Background Different types of disease activity measures available in axial spondyloarthritis (axSpA) and there is no gold standard for all individual patients. Ankylosing Spondylitis Disease Activity Score (ASDAS) as a composite index is highly discriminatory, sensitive to change and associated with the radiographic progression. A simplified version of the ASDAS (SASDAS) was proposed and found to be simple and practical tool to assess disease activity in AS patients. However it is the only simplified version of ASDAS-ESR and may not improve discriminative ability since the ASDAS-CRP is currently preferred assessment tool for axSpA.
Objectives Our aim was to test the performance characteristics of SASDAS and compare it validated tools.
Methods In total 98 consecutive AS patients (77 males [79%]; with a mean age of 39.3±10.0 years) according to the modified New York criteria were included in the study. Disease activity was assessed by ASDAS-ESR, ASDAS-CRP, BASDAI and SASDAS. SASDAS was calculated by the simple linear addition of ASDAS five components: patient global assessment, back pain, peripheral pain and swelling, duration of morning stiffness, and ESR in mm/h, divided by 10. The cut-off values for SASDAS were reported to be as the following: inactive disease from 0 to 7.8, moderate disease activity from 7.9 to 13.8, high disease activity from 13.9 to 27.6 and very high activity above 27.6. The relationship among ASDAS-CRP, ASDAS-ESR, and SASDAS were tested by Spearman's correlation and the level of agreement of different activity categories according to ASDAS and SASDAS were assessed by using Cohen's weighted Kappa coefficients. Receiver operating characteristic (ROC) curves was constructed to assess the cut-off points of SASDAS values that correspond to ASDAS-ESR>2.1 in our cohort. Cut-off point was calculated on the basis of the best trade-off values between sensitivity and specificity.
Results Mean disease duration was 7.0 years (range 1-47 years). Mean (±SD) BASDAI, BASFI and BASMI scores were 4.2 (±2.1), 3.3 (±2.4), and 3.8 (±1.7) respectively. HLA B27 was positive in 67.4% of the patients. In our patient group mean (±SD) ASDAS-CRP, ASDAS-ESR, and SASDAS scores were 3.0 (±0.9), 2.8 (±0.9), and 19.3 (±9.0) respectively. There was a strong correlation between the SASDAS and BASDAI, ASDAS-CRP, ASDAS-ESR (. definitions by Spearman's correlation test (r=0.916, p<0.001; r=0.847, p<0.001 and r=0.942, p<0.001respectively). Although the agreement between ASDAS-ESR and SASDAS was good (with a weighted kappa of 0.825 and total agreement of 79%), there was a moderate agreement between ASDAS-CRP and SASDAS (with a weighted kappa of 0.682 and total agreement of 67%). The best trade-off SASDAS value (figure) corresponding to ASDAS-ESR>2.1 was 15.0 (83.7% sensitivity and 97.8% specificity; AUC: 0.95) and it is a bit higher than reported cut-off.
Conclusions The results of the present analysis suggest that simplified version of ASDAS-ESR should be further validated in different settings and populations due to questionable level of agreement between ASDAS-CRP and SASDAS.
Disclosure of Interest None declared