The American College of Rheumatology (ACR) listed criteria for improvement following therapy of rheumatoid arthritis (RA). A change in the core data set of 20% or more is termed the ACR20.
Core data set:
(1) tender joint count on 28 joints or more
(2) swollen joint count on 28 joints or more
(3) patient's assessment of pain on a 10 cm visual analogue scale (VAS)
(4) patient's global assessment of disease activity on a 10 cm visual analogue scale (VAS)
(5) physician's global assessment of disease activity on a 10 cm visual analogue scale (VAS)
(6) patient's self-assessment of physical function using a sensitive instrument such as the AIMS, HAQ, MHIG, Index of Well-Being or MACTAR
(7) measure of acute phase reaction (Westergren erythrocyte sedimentation rate or C-reactive protein level)
Criteria for ACR20:
(1) >= 20% improvement in tender joint count
(2) >= 20% improvement in swollen joint count
(3) >= 20% improvement in >= 3 of the following 5 criteria:
(3a) patient's pain assessment
(3b) patient's global assessment
(3c) physician's global assessment
(3d) patient's self assessment of physical function (disability)
(3e) acute phase reaction
For counts or VAS scales a decrease >= 20% is:
(value after therapy) / (value before therapy) <= 0.80
For the ESR and CRP, the score may need to be adjusted for the increase over the upper limit of normal:
((value after therapy) - (upper limit of normal)) / ((value before therapy) - (upper limit of normal)) <= 0.80
