Psoriasis Area and Severity Index (PASI)
DESCRIPTION:
The Psoriasis Area and Severity Index is a widely used tool to assess psoriatic disease severity at a specific point in time. The PASI evaluation involves assessing 4 body regions—head, trunk, and upper and lower extremities—across 2 domains of disease activity, percent of surface area affected, and severity of the psoriatic lesions. Percent of surface area affected is based on a 6-point scale and severity of psoriatic lesions is based on a 5-point scale consisting of values assigned to erythema, induration, and desquamation.1,2
DISEASE STATES:
Psoriasis
VALIDATED USES:
Screening, treatment monitoring in conjunction with Physician Global Assessment (PGA)
ADMINISTRATION METHOD:
Clinician
COMMONLY USED IN:
Clinical trials, research
DETAILED DESCRIPTION:
PASI was originally developed in 1978 by Fredriksson and Pettersson to assess the effects of retinoids in psoriasis.1 PASI is now considered the gold standard tool for measuring disease severity and is broadly used in clinical trials to assess primary efficacy endpoints in patients with moderate-to-severe psoriasis.3
PASI quantifies disease severity as a single number that may range from 0.0 (no disease activity) to 72.0 (the most severe disease).1-3 Because there is one value for disease severity, clinical trials often compare baseline PASI scores with clinical endpoint PASI scores. These clinical trial endpoints are represented as a proportion of patients who achieve a predetermined reduction in PASI score, or disease severity:
- PASI 100: Proportion of subjects achieving 100% reduction in PASI score
- PASI 90: Proportion of subjects achieving ≥90% reduction in PASI score
- PASI 75: Proportion of subjects achieving ≥75% reduction in PASI score
The PASI evaluation involves assessing the percentage of body surface area affected by psoriasis and assigning a score for each body region based on the severity of the psoriatic lesions. The score for each region is then multiplied by the percentage of body surface area affected, and the scores for all regions are added together to give the final PASI score. The four main body areas are assessed: head (h), trunk (t), upper extremities (u), and lower extremities (l).1,2*
The PASI score for each body region is obtained by multiplying the sum of the severity scores by the area score, then multiplying the result by the constant weighted value assigned to that body region. PASI score is calculated using the formula:1,2*
PASI = 0.1(Eh + Ih + Dh)Ah + 0.2(Eu + Iu + Du)Au + 0.3(Et + It + Dt)At + 0.4(El + Il + Dl)Al
A = surface area; D = desquamation; E = erythema; I = induration.
*Other specific body parts assignments are as follows: neck = head domain; buttocks = lower extremities; axillae and genitals = trunk; the inguinal canal separates the trunk and legs anteriorly.
PASI SCORING
| Severity Score | |
|---|---|
| 0 | None |
| 1 | Slight |
| 2 | Moderate |
| 3 | Severe |
| 4 | Very severe |
| Area of Involvement Score | |
|---|---|
| 0 | None |
| 1 | 1%–9% |
| 2 | 10%–29% |
| 3 | 30%–49% |
| 4 | 50%–69% |
| 5 | 70%–89% |
| 6 | 90%–100% |
| Plaque Characteristic | Head (h) | Upper Limbs (u) | Trunk (t) | Lower Limbs (l) |
|---|---|---|---|---|
| Erythema (E) | 0–4 | 0–4 | 0–4 | 0–4 |
| Induration (I)+ | 0–4 | 0–4 | 0–4 | 0–4 |
| Desquamation (D)+ | 0–4 | 0–4 | 0–4 | 0–4 |
| Severity Score= | ||||
| Weighting Factor*x | 10% (0.1) | 20% (0.2) | 30% (0.3) | 40% (0.4) |
| Severity Subtotal= | ||||
| Area Scorex | 0–6 | 0–6 | 0–6 | 0–6 |
| Region Score= | ||||
| PASI SCORE (Sum of Region Scores)= | 0–72 |
VALIDITY:
Bożek and Reich found that the PASI components had very good reproducibility, except for scaling, which had good reproducibility. Overall, PASI demonstrated acceptable intrarater and interrater reliability, sensitivity, and the ability to detect change in clinical trials.4,5
LIMITATIONS:
PASI is resource-intensive and complex, therefore often reserved solely for clinical trials and research. Clinicians’ unfamiliarity with PASI tool can lead to large intrarater and interrater variability in scores, leading to subjective measures and low accuracy.3,4
This resource is intended for educational purposes only and is intended for US healthcare professionals. Healthcare professionals should use independent medical judgment. All decisions regarding patient care must be handled by a healthcare professional and be made based on the unique needs of each patient.
References: 1. Fredriksson T, Pettersson U. Severe psoriasis--oral therapy with a new retinoid. Dermatologica. 1978;157(4):238-244. doi:10.1159/000250839. 2. Marks R, Barton SP, Shuttleworth D, Finlay AY. Assessment of disease progress in psoriasis. Arch Dermatol. 1989;125(2):235-240. 3. Leonardi C, See K, Gallo G, et al. Psoriasis Severity Assessment combining physician and patient reported outcomes: the optimal psoriasis assessment tool. Dermatol Ther (Heidelb). 2021;11(4):1249-1263. doi: 10.1007/s13555-021-00544-6 4. Bożek A, Reich A. The reliability of three psoriasis assessment tools: psoriasis area and severity index, body surface area and physician global assessment. Adv Clin Exp Med. 2017;26(5):851-856. doi:10.17219/acem/69804 5. Langley RG, Ellis CN. Evaluating psoriasis with Psoriasis Area and Severity Index, Psoriasis Global Assessment, and Lattice System Physician’s Global Assessment. J Am Acad Dermatol. 2004;51(4):563-569. doi:10.1016/j.jaad.2004.04.012
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