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The Ankylosing Spondylitis Disease Activity Score (ASDAS) is a new composite index to assess disease activity in Ankylosing Spondylitis (AS)1. It combines five disease activity variables with only partial overlap, resulting in one single score with better truth (validity), enhanced discriminative capacity and improved sensitivity to change as compared to single-item variables (1,2).
Table 1. The two ASDAS formulas: ASDAS-CRP (preferred) and ASDAS-ESR (alternative).
|ASDAS-CRP||0.12 x Back Pain + 0.06 x Duration of Morning Stiffness + 0.11 x Patient Global + 0.07 x Peripheral Pain/Swelling + 0.58 x Ln(CRP+1)|
|ASDAS-ESR||0.08 x Back Pain + 0.07 x Duration of Morning Stiffness + 0.11 x Patient Global + 0.09 x Peripheral Pain/Swelling + 0.29 x √(ESR)|
|ASDAS, Ankylosing Spondylitis Disease Activity Score; √(ESR), square root of the erythrocyte sedimentation rate (mm/h); Ln(CRP+1), natural logarithm of the C-reactive protein (mg/L) + 1. Back pain, patient global, duration of morning stiffness and peripheral pain/swelling are all assessed on a visual analogue scale (from 0 to 10cm) or on a numerical rating scale (from 0 to 10). Back pain, BASDAI question 2: "How would you describe the overall level of AS neck, back or hip pain you have had?". Duration of morning stiffness, BASDAI question 6: "How long does your morning stiffness last from the time you wake up?". Patient global: "How active was your spondylitis on average during the last week?" Peripheral pain/swelling, BASDAI question 3: "How would you describe the overall level of pain/swelling in joints other than neck, back or hips you have had?"|
The Assessment of SpondyloArthritis international Society (ASAS) membership has selected the ASDAS containing C-reactive protein (CRP, mg/l) as acute phase reactant as the preferred version, and the one with erythrocyte sedimentation rate (ESR, mm/hr) as the alternative version. Apart from the value of CRP or ESR, the four additional self-reported items included in this index are back pain (0-10cm, visual analogue scale [VAS] or 0-10, numerical rating scale [NRS]), duration of morning stiffness (VAS/NRS), peripheral pain/swelling (VAS/NRS) and patient global assessment of disease activity (VAS/NRS) (table 1) (1,2).
The next step to consolidate the ASDAS as an instrument to measure disease activity in AS was the development of cut-offs for disease activity states and improvement scores. During the 2010 ASAS workshop in Berlin, Germany, cut-offs for the ASDAS were proposed, and cross-validation studies were presented. The methodology and the results were debated by ASAS members and four disease activity states were chosen by consensus: "inactive disease", "moderate disease activity", "high disease activity" and "very high disease activity".
The 3 cut-offs selected to separate these states (figure 1) were: <1.3 between "inactive disease" and "low disease activity", <2.1 between "moderate disease activity" and "high disease activity", and >3.5 between "high disease activity" and "very high disease activity" (3,4).
Selected cut-offs for improvement scores (figure 2) were: a change ≥1.1 units for "clinically important improvement" and a change ≥2.0 units for "major improvement" (3).
Figure 1. Selected cut-offs for disease activity states.
Figure 2. Selected cut-offs for improvement scores.
At the Outcome Measures in Rheumatology (OMERACT) 10 conference, in Kota Kinabalu, Malaysia, the ASDAS disease activity states and response criteria also obtained the endorsement from OMERACT (5).
Using the ASDAS and the newly validated cut-off values, we hope that clinicians can better assess the effectiveness of treatments and determine whether they are providing clinically meaningful improvement. The higher discriminatory capacity of the ASDAS compared to classical response criteria in AS may have important implications in reducing sample size calculation for clinical trials. The ASDAS will also allow clinicians, investigators, regulators, and patients to continue communicating about treatment response using the same metric.