Psoriasis Area Severity Index

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Psoriasis Area Severity Index (PASI) is the most widely used tool for the measurement of severity of psoriasis. PASI combines the assessment of the severity of lesions and the area affected into a single score in the range 0 (no disease) to 72 (maximal disease).^[1]

Contents 1 Calculation 2 Modifications 2.1 Objectivisation 2.2 Simplification 3 References

[edit] Calculation

The body is divided into four sections (Legs, which have 40% of a person's skin; the Body (trunk area: stomach, chest, back, etc.) with 30%; the Arms (20%); and the Head (10%)). Each of these areas is scored by itself, and then the four scores are combined into the final PASI. For each section, the percent of area of skin involved, is estimated and then transformed into a gradefrom 0 to 6:

• 0% of inwolved area, grade: 0
• < 10% of involved area, grade: 1
• 10-29% of involved area, grade: 2
• 30-49% of involved area, grade: 3
• 50-69% of involved area, grade: 4
• 70-89% of involved area, grade: 5
• 90-100% of involved area, grade: 6

The severity is estimated by four different parameters: Itching, Erythema (redness), Scaling and Thickness (psoriatic skin is thicker than normal skin). Severity parameters are measured on a scale of 0 to 4, from none to maximum.

The sum of all four severity parameters is than calculated for each section of skin, multiplied by the area score for that area and multiplied by weight of respective section (0.1 for head, 0.2 for arms, 0.3 for body and 0.4 for legs). Example: (I_head+E_head+S_head+T_head) x A_head x 0.1 = Total_head

At the end the total PASI is calculated as a sum of PASIs for all four skin sections.

[edit] Modifications

[edit] Objectivisation

Psoriatic area assessment and severity assessments were found to be un-reproducible. Several automated procedure for more reproducible measurement of psoriatic area were developed, but were not suitable for large-scale trials. A method where the advantage of accurate computerized measurement of the area on the digital photograph was combined with physician's proficiency in determination of the edge of psoriatic lesion was published recently^[2]. The patients were examined and photographed before and after the therapy with calcipotriol oinment or placebo. The psoriatic area was manually outlined on the patient's photographs by physician and the area was automatically measured by a computer. For comparison, the physicians also made standard prosiatic area assement. Computer-aided measurement of psoriatic lesion area was found to improve the power of the clinical trial, compared to the standard approach. The physician's estimations of the psoriatic lesion area tend to overstimate. The adapted PASI index, where the psoriatic area was not converted into an area grade, but was maintained as a continuous variable, also improved the power of the clinical trial. The modified PASI which involves computer aided area measurement as a continuous varable is named: Computer aided psoriasis contious area and severity score cPcASI.

[edit] Simplification

PASI can be too unwieldy to use outside of trials, which has led to attempts to simplify the index (SPASI) for clinical use.^[3]

[edit] References