PASI: Psoriasis Area and Severity Index (Fredriksson 1978)

PASI, the Psoriasis Area and Severity Index, puts the extent and severity of plaque psoriasis into one number from 0 to 72. The body is split into four regions: head, arms, trunk and legs. In each region the redness, thickness and scaling of the plaques are graded 0 to 4, the area involved is graded 0 to 6, and the region's result is weighted by its share of the body surface. Magent's PASI calculator applies those weights to the grades you enter and returns the total and each region's part.

Not a medical device. Not a diagnosis or dosing recommendation.

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Fredriksson T & Pettersson U · 1978
The PASI formula
Worked examples
Frequently asked questions

What is PASI?

Fredriksson and Pettersson described the index in 1978, in a study of an oral retinoid for severe psoriasis. They needed a way to follow change in the skin over time that took in both how much skin was affected and how marked the plaques were. PASI is now used in many psoriasis trials.

The calculator uses the original adult weights and whole-number grades. It returns the number and doesn't put it into severity bands.

The PASI formula

For each region:  (erythema + induration + scaling) x area grade x weight

Region               weight
Head                 0.1
Upper extremities    0.2
Trunk                0.3
Lower extremities    0.4

Erythema, induration, scaling: 0 none, 1 slight, 2 moderate,
                               3 severe, 4 very severe
Area grade: 0 none, 1 <10%, 2 10-29%, 3 30-49%,
            4 50-69%, 5 70-89%, 6 90-100%

PASI = sum of the four regions, 0 to 72, one decimal place

Exact expression the calculator uses:

PASI = 0.1*(head_erythema + head_induration + head_scaling)*head_area + 0.2*(upper_extremities_erythema + upper_extremities_induration + upper_extremities_scaling)*upper_extremities_area + 0.3*(trunk_erythema + trunk_induration + trunk_scaling)*trunk_area + 0.4*(lower_extremities_erythema + lower_extremities_induration + lower_extremities_scaling)*lower_extremities_area; each area integer 0-6 (0 none, 1 <10%, 2 10-29%, 3 30-49%, 4 50-69%, 5 70-89%, 6 90-100%); each of erythema, induration, and scaling integer 0-4 (0 none, 1 slight, 2 moderate, 3 severe, 4 very severe); adult region weights head 0.1, upper extremities 0.2, trunk 0.3, lower extremities 0.4; max 72; magent uses these adult weights only; this paper published no Investigator Global Assessment or mild-moderate-severe PASI cutoffs

Inputs

For each of the four regions (head, upper_extremities, trunk, lower_extremities):

Input	Meaning	Range
`{region}_area`	Area grade for the share of that region involved	0 to 6
`{region}_erythema`	Redness	0 to 4
`{region}_induration`	Thickness of the plaques	0 to 4
`{region}_scaling`	Scaling	0 to 4

That's 16 inputs in all. Grades are whole numbers that you assign after looking at the skin. A grade outside these ranges returns the error invalid_pasi.

How the calculation works

Grade the signs. In each region, grade redness, thickness and scaling 0 to 4 and add them (0 to 12).
Grade the area. Convert the share of the region involved into a 0 to 6 grade.
Multiply. Sign total × area grade × the region's weight.
Add. The four region results add up to PASI.

The weights reflect the share of body surface: legs most, head least.

Worked examples

These use Magent's own expected values for the calculator.

Example 1: clear skin

All grades 0 in every region.
PASI 0.0.

Example 2: four regions with different grades

Head: area 2; redness 1, thickness 1, scaling 0. 0.1 × 2 × 2 = 0.4.
Arms: area 3; 2, 1, 1. 0.2 × 3 × 4 = 2.4.
Trunk: area 1; 0, 2, 1. 0.3 × 1 × 3 = 0.9.
Legs: area 4; 1, 0, 2. 0.4 × 4 × 3 = 4.8.
PASI 8.5.

Example 3: the head alone at the top grades

Head area 6; redness, thickness and scaling 4 each. Other regions 0.
0.1 × 6 × 12 = 7.2, so PASI 7.2.

Example 4: every region at the top grades

Area 6 and all signs 4 everywhere.
Regions 7.2, 14.4, 21.6 and 28.8.
PASI 72.0, the maximum.

History of PASI

Fredriksson T and Pettersson U introduced the index in Dermatologica in 1978, in "Severe psoriasis--oral therapy with a new retinoid". Later versions changed some details, including weights for children and half-point grades. Thresholds such as percentage improvement from baseline also came later. This calculator follows the original adult form.

When it's used

PASI is recorded at a visit to describe how extensive and marked the psoriasis is, and repeated to follow change.

Where it's used

Dermatology clinics and psoriasis trials.

Who uses it

Dermatologists, dermatology nurses and trial assessors who examine the skin. The calculator does the weighting and adds the regions.

Limits of the calculation

Grading is subjective, and assessors can differ, especially for area.
Area grades are broad, so a small change in area may not move the grade.
It covers plaque features and doesn't reflect itch, quality of life or nail and joint disease.
The calculator returns a number with no severity bands.
This page doesn't say what a result means for a particular person.

Clinical decisions belong to a qualified clinician who knows the patient.

EASI follows the same pattern for eczema, with four signs graded 0 to 3, including swelling.
UAS scores hives and itch in urticaria.

Glossary

Erythema: redness.
Induration: thickness or raised feel of the plaques.
Scaling: flaking on the surface of the plaques.
Area grade: a 0 to 6 code for the share of the region involved.

Common mistakes

Entering the percentage instead of the grade. Area is entered as 0 to 6.
Using half-points. The calculator takes whole numbers.
Weighting regions equally. Head 0.1, arms 0.2, trunk 0.3, legs 0.4.
Applying children's weights. Those come from later versions and aren't used here.

Source

Fredriksson T, Pettersson U. Severe psoriasis--oral therapy with a new retinoid. Dermatologica. 1978;157(4):238-244. DOI: 10.1159/000250839. PMID: 357213

Frequently asked questions

What is the PASI range?

0 to 72.

Why is the highest PASI 72?

Each region can reach 12 sign points × area grade 6 = 72 before weighting, and the four weights add up to 1.

Which body regions does PASI use?

Head, upper extremities, trunk and lower extremities.

Can I use half-point grades?

Not in this calculator. Grades are whole numbers.

Does the calculator say whether psoriasis is mild or severe?

No. It returns the number and the four region results.