Psoriasis Area and Severity Index (PASI) Score

Evaluates the severity of psoriasis based on type of lesions and affected skin area.

Refer to the text below the calculator for more information on the PASI score and its usage.

PASI is mainly used to assess psoriasis disease severity in clinical trials and is based on the model released by Fredericksson and Pettersson in 1978.

PASI consists in evaluating the body in four sections:

  • Head (10% of skin surface);
  • Arms/Upper extremities (20% of skin surface);
  • Trunk (30% of skin surface);
  • Legs/Lower extremities (40% of skin surface).

The severity of the psoriasis lesions is evaluated based on three criteria (erythema, induration and desquamation) whilst the percentage of skin area affected is rated in grades from 0 to 6:

  • Grade 0 – 0% involvement;
  • Grade 1 – less than 10% involvement;
  • Grade 2 – 10 to 29% involvement;
  • Grade 3 – 30 to 49% involvement;
  • Grade 4 – 50 to 69% involvement;
  • Grade 5 – 70 to 89% involvement;
  • Grade 6 – 90 to 100% involvement.

In order to produce the PASI score, for each section of the body, the points awarded to the three severity signs are summed and then multiplied by the involvement grade and the weight of that section (i.e. 0.1 for head, 0.2 for arms, 0.3 for trunk and 0.4 for legs).

PASI = [0.1 x (Eh + Ih + Dh) x Gh] + [0.2 x (Ea + Ia + Da) x Ga] + [0.3 x (Et + It + Dt) x Gt] + [0.4 x (El + Il + Dl) x Gl]

The PASI scores range from 0 to 72, where the higher the score, the higher the severity of the disease.


Grade of affected skin area
Erythema (redness)
Induration (thickness)
Desquamation (scaling)

2Arms/Upper extremities

Grade of affected skin area
Erythema (redness)
Induration (thickness)
Desquamation (scaling)


Grade of affected skin area
Erythema (redness)
Induration (thickness)
Desquamation (scaling)

4Legs/Lower extremities

Grade of affected skin area
Erythema (redness)
Induration (thickness)
Desquamation (scaling)
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About PASI

The Psoriasis Area and Severity Index (PASI) is the most widely used model for measuring psoriasis severity and is based on the degree of severity and degree of skin area affected. The score ranges from 0 to 72, where 72 is maximal disease.

The index was developed in 1978 by Fredriksson and Pettersson as a method of assessing efficiency of retinoid treatment for psoriasis and is now often used in clinical trials of therapies to treat psoriasis, not only for establishing initial dose but also as trended over time to evaluate treatment efficacy.

In clinical trials, a 75% improvement in PASI (PASI 75) has been set as showing clinical meaningful improvement – indication of treatment success.

One of the criticisms received by the model concerns the fact that the clinical severity of the disease cannot be explicitly described through a PASI result and that it needs to be used alongside other evaluation systems such as the Psoriasis Global Assessment (PGA).

Criterion variability for PASI is restricted, as there is lack of a true gold standard measure of psoriasis severity.

Subsequent validation studies such as that by Puzenat et al. found PASI to have good internal consistency and moderate interobserver variation, and to be highly reproducible when performed by trained individuals. However, since PASI is neither a linear nor normally distributed score it is less sensitive at the lower end and to improve its accuracy, it requires some level of clinical judgments when scoring.

Simpson et al. found that PASI has good construct validity, with positive correlation with quality of life measures.


Help scoring PASI

The body is divided in four sections:

  • Head (H): 10% of skin area;
  • Arms (A): 20% of skin area;
  • Trunk (T): 30% of skin area;
  • Legs (L): 40% of skin area.

For each of the four sections above, the percent of skin area affected by psoriasis is estimated then associated to a grade, from 0 to 6, according to the below:

  • Grade 0: 0% of involved area;
  • Grade 1:< 10% of involved area;
  • Grade 2: 10–29% of involved area;
  • Grade 3: 30–49% of involved area;
  • Grade 4: 50–69% of involved area;
  • Grade 5: 70–89% of involved area;
  • Grade 6: 90–100% of involved area;

Also, within each section, the severity of the skin lesion is estimated by three clinical signs, each on a scale from 0 to 4.

  • Erythema (redness);
  • Induration (thickness);
  • Desquamation (scaling);

The sum of all three skin lesion severity clinical signs is then calculated for each of the four sections. The sum above is multiplied by the area score for that area and multiplied by weight of respective section (0.1 for head, 0.2 for arms, 0.3 for body and 0.4 for legs).

PASI for each body region = lesion severity x percentage of area affected x body surface area; Total PASI = addition of PASI for each body region;

PASI = [0.1 x (Eh + Ih + Dh) x Gh] + [0.2 x (Ea + Ia + Da) x Ga] + [0.3 x (Et + It + Dt) x Gt] + [0.4 x (El + Il + Dl) x Gl]



Original reference

Fredriksson T, Pettersson U. Severe psoriasis--oral therapy with a new retinoid. Dermatologica. 1978;157(4):238-44.


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):563-9.

Schmitt J, Wozel G. The Psoriasis area and severity index is the adequate criterion to define severity in chronic plaque-type psoriasis. Clinical and Laboratory Investigations. 2005; 210: 194-199.

Feldman SR, Krueger GG. Psoriasis assessment tools in clinical trials. Ann Rheum Dis. 2005; 64: 65-68.

Puzenat E, Bronsard V, Gourraud PA et al. What are the best outcome measures for assessing plaque psoriasis severity? A systematic review of the literature. European Academy of Dermatology and Venereology. 2010; 24: 10-16.

Simpson MJ, Chow C, Morgenstem H et al. Comparison of 3 methods for measuring psoriasis severity in clinical studies (Part 2 of 2): use of quality of life to assess construct validity of LS-PGA, PASI and Static Physician’s Global assessment. European Academy of Dermatology and Venereology. 2015; 29: 1415-1420.

Specialty: Dermatology

Objective: Assessment

No. Of Items: 16

Year Of Study: 1978

Abbreviation: PASI

Article By: Denise Nedea

Published On: April 27, 2020

Last Checked: April 27, 2020

Next Review: April 27, 2025