Psoriasis Area and Severity Index (PASI) is the most widely used tool for the measurement of severity of
psoriasis
Psoriasis is a long-lasting, noncontagious autoimmune disease characterized by raised areas of abnormal skin. These areas are red, pink, or purple, dry, itchy, and scaly. Psoriasis varies in severity from small, localized patches to complete ...
. PASI combines the assessment of the severity of
lesion
A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals.
Types
There is no designated classifi ...
s and the area affected into a single score in the range 0 (no disease) to 72 (maximal disease).
Calculation
The body is divided into four sections (
head
A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may ...
(H) (10% of a person's
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
);
arm
In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between the ...
s (A) (20%);
trunk (T) (30%);
leg
A leg is a weight-bearing and animal locomotion, locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single ...
s (L) (40%)). 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 grade from 0 to 6:
Within each area, the severity is estimated by three clinical signs:
erythema
Erythema (from the Greek , meaning red) is redness of the skin or mucous membranes, caused by hyperemia (increased blood flow) in superficial capillaries. It occurs with any skin injury, infection, or inflammation. Examples of erythema not assoc ...
(redness),
induration
A skin condition, also known as cutaneous condition, is any medical condition that affects the integumentary system—the organ system that encloses the body and includes skin, nails, and related muscle and glands. The major function of this sy ...
(thickness) and
desquamation
Desquamation occurs when the outermost layer of a tissue, such as the skin, is shed. The term is . Physiologic desquamation
Keratinocytes are the predominant cells of the epidermis, the outermost layer of the skin. Living keratinocytes reside in ...
(scaling). Severity parameters are measured on a scale of 0 to 4, from none to maximum.
The sum of all three severity parameters is then 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).
Response rate
PASI is widely used in clinical trials of therapies to treat psoriasis. Although absolute PASI score is often used to define entry into a trial, it is response to treatment that is important to measure efficacy and outcomes. This is usually presented as a percentage response rate; e.g., PASI 50, PASI 75, PASI 90, PASI 100.
PASI 75, for example, represents the percentage (or number) of patients who have achieved a 75% or more reduction in their PASI score from baseline. PASI 100 indicates patients who have achieved a complete resolution of all disease. By definition a patient who has achieved PASI 75 has also achieved PASI 50. There is a gradual tendency, as therapies become more effective to report higher efficacy rates in clinical trials - but aiming high may not be without risk of over-treating some patients.
Modifications
Objectivisation
Psoriatic
area
Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape
A shape or figure is a graphics, graphical representation of an obje ...
assessment and
severity assessments were found to be non-reproducible. Several automated procedure for more reproducible measurement of psoriatic area were developed, but were not suitable for large-scale
trials
In law, a trial is a coming together of parties to a dispute, to present information (in the form of evidence) in a tribunal, a formal setting with the authority to adjudicate claims or disputes. One form of tribunal is a court. The tribunal, w ...
. 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. The patients were examined and photographed before and after the therapy with calcipotriol ointment 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 psoriatic area assessment. 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 overestimate. 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 variable is named ''Computer aided'' psoriasis ''continuous'' area and severity score cPcASI.
Simplification
PASI can be too unwieldy to use outside of trials, which has led to attempts to simplify the index (SPASI) for clinical use.
Alternate approaches
While higher PASI scores indicate more severe psoriasis, it is difficult for patients or doctors to describe the clinical severity for any specific PASI number. Attempts have been made to address this problem by providing descriptions of psoriasis severity in evaluation systems, including the Psoriasis Global Assessment (PGA) and the Lattice-System Physician's Global Assessment (LS-PGA); a study compared and validated PASI, PGA, and LS-PGA.
[Langley RG, Ellis CN: Evaluating psoriasis with Psoriasis Area and Severity Index, Psoriasis Global Assessment, and Lattice System Physician’s Global Assessment]
J Am Acad Dermatol 2004;51:563-9
See also
*
Eczema Area and Severity Index
References
{{Reflist, 2
External links
New Zealand Dermatological Society page on PASI with scoring picturesOnline PASI calculatorOnline PASI calculatorInteractive PASI calculator
Medical scales
Dermatologic terminology