How to Calculate the Simplified Disease Activity Index (SDAI)
The SDAI is calculated by adding five components together: tender joint count (28 joints) + swollen joint count (28 joints) + patient global assessment (0-10 cm VAS) + provider global assessment (0-10 cm VAS) + C-reactive protein in mg/dL. 1, 2
Components Required for Calculation
Tender Joint Count (TJC-28): Count the number of tender joints out of 28 specific joints assessed 1, 2
Swollen Joint Count (SJC-28): Count the number of swollen joints out of 28 specific joints assessed 1, 2
Patient Global Assessment (PGA): Patient rates their disease activity on a 0-10 cm visual analog scale (or 0-100 mm scale) 1, 2
Provider/Physician Global Assessment (PhGA/EGA): Clinician rates the patient's disease activity on a 0-10 cm visual analog scale (or 0-100 mm scale) 1, 2
C-Reactive Protein (CRP): Laboratory value expressed in mg/dL 1
The Calculation Formula
SDAI = TJC-28 + SJC-28 + PGA + PhGA + CRP (mg/dL) 2, 3
This is simple arithmetic addition—no complex weighting or formulas required, unlike DAS28 which requires differential weighting and typically needs a calculator 2, 4
Critical Technical Details
CRP must be in mg/dL: If your laboratory reports CRP in other units, convert to mg/dL before calculating 1
For undetectable CRP values (reported as <0.3 mg/dL): Input 0.29 mg/dL for the calculation 1
Visual analog scales: If using 0-100 mm scales instead of 0-10 cm, divide by 10 to convert to the 0-10 scale 1
Interpreting the SDAI Score
The total SDAI score ranges from 0 to 86, with the following disease activity categories 1, 2:
- Remission: ≤3.3
- Low disease activity: >3.3 to ≤11.0
- Moderate disease activity: >11.0 to ≤26.0
- High/severe disease activity: >26.0
Clinical Application for Your Patient
For a patient with RA on high-dose prednisone, hydroxychloroquine, and experiencing increased arthritis pain:
Perform a 28-joint examination to count tender and swollen joints 1, 2
Obtain CRP level from laboratory (essential since this patient likely has elevated acute-phase reactants given active symptoms) 1
Record both patient and provider global assessments on 0-10 scales at the same visit 1, 2
Calculate SDAI immediately once CRP results are available—this provides objective inflammatory marker data when CRP is elevated 2
Important Clinical Considerations
SDAI is preferred when acute-phase reactants are elevated because it includes the objective CRP measurement, making it particularly appropriate for patients with active inflammatory disease 1
Proper training in 28-joint count assessment is essential for accurate calculation—inconsistent joint examination technique will compromise the validity of serial measurements 2
SDAI remission criteria (≤3.3) are more stringent than DAS28 remission (≤2.4), meaning SDAI is superior for treatment decisions when aiming for true remission 2, 5
The main limitation is waiting for CRP results, which delays point-of-care calculation—if immediate assessment is needed, consider using CDAI instead (which excludes CRP) 2
Monitor SDAI every 1-3 months during active disease until remission is achieved, then every 3-6 months once remission or low disease activity is maintained 2
Common Pitfalls to Avoid
Don't use ESR instead of CRP: The SDAI specifically requires CRP in mg/dL, not ESR 1
Don't forget unit conversion: Laboratories often report CRP in mg/L—you must divide by 10 to convert to mg/dL 1
Don't use different joint counts: The SDAI requires the standardized 28-joint count, not other joint count systems 1, 2
Don't interpret SDAI and DAS28 remission as equivalent: A patient can be in DAS28 remission while having an SDAI score indicating moderate disease activity 2, 5