PASI Calculator (Psoriasis Area and Severity Index)
PASI Calculator (Psoriasis Area and Severity Index)
Four body regions weighted 0.1, 0.2, 0.3 and 0.4, each scored for erythema, induration and desquamation and for the area involved. The index runs 0 to 72 — and it is blind to site, symptoms and small areas of devastating disease.
Psoriasis Area and Severity Index
4 regions, 16 inputs to 0–72Head and neck E2 I1 D2, area 10–29% (2); upper limbs E2 I2 D2, area 30–49% (3); trunk E3 I2 D2, area 10–29% (2); lower limbs E3 I2 D3, area 50–69% (4)
Formula
E, I, D = erythema, induration, desquamation, each 0–4 · A = area category 0–6 · total 0–72
- the four weights
- 0.1 head and neck, 0.2 upper limbs, 0.3 trunk, 0.4 lower limbs — the regions’ approximate shares of body surface, summing to exactly 1.0. That is what makes the maximum 72: three signs at 4 is 12, times an area category of 6 is 72, times one whole body
- the area category
- 0 for none, 1 for 1 to 9 per cent, 2 for 10 to 29, 3 for 30 to 49, 4 for 50 to 69, 5 for 70 to 89, 6 for 90 to 100 — of THAT REGION, not of the body. DermNet prints the first band as 1 to 9 per cent and the Wikipedia article as under 10; for whole percentages those are the same set
- insensitive at the bottom
- the CHAMPION post hoc analysis states flatly that “PASI is a less sensitive measurement in patients with low BSA involvement” and that it “is a non-linear scale that does not allow reliable assessment of subtle variations of its components”. The category jumps a whole integer at a time, so the smallest detectable change in one region is 0.1 to 0.4 points times the sign sum
- inter-observer variability
- an inter-rater study in Anais Brasileiros de Dermatologia scored 20 patients with three first-year dermatology postgraduates: intraclass correlation 0.729 (95% CI 0.440 to 0.882) for the total, 0.817 for the best-agreeing pair. A lower bound of 0.44 is the honest figure to hold
- no site term, no symptom term
- the same CHAMPION analysis notes that “the scoring does not take into account the disproportionate disease burden reflected in the more visible” areas “or the impact on patient quality of life”. That is the single most important thing to know about this number, and why the European consensus added upgrade criteria. The four regional contributions under the headline are shown because a PASI of 20 concentrated in the lower limbs is a different disease from a PASI of 20 spread evenly
Worked example
Head and neck E2 I1 D2, area 10–29% (2); upper limbs E2 I2 D2, area 30–49% (3); trunk E3 I2 D2, area 10–29% (2); lower limbs E3 I2 D3, area 50–69% (4)
Head and neck: (2 + 1 + 2) × 2 × 0.1 = 1.0
Upper limbs: (2 + 2 + 2) × 3 × 0.2 = 3.6
Trunk: (3 + 2 + 2) × 2 × 0.3 = 4.2
Lower limbs: (3 + 2 + 3) × 4 × 0.4 = 12.8
1.0 + 3.6 + 4.2 + 12.8 = 21.6 points
Nearly three-fifths of that total is the lower-limb term, which carries the largest weight and the largest area category at once
Move the head and neck area from 10–29% to 90–100% — category 2 to category 6 — and the total rises by (2+1+2) × 4 × 0.1 = 2.0 to 23.6. Do the same to the lower limbs and the total rises by (3+2+3) × 2 × 0.4 = 6.4. The same change in area category is worth four times as much on the legs
Swap the head and lower-limb weights by mistake and this case returns 16.2 rather than 21.6 — a 25 per cent error, in a number that looks entirely plausible
The four regions, their weights and what each is worth
| Region | Weight | Maximum contribution | Per area category, at maximum intensity |
|---|---|---|---|
| Head and neck | 0.1 | 7.2 points | 1.2 points |
| Upper limbs | 0.2 | 14.4 points | 2.4 points |
| Trunk | 0.3 | 21.6 points | 3.6 points |
| Lower limbs | 0.4 | 28.8 points | 4.8 points |
| Total | 1.0 | 72 points | — |
The area category, and the edges that move the answer
| Per cent of the REGION involved | Category | Width of the band |
|---|---|---|
| 0 | 0 | a point, not a range |
| 1 to 9 | 1 | 9 percentage points |
| 10 to 29 | 2 | 20 |
| 30 to 49 | 3 | 20 |
| 50 to 69 | 4 | 20 |
| 70 to 89 | 5 | 20 |
| 90 to 100 | 6 | 11 |
The most-used index in dermatology, and the most criticised
PASI came out of a 1978 retinoid trial, where Fredriksson and Pettersson needed one number for severe psoriasis across a whole body. Nearly fifty years later it is still the primary endpoint of essentially every psoriasis registration trial: four regions weighted by their share of body surface, each scored for redness, thickness and scale from 0 to 4 and multiplied by a 0 to 6 area category. The weights sum to one body, so the index runs 0 to 72.
