (RMSEA) and standardized root mean residuals (SRMR) values were 0.943, 0.886, 0.096 and 0.038, respectively.

This was not true for the one-factor BASDAI model, yielding CFI, TLI, RMSEA and SRMR values of 0.876, 0.794, 0.210 and 0.063, respectively. Optimal cutoffs for good fit were >0.95 for both CFI and TLI, <0.06 for RMSEA, and <0.08 for SRMR.

“To our knowledge, this is the first psychometric study to validate the use of ASDAS‐CRP and BASDAI in measuring disease activity in patients with axSpA in Singapore,” the researchers said. “Furthermore, our study is the first to use [confirmatory factor analysis] to assess structural validity of ASDAS‐CRP and BASDAI.”

“Formulating hypotheses a priori for construct validity and following [the consensus-based standards for the selection of health measurement instruments] framework were key strengths of this study,” they added.