ACR20 Criteria Explained

ACR stands for American College of Rheumatology, the organization that developed the ACR20 criteria. ACR20 measures how much a person with rheumatoid arthritis (RA) improves from when they start taking a medication to a certain point of time later (for example, 32 weeks).

The ACR20 measurement has five (5) criteria:

  1. How active the person thinks their RA is.
  2. How well their doctor thinks they’re responding to the treatment.
  3. How easy it is for them to do daily activities like walking, dressing, and eating.
  4. How much pain they’re in.
  5. How much inflammation they have in their body, based on the results of blood tests like C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR).

The 20 after ACR represents 20%, which is what a patient needs to “pass” the ACR20.  That is, for a patient to achieve an ”ACR20,” they have shown a 20% improvement in any three of the five factors.

The “20” number also refers to the fact that researchers need to show a 20% improvement in the number – the unit count – of swollen and painful joints of their participants for a medication to be considered valid.

A major use of the ACR20 is in clinical trials of new RA treatments. Before the ACR20, studies lacked a standardized way to report upon the effectiveness of a new treatment. The fact that different studies used varying metrics made it very difficult to compare the 

An international group of rheumatologists met in1993, and developed a comprehensive set of measures titled the ACR20. The FDA became something of a proponent thereafter, recommending the ACR20 as the outcome measurement of choice for clinical trials studying the effectiveness of any new RA drug. It’s today’s most used outcomes measure in clinical trials for new RA treatments.

It requires consistent longitudinal care; that is, scheduled appointments, because these are comparative measures – a series of 2-n is required.

The common “point in time” measures are the Clinical Disease Activity Index (CDAI), which is based on the number of swollen, tender joints that you and your doctor count. The Disease Activity Score DAS-28 is generated from this Index, and is utilized in both clinical trials and provider’s offices/video visits. In addition to the count of tender and swollen joints, it contemplates the results of your ESR blood test (a test that measures inflammation in your body), and your rating of how you feel on a scale from one to 100, aka Patient Reported Outcomes..

In Clinical Trials. Every ACR number represents a different degree of improvement in three of the five measurements mentioned above, per the Mapi Research Trust.

  • ACR20 = 20% improvement
  • ACR50 = 50% improvement
  • ACR70 = 70% improvement

The higher the number, the larger the improvement that participants experienced during the study.