Developed in 2001.
The ASAS Response Criteria (ASAS 20) is defined as an improvement of at least 20% and an absolute improvement of at least 10 units on a 0-100 scale in at least three of the following domains: Patient global assessment, Pain assessment, Function (BASFI), and Inflammation (last 2 questions of BASDAI).
ASAS 40, 50 and 70 are defined as above with improvements of at least 40%, 50% and 70% respectively.
Anderson JJ; Baron G; van der Heijde D; Felson DT; Dougados M
Anderson JJ, Baron G, van der Heijde D, Felson DT, Dougados M. Ankylosing spondylitis assessment group preliminary definition of short-term improvement in ankylosing spondylitis. Arthritis Rheum. 2001 Aug;44(8):1876-86 (Full text article)