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If assessing the severity of deterioration in quality of life and any improvement in it under treatment represents major progress in assessing the real-term severity of psoriasis and therapeutic effectiveness, measuring the development under treatment of the skin areas affected by psoriasis and the modifications of the clinical characteristics of the plaques remains a major and irreplaceable means of assessment in clinical pharmacology. It is noteworthy that if there is no individual correlation between the decrease in affected areas and improvement in the quality of life, this correlation is particularly close when large populations are studied, as has recently been shown by the EUROPSO survey, relating to more than 18,000 patients in seven European countries.

2.1 Psoriasis Area Severity Index (PASI)

Suggested by Frederickson in 1975, this index remains, in spite of all its flaws, the reference index because it has been used to evaluate a large number of drugs, thus allowing any new treatment to be ranked by its efficacy. However, it is imperative to take advantage of the experience gained from using it so to replace it with a more suitable index. This substitution can only be made progressively, in order to find an index accepted by the psoriasis experts, on the one hand, and to validate it by using it systematically in parallel with PASI, on the other.

PASI combines an assessment of the clinical characteristics of psoriasis plaques with an assessment of the areas affected.

The assessment of plaque characteristics satisfies most users. It concerns erythema, desquamation and infiltration. The analysis is done comprehensively on four areas of the body: the head, the trunk, the upper limbs and the lower limbs. In each of these four areas, the examiner subjectively assesses the percentage of area affected. Finally, the score obtained in each of these four areas is multiplied by a coefficient, taking into account the percentage of total skin surface represented by each of these four areas.

The plaque characteristics are noted from 0 to 4, as follows:

The descriptions of these three clinical signs vary somewhat according to the studies, but agreement among experts is easy to establish.

Each regional score is collected in a spreadsheet and linked to a percentage of area affected. This affected area is assessed highly subjectively among experts holding two extreme positions: a functional assessment, in which each anatomical territory displaying a plaque (forearm, leg, lumbar region etc.) is counted in full, since this corresponds to the patient’s experience; and a mathematical assessment, in which the examiner attempts to morph into an image analyst and quantify the affected area by adding together the area of each plaque. As a guide, he recalls that the palm of the hand represents approximately 1% of the body’s surface. Thus, according to the examiner’s sensitivity, the PASI for generalized guttate psoriasis ranges from 1 to 20! This is why, 28 years since the first time PASI was used, it is still indispensable to gather together the experts, before each clinical trial, to reach agreement on how to calculate PASI.

PASI formula
PASI = 0.1 (Eh + Ih + Sh) x Ah + 0.3 (Et + It + St) x At + 0.2 (Eu + Iu + Su) x Au + 0.4 (Ei + II + SI) x AI

PASI’s limitations are thus understood: arbitrary assessment of the areas affected, body regions too extensive to distinguish between resistant regions and regions sensitive to treatment, use of irrelevant multiplier coefficients, complete assimilation of the plaques’ surface and clinical characteristics, maximum score of 72, very weak sensitivity for PASIs lower than 12 due to the absence of linearity in the response: It is much easier to go from a PASI of 20 to a PASI of 10 than from a PASI of 10 to a PASI of 5. Moving from minimal erythema to pink erythema over 30% of the body’s surface, in the absence of scaling and infiltration, causes the PASI to go from 2 to 4!
A mainly noninflammatory psoriasis highly sensitive to treatment but affecting 70% of each region of the body will have a PASI of 30, whereas “chalky psoriasis”, highly resistant to treatment and affecting 85% of the surface of the lower limbs, will have a PASI of 24.
Thus the best way to get spectacular therapeutic results is to select patients with a high initial PASI!
A number of efforts have therefore been made to find something better.

2.2 Static Investigator’s Global Assessment Scale (IGA)

For your assessment, please choose the category that best describes the patient’s general appearance:

This highly generalized index confines itself therefore to recording the situation at the time of each visit. It also goes to show that the affected area is too subjective to be taken into consideration and that separating the body into four zones is useless, since a priori the improvement in all four zones is parallel.

The index is a sound one, therefore reproducible from one investigator to another, but particularly unsophisticated and scant on information. As the PASI, it does not allow to discriminate between skin areas sensitive or resistant to treatments.

2.3 Lattice System Global Psoriasis Score (LS-GPS)

Another test, used in the USA, has attempted to integrate the affected area. It also concerns the whole body: this is the Lattice System Global Psoriasis Score (LS-GPS).

This involves a three-stage process for qualifying psoriasis. The first stage assesses the total area affected, the second the clinical characteristics of the plaques, assuming them all to be similar, thus arriving at the third stage, determination of the overall severity of the psoriasis, or LS-GPS, according to 8 options, ranging from clear to very severe, allowing development to be monitored under treatment.

This index is an enhanced variation of the Investigator’s Global Assessment scale (IGA); it takes into account the affected area, but remains extremely unsophisticated.

2.4 Psoriasis Score

A final score, already validated in a simplified form in an article in the British Journal of Dermatology of 1985, has been recently updated. It uses the same grading for the clinical description of plaques as the PASI, adding grading for pruritus and impact on the quality of life. Completely unlike the two preceding scores, its originality lies in making it possible to study analytically the evolution of psoriasis under treatment while taking into account all the territories of the body. This has the merit taking into account one of the main characteristics of psoriasis: the variety of clinical characteristics and of the plaques’ response to treatment from one body area to another. This method allows to avoid the utopian and scarcely reproducible assessment of the affected area by replacing it with an assessment of the number of skin areas affected - a measure far closer to the reality experienced by the patient. With far greater precision than the PASI, it allows regression of the illness to be objectivized, as attested to by the decrease in the number of areas affected and by the alteration in the clinical characteristics of the plaques, area by area. It allows comparison of the kinetics of improvement under treatment for each skin area and hence the identification of those zones that are responsive and those that are resistant. It takes into account the impact of each psoriasis site on the quality of life and its development under treatment. Finally, this system of assessment is much more sensitive than PASI because the maximum score is 368.

The characteristics of the plaques are recorded from 0 to 4, as follows:

Two simple scales can be added to this:

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