PASI Score Calculator — Psoriasis Area and Severity Index
The Psoriasis Area and Severity Index (PASI) is the most widely used tool for assessing psoriasis severity in clinical trials and practice. It combines the assessment of body surface area involvement with the severity of three clinical features — erythema, induration, and desquamation — across four body regions.
About
The PASI score was developed in 1978 by Fredriksson and Pettersson for the assessment of psoriasis severity in clinical trials. It has become the gold standard endpoint in psoriasis clinical trials worldwide, particularly as the PASI 75 (≥75% reduction from baseline) and PASI 90 (≥90% reduction) response criteria. The score ranges from 0 (no disease) to 72 (maximal severity). Four body regions are assessed: head and neck (10% of body surface), upper extremities (20%), trunk (30%), and lower extremities (40%). Within each region, three parameters are scored on a 0-4 scale: erythema (redness), induration (thickness), and desquamation (scaling). The area of involvement is scored 0-6 (0 = no involvement, 1 = <10%, 2 = 10-29%, 3 = 30-49%, 4 = 50-69%, 5 = 70-89%, 6 = 90-100%). The PASI has been validated extensively and remains the primary efficacy endpoint in most psoriasis clinical trials, though its complexity has led to the development of simplified tools like the Physician Global Assessment (PGA) and the Simplified Psoriasis Index (SPI).
Formula
PASI = 0.1(E_h+I_h+D_h)A_h + 0.2(E_u+I_u+D_u)A_u + 0.3(E_t+I_t+D_t)A_t + 0.4(E_l+I_l+D_l)A_l
PASI combines assessment of four body regions (head 10%, upper extremities 20%, trunk 30%, lower extremities 40%). For each region, three parameters are scored 0-4: erythema (redness), induration (thickness), and desquamation (scaling). The area score (0-6) reflects the percentage of involvement. Region score = (E + I + D) × Area × Weight. Total PASI = sum of all four region scores (range 0-72).
Score Interpretation
PASI is the gold standard for assessing psoriasis severity in clinical trials and is the most commonly used primary endpoint for phase 2 and 3 trials of psoriasis treatments. A PASI 75 response has been the standard regulatory endpoint, though PASI 90 and PASI 100 are increasingly used. The minimal clinically important difference (MCID) in PASI is approximately 2-3 points or a 50% improvement from baseline.
Mild — 0–9.9
Mild psoriasis severity. Topical therapy is typically sufficient.
Management: Topical therapy with corticosteroids or vitamin D analogues.
Moderate — 10–19.9
Moderate psoriasis. Phototherapy or systemic therapy may be indicated.
Management: Consider phototherapy (UVB) or systemic agents (methotrexate, cyclosporine).
Severe — 20+
Severe psoriasis. Systemic or biologic therapy is typically required.
Management: Biologic therapy (TNF-alpha, IL-17, IL-23 inhibitors) recommended.
Reference Ranges
| Population | Normal Range | Notes |
|---|---|---|
| Plaque psoriasis patients | 0 (no disease) to 72 (maximal severity) | PASI < 10: mild; 10-20: moderate; > 20: severe |
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Expert dermatology review panel.
View medical review board & editorial policy →Example Calculation
A patient with head: E=2, I=2, D=1, area=3 (30-49%); upper extremities: E=3, I=2, D=2, area=4 (50-69%); trunk: E=3, I=3, D=2, area=4; lower extremities: E=2, I=2, D=2, area=3. PASI = 0.1×(2+2+1)×3 + 0.2×(3+2+2)×4 + 0.3×(3+3+2)×4 + 0.4×(2+2+2)×3 = 1.5 + 5.6 + 9.6 + 7.2 = 23.9 (severe).
Related Conditions
Related Medications
Common Mistakes
Using BSA alone instead of full PASI calculation.
BSA measures only extent without severity. PASI combines both for a comprehensive assessment.
Frequently Asked Questions
What PASI score indicates severe psoriasis?
References
- Fredriksson T, Pettersson U. Severe psoriasis — oral therapy with a new retinoid. Dermatologica. 1978;157(4):238-44.