The index PASI is calculated in a different way according to
the parameters to measure in each study. An habitual and
standardized form it can be the following one:
The extension and severity is calculated in a separate way:
The extension (E) is evaluated for one of the following
corporal areas, superior extremities , trunk and inferior
extremities that correspond to 20%, 30% and 40% of the total
corporal surface and a scale is used from 0 to 6 points
0 = without affectation
1 = < 10 %
2 = 10-29 %
3 = 30-49 %
4 = 50-69 %
5 = 70-89 %
6 = 90-100 %
The severity of the psoriasic lesions registers in function
of the studied parameters, Itching, Erythema, Scaling,
Thickness, according to a scale of values from a total
absence of cutaneous affectation, until a maximum possible
severity according to the following chart:
0 = Total absence of cutaneous affectation.
1 = Slight affectation.
2 = Moderate affectation.
3 = Severe affectation.
4 = Maximum severity possible of affectation.
The index PASI is calculated independently for each
localization, according to the following formulas:
Arms: 0.2 (I+R+S+T) x E = X
Trunk: 0.3 (I+R+S+T) x E = Y
Legs: 0.4 (I+R+S+T) x E = Z
(I=Itching, R=Erythema, S=Scaling, T=Thickness, E=Extension)
The index PASI corresponds to the total sum, X+Y+Z, and it
can vary from a study to another.In this case from 0 to
86.4.
Sorry my english is not good enough to better explain.
Alberto