PASI: The gold standard for measuring psoriasis severity and treatment response.
Head (10%)
Value: 0
Trunk (30%)
Value: 0
Upper Limbs (20%)
Value: 0
Lower Limbs (40%)
Value: 0
Calculate Severity
Enter the area, erythema, induration, and scaling for all four body regions to compute the total PASI index.
Guidelines & Evidence
Verified
Last Review: 2026-07-17
When to Use
When to Use
The PASI is the gold-standard composite measure of psoriasis severity in clinical trials and a key tool in routine practice for determining eligibility for, and monitoring response to, systemic and biologic therapies. It evaluates both the extent of skin involvement and the intensity of lesional features.
Primary Indications
Assessing severity of chronic plaque psoriasis (psoriasis vulgaris) at baseline
Determining eligibility for biologic therapies (typically requires PASI ≥ 10 with DLQI > 10 per NICE/BAD guidelines)
Monitoring treatment response — tracking PASI 50, PASI 75, PASI 90, and PASI 100 response thresholds
Clinical trial endpoint: virtually all pivotal psoriasis biologic trials use PASI 75 as the primary endpoint
Documenting disease activity for insurance or prior authorization purposes
When NOT to Use
Psoriatic arthritis — PASI captures only skin; use PsARC or DAS28 for joint disease
Pustular or erythrodermic psoriasis — PASI was validated for plaque-type; scores may not reflect clinical severity
Scalp-only or nail-only psoriasis — use Nail Psoriasis Severity Index (NAPSI) or Scalp PGA instead
Routine brief monitoring visits — PASI is time-consuming (5–10 min); IGA is faster for routine use
Pediatric psoriasis — less validated; consider Children's Dermatology Life Quality Index (CDLQI) adjunct
Guideline Threshold
BAD (British Association of Dermatology) and NICE guidelines define severe psoriasis requiring biologic therapy as PASI ≥ 10 AND DLQI > 10, OR PASI ≥ 10 with inadequate response to ≥ 2 conventional systemic therapies.
How it Works
Formula Overview
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. The four body regions are weighted by their surface area proportion: head/neck (10%), upper limbs (20%), trunk (30%), lower limbs (40%).
Lesional Severity Variables (E, I, D)
| E — Erythema | 0 = none | 1 = slight | 2 = moderate | 3 = marked | 4 = very marked |
| I — Induration (thickness) | 0 = none | 1 = slight | 2 = moderate | 3 = marked | 4 = very marked |
| D — Desquamation (scaling) | 0 = none | 1 = slight | 2 = moderate | 3 = marked | 4 = very marked |
Area of Involvement (A) — per Region
| 0 | No involvement (0%) |
| 1 | 1–9% of region affected |
| 2 | 10–29% |
| 3 | 30–49% |
| 4 | 50–69% |
| 5 | 70–89% |
| 6 | 90–100% |
Scoring Notes
Subscripts: h = head/neck, u = upper limbs, t = trunk, l = lower limbs
Maximum score: 72 (theoretical); scores > 40 are exceedingly rare in clinical practice
The score is a continuous measure — there is no natural "cut point"; severity bands are consensus-derived
PASI 75 denotes a 75% reduction from baseline score (not an absolute value of 75)
Clinical Pearls
Key Limitations
PASI is heavily weighted toward body surface area — small but severely inflamed plaques on cosmetically significant sites (face, hands, genitalia) may yield a deceptively low score despite high patient burden.
Inter-rater variability is moderate to high without formal training; studies show up to 20–30% disagreement between assessors in the same patient.
PASI does not capture patient-reported burden — always use in conjunction with a PRO measure (DLQI) for treatment decisions.
PASI 75 remains the industry standard trial endpoint, but real-world targets have shifted: PASI 90 and PASI 100 (complete clearance) are now clinical treatment goals for most biologic agents.
The Simplified PASI (sPASI) and Lattice System Physician's Global Assessment (LS-PGA) are alternative instruments validated for reducing inter-rater variability.
Comparator Tools
| IGA (Investigator Global Assessment) | Faster, ordinal 0–4 scale; less granular than PASI; preferred for routine clinic visits |
| DLQI | Patient-reported quality-of-life measure; required alongside PASI for biologic eligibility decisions |
| BSA % | Simpler area assessment only; lacks lesional severity weighting |
Guideline Position
AAD (2019) and EuroGuiDerm (2022) psoriasis guidelines recommend PASI as the standard severity measure for systemic therapy decisions. Both guidelines use PASI 75 as the minimum acceptable biologic response at 16–24 weeks, with PASI 90 as the preferred benchmark for modern biologics.
Next Steps
Severity Tiers and Actions
| PASI 0 | Clear — maintain current therapy; document remission |
| PASI < 5 | Mild — topical therapies (corticosteroids, vitamin D analogues, calcineurin inhibitors); phototherapy if extensive |
| PASI 5–10 | Moderate — topical + phototherapy (NB-UVB); consider conventional systemics (methotrexate, acitretin, cyclosporine) if failing topicals |
| PASI > 10 + DLQI > 10 | Severe — eligible for biologic therapy per NICE/BAD (after failure of ≥ 2 conventional systemics or contraindication) |
Treatment Response Assessment
01
Establish a baseline PASI before initiating any new systemic therapy
02
Reassess at 12–16 weeks (biologic induction period) to assess PASI 75/90/100 response
03
Continue biologic if PASI 75 achieved; escalate or switch if PASI 50–75 with inadequate QoL improvement
04
Discontinue biologic if < PASI 50 response at 16 weeks (per NICE guidance)
05
For patients achieving PASI 100 on biologic — consider dose spacing (gap therapy) as per drug SmPC
Documentation
Always document PASI alongside DLQI at baseline and each reassessment. Both scores are required for biologic eligibility and continuation criteria under NHS and most international payer protocols.
The Evidence
Original Derivation
Severe psoriasis--oral therapy with a new retinoid.
Fredriksson T et al. • Dermatologica. 1978;157(4):238-244. Derivation paper for the PASI in a trial of 27 patients with severe plaque psoriasis treated with oral retinoid Ro 10-9359.
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Origins & History
Origins
The Psoriasis Area and Severity Index (PASI) was developed by Torsten Fredriksson and Ulla Pettersson at the Karolinska Institute, Stockholm, Sweden. It was introduced incidentally within a clinical trial evaluating an oral retinoid (Ro 10-9359) for severe psoriasis — the scoring system was not the primary aim of the paper but was created to standardize assessment of treatment response. Despite its known limitations, PASI has remained the international standard for five decades, largely due to its early adoption in pivotal biologic trials and regulatory precedent.
Last Comprehensive Review: 2026-07-17