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Printed on 5/15/2026
For informational purposes only. This is not medical advice.
The Psoriasis Area and Severity Index (PASI) is the most widely used clinical measurement tool for assessing psoriasis severity and treatment response. It evaluates four body regions (head, trunk, upper limbs, and lower limbs) based on three clinical signs — erythema (redness), induration (thickness), and desquamation (scaling) — each graded 0-4, combined with the percentage of area affected in each region (graded 0-6). The PASI score ranges from 0 (no disease) to 72 (maximum severity). A PASI 75 response (75% improvement from baseline) is the standard primary endpoint in psoriasis clinical trials. Pair with [DLQI Score](/tools/dlqi) to document quality of life impact for biologic eligibility. Psoriasis is associated with increased cardiovascular risk — assess with [ASCVD Risk Calculator](/tools/ascvd-risk) and [Framingham Risk Score](/tools/framingham-risk). Monitor renal function during biologic therapy with [eGFR Calculator](/tools/egfr-calculator). For body surface area context, see [BSA Calculator](/tools/bsa-calculator).
Formula: PASI = 0.1 × (Eh + Ih + Sh) × Ah + 0.3 × (Et + It + St) × At + 0.2 × (Eu + Iu + Su) × Au + 0.4 × (El + Il + Sl) × Al
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For each of the four body regions (head, trunk, upper limbs, lower limbs), grade erythema, induration, and desquamation from 0 (none) to 4 (very severe). Then grade the percentage of area involved from 0 (none) to 6 (90–100%).
For each region, sum the three severity grades and multiply by the area score and the region's body weight factor: head ×0.1, trunk ×0.3, upper limbs ×0.2, lower limbs ×0.4.
Add the four regional subscores for the total PASI (0–72). Scores ≤5 indicate mild disease, 5–10 mild-to-moderate, and above 10 moderate-to-severe warranting systemic or biologic therapy. Track from baseline to assess PASI 75, 90, or 100 response.
Dermatologists
PASI ≥10 combined with [DLQI Score](/tools/dlqi) >10 is the standard NICE threshold for biologic therapy eligibility for adalimumab, secukinumab, ixekizumab, and risankizumab. Document baseline PASI before starting any systemic therapy.
Clinical researchers, dermatology investigators
PASI 75 is the primary endpoint in virtually all pivotal psoriasis trials. PASI 90 and PASI 100 are increasingly used as co-primary endpoints for newer IL-17 and IL-23 inhibitors that routinely achieve near-complete clearance.
Dermatologists, practice administrators
Most payers require documented PASI score plus DLQI for biologic prior authorization. A PASI score at baseline and at 12–16 weeks provides objective evidence of treatment response for continued authorization.
Rheumatologists, MFM physicians
Psoriasis severity assessed by PASI informs psoriatic arthritis (PsA) management. Patients with PASI >10 have higher rates of PsA. Systemic inflammation in severe psoriasis increases cardiovascular risk — assess with [ASCVD Risk Calculator](/tools/ascvd-risk).
Dermatology nurses, physician assistants
Serial PASI scoring at weeks 12, 24, and 52 of biologic therapy objectively tracks whether treatment targets are met. A less than 50% improvement at week 16 is typically grounds for treatment change.
Medical students, dermatology residents
PASI scoring standardizes psoriasis severity assessment across trainees. The structured approach to grading erythema, induration, and scaling builds systematic lesion evaluation skills applicable across dermatology.
The lower limbs have the highest regional weight factor (0.4), meaning extensive leg involvement dominates the total score. A patient with 90% of their lower legs affected scores far higher than one with similar-looking but more limited involvement.
Older biologics (etanercept, ustekinumab) typically achieved PASI 75. Modern IL-17 inhibitors (secukinumab, ixekizumab) and IL-23 inhibitors (risankizumab, guselkumab) routinely achieve PASI 90 or PASI 100. Set appropriate patient expectations based on the chosen treatment.
A baseline PASI is legally and clinically essential. Without a documented starting score, PASI 75/90 response cannot be objectively demonstrated at follow-up, complicating insurance renewals and clinical trial eligibility.
