PASI Score Calculation
The PASI score is calculated by assessing three plaque characteristics (erythema, induration, and scaling) plus body surface area (BSA) involvement across four anatomical regions, yielding a total score ranging from 0 to 72. 1
Core Components of PASI Calculation
The PASI evaluates four anatomical regions separately 1:
- Head and neck
- Upper limbs
- Trunk
- Lower limbs
For each region, assess the following:
Severity Parameters (scored 0-4 for each)
Scoring scale for each parameter:
- 0 = None
- 1 = Slight
- 2 = Moderate
- 3 = Marked
- 4 = Very marked
Area Score (scored 0-6 for each region)
Percentage of BSA affected within each anatomical region 1:
- 0 = 0%
- 1 = 1-9%
- 2 = 10-29%
- 3 = 30-49%
- 4 = 50-69%
- 5 = 70-89%
- 6 = 90-100%
Calculation Formula
For each anatomical region: (Erythema + Induration + Scaling) × Area Score × Regional Weight
Regional weights 1:
- Head: 0.1 (10% of total BSA)
- Upper limbs: 0.2 (20% of total BSA)
- Trunk: 0.3 (30% of total BSA)
- Lower limbs: 0.4 (40% of total BSA)
Total PASI = Sum of all four regional scores
Clinical Interpretation
- PASI ≥10: Generally considered moderate to severe psoriasis 1
- PASI <7: Typically classified as mild disease 2
- PASI 7-15: Moderate disease (may be severe if difficult-to-treat sites affected or significant psychosocial impact) 2
- PASI >15: Severe disease, independent of quality of life impact 2
Important Clinical Caveats
PASI is primarily a research tool and seldom used in routine clinical practice despite being the gold standard in clinical trials 1. The AAD-NPF guidelines note that while PASI is commonly used in clinical trials, its use in clinical practice is infrequent 1.
Key limitations to recognize 1, 3:
- Less sensitive when BSA involvement is <10% in any given region, as the area score defaults to "1" for any involvement between 0-10%, reducing precision 3
- Inter-rater and intra-rater variability exists, though reliability improves with rater experience 1
- Does not capture location-specific impact: Head and upper limb involvement may have greater quality of life impact than trunk or lower limbs, but standard PASI weights these differently 4
- Time-consuming nature limits practical clinical use 1
For clinical practice, consider using simpler measures like BSA alone (recommended for risk stratification and treatment response) or Physician Global Assessment (PGA), which are more practical while maintaining good reliability 1.