What methods are used to measure rheumatoid arthritis activity?

Measuring Rheumatoid Arthritis Disease Activity

The American College of Rheumatology recommends six validated composite measures for assessing rheumatoid arthritis disease activity: CDAI, DAS28 (ESR or CRP), PAS, PAS-II, RAPID-3, and SDAI. 1 These measures accurately reflect disease activity, are sensitive to change, and discriminate between different disease activity states.

Composite Disease Activity Measures

Patient-Reported Measures

These are easiest to implement in clinical practice:

1. RAPID-3 (Routine Assessment of Patient Index Data with 3 measures)

   • Formula: (MDHAQ × 3.33 + pain VAS + PtGA VAS)/3
   • Scale: 0-10
   • Disease activity categories:
     • Remission: 0-1.0
     • Low: >1.0 to 2.0
     • Moderate: >2.0 to 4.0
     • High: >4.0 to 10
   • Advantages: Takes <3 minutes to complete, tested in multiple rheumatic diseases

2. PAS (Patient Activity Scale)

   • Formula: (HAQ × 3.33 + pain VAS + PtGA VAS)/3
   • Scale: 0-10
   • Disease activity categories:
     • Remission: 0.00-0.25
     • Low: 0.26-3.70
     • Moderate: 3.71 to <8.0
     • High: 8.00-10.00

3. PAS-II (Patient Activity Scale-II)

   • Formula: (HAQ-II × 3.33 + pain VAS + PtGA VAS)/3
   • Scale: 0-10
   • Disease activity categories: Same as PAS

Provider and Patient Measure

1. CDAI (Clinical Disease Activity Index)
   • Formula: 28SJC + 28TJC + PrGA + PtGA
   • Scale: 0-76
   • Disease activity categories:
     • Remission: ≤2.8
     • Low: >2.8 to 10.0
     • Moderate: >10.0 to 22.0
     • High: >22.0
   • Advantages: Simple addition, no laboratory tests needed

Provider, Patient, and Laboratory Measures

1. DAS28 (Disease Activity Score with 28-joint counts)

   • Two versions: DAS28-ESR and DAS28-CRP
   • Scale: 0-9.4
   • Disease activity categories:
     • Remission: <2.6
     • Low: ≥2.6 to <3.2
     • Moderate: ≥3.2 to ≤5.1
     • High: >5.1
   • Requires complex calculation with weighted components

2. SDAI (Simplified Disease Activity Index)

   • Formula: 28SJC + 28TJC + PrGA + PtGA + CRP
   • Scale: 0-86
   • Disease activity categories:
     • Remission: ≤3.3
     • Low: >3.3 to ≤11.0
     • Moderate: >11.0 to ≤26
     • High: >26

Key Components of Disease Activity Assessment

1. Joint Assessment

   • 28-joint count for swelling (28SJC)
   • 28-joint count for tenderness (28TJC)
   • Includes: proximal interphalangeal joints, metacarpophalangeal joints, wrists, elbows, shoulders, and knees 1

2. Patient-Reported Outcomes

   • Patient Global Assessment (PtGA): Visual analog scale (0-10cm)
   • Pain assessment: Visual analog scale (0-10cm)
   • Functional assessment: HAQ, HAQ-II, or MDHAQ

3. Provider Assessment

   • Provider Global Assessment (PrGA): Visual analog scale (0-10cm)

4. Laboratory Measures

   • C-reactive protein (CRP)
   • Erythrocyte sedimentation rate (ESR)

Selecting the Appropriate Measure

When choosing which measure to use, consider:

1. Practice Setting and Resources

   • Patient-only measures (PAS, PAS-II, RAPID-3) are easiest to implement
   • CDAI doesn't require laboratory tests but needs joint counts
   • DAS28 and SDAI require laboratory values

2. Measurement Accuracy

   • DAS28, SDAI, and CDAI have been validated in clinical trials 1, 2
   • Patient-reported measures may be influenced by factors like mood and cultural beliefs

3. Time Constraints

   • Patient-reported measures can be completed in waiting rooms
   • Joint counts add time to clinical assessment

Important Considerations and Pitfalls

• Laboratory Values: CRP may be preferred over ESR as it's simpler, more reliable, and not age-dependent 3

• Comorbidities: Conditions like fibromyalgia can affect tender joint counts and patient-reported outcomes, potentially overestimating disease activity 1

• Biologic Therapy: Newer biologic agents may disproportionately affect ESR and CRP, potentially underestimating disease activity 1

• Consistency: Use the same measure consistently for a given patient to track disease activity over time

• Remission Goals: While remission is the ultimate goal, it may not be appropriate for all patients due to comorbidities, treatment toxicity, or patient preference 1

By systematically assessing disease activity using these validated measures, clinicians can effectively implement treat-to-target strategies, leading to improved outcomes for patients with rheumatoid arthritis.