Scalp and facial psoriasis contribute little to the PASI total but profoundly affect quality of life. A patient with extensive scalp plaques may have a low PASI but a high [DLQI Score](/tools/dlqi). Always assess DLQI alongside PASI.
Inter-rater variability for PASI is 15–20%, driven by subjective grading of erythema and induration. Standardized serial assessments by the same clinician reduce variability and make treatment response comparisons more reliable.
Dermatologists often focus on redness and scaling while underestimating plaque thickness (induration). Accurate palpation to distinguish slight thickening (grade 1) from moderate (grade 2) plaque elevations is essential for an accurate score.
Psoriasis is an independent cardiovascular risk factor. Patients with PASI >10 have a 1.5× higher risk of major adverse cardiovascular events. Always screen with [ASCVD Risk Calculator](/tools/ascvd-risk) and [Framingham Risk Score](/tools/framingham-risk).
Nail psoriasis (present in 30–50% of patients) and psoriatic arthritis require separate assessments (NAPSI, DAS28). A 'normal' PASI does not mean the full disease burden is controlled.
PASI was developed by Fredriksson and Pettersson (1978) and remains the global standard for psoriasis severity assessment. PASI 75 response was established as the primary efficacy endpoint in the pivotal phase III trials for biologics (NICE TA 103, 134). NICE requires PASI ≥10 plus DLQI >10 for biologic eligibility in England and Wales.
Your PASI score reflects the overall severity of psoriasis by combining the extent of body surface area involvement with the intensity of three clinical signs — redness, thickness, and scaling — across four body regions. A PASI score of 0 indicates complete clearance, while a score of 1 to 5 generally corresponds to mild psoriasis. Scores between 5 and 10 suggest mild-to-moderate disease, and scores above 10 are typically classified as moderate-to-severe psoriasis warranting systemic therapy or biologic treatment.
Scores above 20 indicate severe disease with significant skin involvement. In clinical practice, the PASI score is most valuable when tracked over time to measure treatment response. The standard treatment endpoints are PASI 75 (75% improvement from baseline), PASI 90 (90% improvement), and PASI 100 (complete clearance). Modern biologic therapies frequently achieve PASI 90 or PASI 100 responses.
It is important to note that the PASI score may not fully capture disease burden. Low PASI scores can still reflect significant impairment if psoriasis affects highly visible or functionally important areas such as the face, palms, soles, genitals, or nails. The PASI score should be interpreted alongside quality of life measures like the Dermatology Life Quality Index (DLQI) for a complete picture of disease impact.
The PASI score should be calculated when assessing psoriasis severity for treatment decisions, particularly when considering systemic therapies, biologics, or entry into clinical trials. Most clinical guidelines and insurance authorization processes require a documented PASI score (often along with body surface area and DLQI) to qualify patients for biologic treatment, typically requiring a PASI of 10 or greater.
PASI is also essential for monitoring treatment response over time. It should be calculated at baseline before starting a new therapy and at regular follow-up intervals (commonly at weeks 12, 16, 24, and 52) to determine whether treatment goals such as PASI 75 or PASI 90 have been achieved. If treatment targets are not met, the PASI score provides objective evidence to support therapy changes.
The PASI score has notable inter-rater variability, particularly in grading the severity of erythema, induration, and scaling, which are subjective assessments. Different clinicians may assign different severity grades for the same lesion, leading to inconsistent scores. Training and standardization can reduce but not eliminate this variability.
The PASI scoring system gives disproportionate weight to the lower limbs (40% weighting) relative to the head (10% weighting), which means that extensive scalp or facial psoriasis may produce a deceptively low PASI score. Similarly, the area scoring uses broad percentage ranges (e.g., 1-9% and 10-29%), which reduces sensitivity to small changes in affected area, particularly in mild disease.
PASI does not assess nail psoriasis, psoriatic arthritis symptoms, or the psychological impact of the disease. A comprehensive psoriasis assessment should include additional tools such as the Nail Psoriasis Severity Index (NAPSI), joint assessment, and quality of life questionnaires. The minimum clinically important difference for PASI has not been definitively established, making it difficult to interpret small score changes.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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