In order to properly assess the severity of such disease as psoriasis, in 1978 the Psoriasis Area and Severity Index or PASI for shot had been introduced by Dr.Fredriksson & Patterson.

The essence of this method of evaluation is that the percentage ratio of affected body areas is calculated compared with healthy ones taking into account the heaviness of lesions as well.

Main body areas are individually assessed and intensity of lesions on each of those body parts provides additional points to exactly estimate the seriousness of the disease.

- Head and neck
- Upper extremities
- Trunk
- Lower extremities

Each of the mentioned body parts is scored by degree of lesions – erythema (or redness), induration (or thickness) and desquamation (or scaling).

As long as different body areas might be differently affected, the PASI evaluate separately each lesion surface and severity i.e. each body part is scored by itself. The results obtained are summed up in order to receive the overall PASI score. A patient’s palm is taken as measuring unit and constitutes 1 % of total Body Surface Area or BSA.

Maximum total BSA is equal to 100% and such score means that psoriasis plaques covered the same surface as 100 patient’s palms.

Arms and shoulders (upper extremities) form 20% or 20 palms of BSA. The third area of trunk include also axillae and genitals and makes up 30% or 30 palms of total surface. The last region of lower extremities is separated from the trunk by the inguinal canal. This area includes legs and buttocks and constitutes 40% or 40 palms of Body Surface Area.

Thereupon the percentage of surface area involved is specified on each body part, each region receives a score from 0 to 6 points according to the following rating scale:

**0 No morbid affection**

**1 1-9% of body affected**

**2 10-29% of affection**

**3 30-49% affected**

**4 50-69% of body is involved**

**5 70-89% involved**

**6 90-100% of body affected**

Let’s take a closer look at the process of scoring.

The same assessment principle applies to the rest of body areas. Specifically, if we have a patient with 1 palm of psoriasis on arms, and we keep in mind that this area makes up 20% or the total BSA, then affected area comes to 1/20 or 5% of Upper Extremities BSA and therefore this patient Upper Extremities score is 1 point. On trunk 1 palm of psoriasis makes up 1/30 or 3.3% of total BSA.

If a patient has lesions surface equal to 10 palms on the trunk, his skin involvement is assessed as 10/30 or 33.3% of Trunk affection, and consequently his score for Trunk area is 3 points. In the same way lower extremities form 40% or 40 palms or the total BSA, and 1 palm of psoriasis on legs and buttocks is equal to 1/40 or 2.5% of total BSA. In case if a patient has 20 palms of psoriasis on Lower Extremities, he has 20/40 or 50% of skin area involved, and this amount corresponds to 4 points on rating scale.

The main characteristics of the plaque heaviness are redness, thickness and scaling. Each of these parameters is evaluated in points from 0 to 4, namely: 0 – for absence, 1- for mild affection, 2 – for moderate affection, 3 – for strong severity and 4 – for very strong severity. Particularly, when it comes to erythema, the intensity of redness is evaluated as follows: 1 – Light red, 2 – red, but not deep, 3 – very red and 4 – extremely red. While assessing induration, the points are given according to the following scoring: 1 – Mild, not exceeding 0.25mm of plaque thickness, 2 – Moderate, within the limits of 0.5mm, 3 – severe thickness below 1 mm, 4 – for Very Severe character of plaque induration up to 1.25mm.

In order to complete the total Head/Neck PASI score computation, it is necessary to add points for redness (0-4) with points for thickness (0-4) and scaling (0-4). The result obtained (0-12) is multiplied by BSA for Head (0-6) and multiplied by 0.1 since this particular area makes up 10% of total body.

The same for total arms PASI score, the points obtained for this area redness, thickness and scaling (0-12) should be multiplied by Area Score for Upper Extremities (0-6) and multiplied by 0.2 because this area constitutes 20 % of total body.

Calculation formula for total Trunk PASI score is as follows: erythema points plus induration points plus scaling points multiply by Trunk BSA and multiply by 0.3 as long as this region makes 30% of total body.

Finally, total Lower Extremities PASI score is calculated as the sum of redness, thickness and scaling points (0-12) multiplied by BSA for Legs (0-6) and multiplied by 0.4 since Legs constitute 40% of total body.

Total PASI score is received by summing up all 4 outcomes for Head, Arms, Trunk and Legs.