Two features of that construction decide how the number should be read. The first is that the area category is a RANGE mapped to an integer, and the ranges are uneven: one to nine per cent of a region is category 1 and ten to twenty-nine is category 2. A single percentage point can move a region by a whole category at the bottom of the scale, while an eighteen-point difference in the middle moves nothing. That is the non-linearity the CHAMPION post hoc analysis — adalimumab against methotrexate — described as a scale “that does not allow reliable assessment of subtle variations of its components”.
The second is that there is no term for anything but area and the three signs: no site, no symptom, no nail, no joint, no quality of life. A psoriasis confined to the palms, the soles or the genitals can be disabling at a PASI of one or two, which is why the 2011 European consensus bolts upgrade criteria on to the outside of the index. Reliability is the third problem: an inter-rater study of twenty patients scored by three first-year dermatology postgraduates found an intraclass correlation of 0.729 (95% CI 0.440 to 0.882).
All of which is why the trial endpoint is a relative improvement rather than an absolute change, and why the PASI response calculator is a separate page. Methotrexate was CHAMPION’s comparator and remains a mainstay of systemic treatment, so the methotrexate unit converter is this page’s laboratory neighbour; for how much ointment a region takes, see the topical dosing calculator. No proprietor, licence portal or permission notice was found for this instrument; it is a journal-published scoring scheme, restated here with attribution rather than reproduced from a licensed questionnaire. No patient-reported outcome measure is computed anywhere on this site. The Dermatology Life Quality Index is owned and licensed by Cardiff University, whose own page states that its measures may not be reproduced in any publication and charges per patient for non-clinical use, so it is named here and never scored.
Frequently asked questions
How is a PASI score calculated?
For each of four regions — head and neck, upper limbs, trunk, lower limbs — add the erythema, induration and desquamation scores (each 0 to 4), multiply by that region’s area category (0 to 6), and multiply by its weight (0.1, 0.2, 0.3, 0.4). The PASI is the sum of the four, 0 to 72.
What PASI score counts as moderate-to-severe psoriasis?
The 2011 European consensus, restated in the EuroGuiDerm chapter, defines MILD disease as PASI 10 or below AND body surface area 10 per cent or below AND a Dermatology Life Quality Index of 10 or below, with upgrade criteria for visible areas, scalp, genitals, two affected fingernails, and itch with recalcitrant plaques. The threshold is not a PASI value on its own.
Why does PASI under-score palm, sole and genital psoriasis?
Because the index has no site term. Those sites are small fractions of their regions, so even complete involvement gives a low area category and a low total. The CHAMPION analysis puts it as the scoring not taking account of “the disproportionate disease burden reflected in the more visible” areas. The European upgrade criteria exist to override the number there.
How reproducible is the PASI between assessors?
Moderately, at best. An inter-rater study of 20 patients scored by three first-year dermatology postgraduates found an intraclass correlation of 0.729 (95% CI 0.440 to 0.882); the best-agreeing pair reached 0.817. The area estimate is the subjective part, and physicians’ estimates of lesion area tend to run high.
Related calculators
References
- Fredriksson T, Pettersson U. Severe psoriasis — oral therapy with a new retinoid. Dermatologica. 1978;157(4):238–44. doi:10.1159/000250839. The derivation publication; the index was that trial’s severity measure.
- DermNet. PASI score — the four regions, the 0–6 area categories and the three signs on 0–4. dermnetnz.org/topics/pasi-score
- Psoriasis Area and Severity Index — each region matched to its weight, the 0–72 range and the published criticisms. en.wikipedia.org/wiki/Psoriasis_Area_and_Severity_Index
- Navarini AA, Poulin Y, Menter A, Gu Y, Teixeira HD. Analysis of body regions and components of PASI scores during adalimumab or methotrexate treatment for patients with moderate-to-severe psoriasis. J Drugs Dermatol. 2014;13(5):554–62. Post hoc analysis of CHAMPION.
- Inter-rater concordance study of the PASI (Psoriasis Area and Severity Index). An Bras Dermatol. 2010 — 20 patients scored independently by three first-year dermatology postgraduates; intraclass correlation 0.729 (95% CI 0.440–0.882).
- EuroGuiDerm. Disease severity and treatment goals, psoriasis vulgaris living guideline chapter, September 2023 — the 2011 European consensus definition and the PASI 50/75 treatment goal algorithm. guidelines.edf.one
- Cardiff University. Quality of life questionnaires — the DLQI’s ownership, the prohibition on reproducing the measures, and the per-patient fee schedule for non-clinical use. cardiff.ac.uk/medicine/resources/quality-of-life-questionnaires
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/