| MEASUREMENT PROPERTIES | |
| Limitations | <5 European languages (English) |
| Observations | |
| 1. RELIABILITY | |
| A. Internal Consistency | Tested |
| Cronbach's (Describe) |
Four of the items in the ASDAS are taken from the BASDAI, and these items in general have good reliability. |
| B. Reliability intraobserver or test-retest | Not Tested |
| Continuous scores: intraclass correlation coefficient (ICC) Dichotomus: Cohen kappa (Describe) |
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| C. Reliability interobserver or Measurement error | Tested |
| Standard error of measurement (SEM), smallest detectable change (SDC) or Limits of agreement (LoA) (Describe) |
Wyrwich SEM = 0.41 SDC = 0.41 to 1.06 depending on the method |
| 2. VALIDITY | |
| A. Content validity: face validity | Tested |
| Expert opinion (relevance and comprehensiveness) (Describe) |
Items were generated by Assessment of Spondyloarthritis international Society (ASAS) members using a 3-round Delphi process. Only those domains receiving 80% agreement were taken to the next round. Items selected were then tested in an international longitudinal cohort study. There were no patients directly involved in item generation
Generation by an international expert group of rheumatologists (ASAS) interested in AS, and the inclusion of serologic markers of inflammation improves the face validity over solely patient-reported domains.
Extensive statistical analysis has minimized redundancy between items. |
| B. Construct Validity: Structural validity |
Tested |
| Hypotheses-testing | Tested |
| Cross-cultural validity | Not Tested |
| Brief Description |
Testing in a three-step statistical approach (principal component analysis, discriminant function analysis and linear regression analsyis) supports structural validity of the ASDAS. The ASDAS has shown construct validity, as assessed by the correlations between the ASDAS-CRP and patient global assessment (Pearson’s correlation coefficient = 0.74; similar for the ASDAS with ESR [ASDAS-ESR]) as well as the ASDAS-CRP and physician global assessment (Pearson’s correlation coefficient = 0.47). |
| C. Criterion validity | Tested |
| Comparison with a 'gold standard' Continuous scores: correlations, ROC curves Dichotomus: Sensitivity & Specificity (Describe) |
Excellent discrimination between high and low disease activity states as defined by the physician global assessment (standardized mean difference [SMD] at baseline 1.33 for ASDAS-CRP and SMD 1.55 for ASDAS-ESR) based on the Norwegian disease-modifying anti-rheumatic drug (NORDMARD) database, and between patients treated with TNF blockers and with placebo (SMD 1.50 for ASDAS-CRP and SMD 1.51 for ASDAS-ESR) based on participants in randomized controlled trials of TNF blockers for AS.
AUC ranging from 0.74 to 0.95 against multiple gold-standards based on physician global assessment, patient global assessment and patient global rating of change after treatment.
Cut-off developed were developed using a robust methodology involving ROC curve analyses and have proven to have external validity and a good performance in cross-validation.
In RCTs with TNF-blockers versus placebo, ASDAS improvement scores perform better (higher Qui-squares) than classical improvement criteria (ASAS20, ASAS40, BASDAI50 and change in BASDAI of at least 2 units) and also that ASDAS inactive disease is more discriminative than ASAS partial remission criteria. This has implication is sample size calculation using ASDAS (lower number of patients required for the same power and α level). |