Psoriasis Area and Severity Index (PASI) Score

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]

References

Original reference

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

Validation

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.