What is the DAS28 (Disease Activity Score) for rheumatoid arthritis, how is it calculated, and what do the score thresholds indicate for disease activity and treatment decisions?

DAS Score (Disease Activity Score)

The DAS28 is a composite measure of rheumatoid arthritis disease activity that ranges from 0-9.4, calculated using a complex formula incorporating 28 tender joint count, 28 swollen joint count, ESR (or CRP), and patient global assessment, with established thresholds defining remission (<2.6), low disease activity (2.6-3.2), moderate disease activity (3.2-5.1), and high disease activity (>5.1). 1

How DAS28 is Calculated

The DAS28 uses two different formulas depending on the inflammatory marker:

DAS28-ESR Formula:

• 0.56 × √(28TJC) + 0.28 × √(28SJC) + 0.70 × ln(ESR) + 0.014 × PtGA 1

DAS28-CRP Formula:

• 0.56 × √(28TJC) + 0.28 × √(28SJC) + 0.36 × ln(CRP + 1) + 0.014 × PtGA + 0.96 1

Where:

• 28TJC = 28 tender joint count (0-28)
• 28SJC = 28 swollen joint count (0-28)
• ESR = erythrocyte sedimentation rate (mm/hr)
• CRP = C-reactive protein (mg/L)
• PtGA = patient global assessment of disease activity (0-10 visual analog scale)

Disease Activity Thresholds

The ACR-recommended cutoffs for DAS28 are 1:

• Remission: <2.6
• Low disease activity: ≥2.6 to <3.2
• Moderate disease activity: ≥3.2 to ≤5.1
• High disease activity: >5.1

Treatment Decision Framework

Use DAS28 to guide treatment escalation according to treat-to-target principles: 2

1. Target remission (DAS28 <2.6) in early disease as the primary goal
2. Accept low disease activity (DAS28 2.6-3.2) as an alternative target, particularly in long-standing disease
3. Assess response at 12 weeks: Require at least 50% improvement in DAS28 score to continue current therapy 3
4. Escalate therapy if DAS28 remains >3.2 despite adequate treatment trial
5. Adjust medications every 3 months based on DAS28 measurements until target is achieved 2

Critical Caveats and Pitfalls

DAS28-CRP vs DAS28-ESR Differences

DAS28-CRP consistently produces lower scores than DAS28-ESR and requires different thresholds. 4, 5 The correlation between the two is strong (r=0.946), but DAS28-CRP thresholds should be:

• Remission: 2.3-2.5 (not 2.6)
• Low disease activity: 2.7-2.9 (not 3.2)
• High disease activity: 4.1-4.6 (not 5.1) 4, 5

Limitations of DAS28 Remission Criteria

The FDA classifies DAS28 <2.6 as low disease activity, not true remission, because significant residual disease activity and joint damage progression can occur at this threshold. 2 The 2016 EULAR task force emphasizes that DAS28 remission criteria are less stringent than SDAI (≤3.3) or Boolean remission definitions 2.

Acute Phase Reactant Confounders

ESR contributes 15% of the DAS28-ESR score and can be misleadingly elevated or suppressed: 1

• High ESR from comorbidities may underestimate remission despite few active joints
• Low ESR states may falsely indicate remission despite significant swollen joint counts
• Biologic agents targeting specific cytokines differentially affect ESR and CRP, potentially deflating scores artificially

Patient-Reported Component Dominance

When the patient-reported components (tender joints and patient global assessment) disproportionately drive the DAS28 score, consider non-inflammatory pain mechanisms (fibromyalgia, osteoarthritis, depression) rather than escalating immunosuppression 6. The DAS28-P index (proportion of patient-reported components) can identify patients who may not benefit from further DMARD escalation.

Joint Count Reliability

Joint counts are examiner-dependent and may be unreliable when assessors change. 1, 7 Standardize your technique and maintain consistency in who performs assessments when possible.

Practical Implementation

Calculate DAS28 at every clinic visit for active RA patients to enable treat-to-target management 7. Online calculators are available at http://www.das-score.nl/ 1.

The DAS28 weighs tender joint count more heavily than swollen joint count due to the differential weighting in the formula 1. This means subjective symptoms have greater influence than objective inflammation, which can be problematic in patients with pain amplification syndromes.

For point-of-care decision making without waiting for lab results, consider CDAI (Clinical Disease Activity Index), which uses the same components as DAS28 except substitutes provider global assessment for the acute phase reactant and uses simple addition rather than complex weighting 1.