The Psoriasis Area and Severity Index (PASI) is a widely used tool for assessing the severity and extent of psoriasis. It combines the assessment of the severity of lesions and the area affected into a single score, ranging from 0 (no disease) to 72 (maximal disease). The PASI is a critical measure in both clinical practice and research to evaluate the effectiveness of treatments.

Psoriasis affects around 125 million people worldwide. Indeed, between 2% and 3% of the total population have psoriasis, according to the World Psoriasis Day consortium. Psoriasis is a chronic skin condition that significantly impacts patients' health and quality of life.

The PASI score helps clinicians determine the severity of their psoriasis and monitor treatment responses. It was created by Dr Torsten Fredriksson and Dr Ulf Pettersson in 1978 as an objective way to measure the effectiveness of psoriasis treatments.

This standardised scoring system allows for consistent assessment and comparison of disease severity, facilitating better patient management and enabling the evaluation of treatment efficacy in clinical trials.

The formula to calculate the PASI is the following:

PASI = 0.1 (E_h + l_h + S_h) A_h + 0.2 (E_u + l_u + S_u) A_u + 0.3 (E_t + l_t + S_t) A_t + 0.4 (E_l + l_l + S_l) A_l

Where:

• E: erythema
• I: induration
• S: scaling
• A: area
• h: head score
• u: upper extremities
• t: trunk score
• l: lower extremities

On this page, you will find the PASI Calculator as well as the cutting-edge APASI, an automatic PASI calculator by Legit.Health that provides quick and accurate results from patient images to measure psoriasis severity.

The PASI score is calculated based on the severity of psoriasis lesions and the area of the body affected. To calculate the PASI score, one must follow these steps:

1. Divide the body into four regions

The body is divided into four regions: head, upper extremities, trunk, and lower extremities, each with a different weight factor.

• Head: weights 10% of the total score.
• Upper extremities: weight 20% of the total score.
• Trunk: weights 30% of the total score.
• Lower extremities: weight 40% of the total score.

This weight is calculated according to the standard of Body Surface Area, also known as BSA.

2. Assess the severity of lesions in each region

The clinical signs that must be assessed are:

• Erythema, also known as redness.
• Induration, also known as thickness.
• Desquamation, also known as scaling.

Each of these signs is scored on a scale from 0 to 4, where:

• 0 is none.
• 1 is mild or slight.
• 2 is moderate.
• 3 is severe or striking.
• 4 is very severe or exceptionally striking.

With this in mind, you must assess the severity of each clinical sign for each of the four body regions. For example, a patient could have the following severity:

• Head:
  • Erythema: 2
  • Induration: 2
  • Desquamation: 3
• Upper extremities:
  • Erythema: 2
  • Induration: 1
  • Desquamation: 2
• Trunk:
  • Erythema: 3
  • Induration: 3
  • Desquamation: 4
• Lower extremities:
  • Erythema: 2
  • Induration: 1
  • Desquamation: 2

3. Estimate the area of affected skin in each region

The affected area is measured in terms of the percentage that is affected, classified across six ranges:

• 0: 0%
• 1: < 10%
• 2: 10-29%
• 3: 30-49%
• 4: 50-69%
• 5: 70-89%
• 6: 90-100%

For example, a patient could have the following severity:

• Head:
  • Affected area: 0
• Upper extremities:
  • Affected area: 4
• Trunk:
  • Affected area: 1
• Lower extremities:
  • Affected area: 2

4. Calculate the PASI for each region and sum the scores

Multiply the area score by the sum of the severity scores for erythema, induration, and desquamation. Then, multiply this result by the weight of the respective region. The final PASI score will range from 0 to 72.

Example PASI calculation

For a patient with the following scores:

• Head: Erythema(2), Induration(2), Desquamation(3), Area(3)
• Upper Extremities: Erythema (2), Induration (1), Desquamation (2), Area (2)
• Trunk: Erythema (3), Induration (3), Desquamation (4), Area (4)
• Lower Extremities: Erythema (1), Induration (2), Desquamation (1), Area (3)

The PASI score would be calculated as follows:

• Head: (2 + 2 + 3) * 3 * 0.1 = 2.1
• Upper extremities: (2 + 1 + 2) * 2 * 0.2 = 2
• Trunk: (3 + 3 + 4) * 4 * 0.3 = 12
• Lower extremities: (1 + 1 + 2) * 3 * 0.4 = 4.8

As a result, the total PASI score would be 2.1 + 2 + 12 + 4.8 = 20.9

Although the Psoriasis Area and Severity Index (PASI) remains the most widely used tool for assessing psoriasis severity, it has several well-documented limitations. Researchers have highlighted various concerns over the years:

1. Correlation with Quality of Life: A study published in 2004, titled "Psoriasis Area Severity Index (PASI) and the Dermatology Life Quality Index (DLQI): the correlation between disease-severity and psychological burden in patients treated with biological therapies," found that "The correlation between the most widely used clinical trial efficacy endpoint, the physician-rendered Psoriasis Area Severity Index (PASI) score, and the most widely used quality of life metric, the Dermatology Life Quality Index (DLQI), is not always high".
2. Measuring Small Areas: The most prominent criticism of the PASI is its insufficiency in accurately measuring psoriasis affecting areas smaller than 10% of a body region. A 2014 study titled "A pilot study on the Psoriasis Area and Severity Index (PASI) for small areas: Presentation and implications of the Low PASI score" from the Journal of Dermatological Treatment concluded: "A minimal involvement (lower than 10% of the body area involved) will always lead, according to the classic PASI six-point area scale classification, to an affected area of 1."

Several limitations of the PASI have been identified. Chief among them are:

• Subjectivity: The scoring of erythema, induration, and desquamation can be subjective and may vary between different clinicians.
• Complexity: The calculation process can be time-consuming and complex, requiring detailed assessments.
• Area Estimation: Estimating the percentage of body surface area affected can introduce variability and potential errors.

In 2019, a multidisciplinary team of scientists from several countries developed the Automatic Psoriasis Area and Severity Index, also called APASI. The APASI automatically measures how much psoriasis a patient suffers from by analysing a photo of the patient through the use of Artificial Intelligence. The APASI is the automatic version of the PASI. Through the use of Artificial Intelligence, the APASI automatically calculates the PASI score.

The main objective of APASI (or Automatic PASI) is to provide a tool to accurately and consistently measure the severity of psoriasis during routine evaluations and clinical studies. The main advantage of the APASI is speed. The APASI requires only 23 seconds, while the manual PASI requires between six and 10 minutes.

Another advantage of APASI is the reduction in inter-observer variability. The APASI outperforms traditional methods, achieving a margin of error below 13%, much lower than the usual 20% observed in traditional manual PASI.

The APASI system represents the future of dermatology. It allows clinicians to benefit from the best tool for diagnosing and measuring the severity of psoriasis.

In essence, the APASI allows health care providers to practise evidence-based medicine and improves communication between doctor and patient.

• Speed and Efficiency: Quick assessment through image analysis reduces the time needed for manual scoring.
• Consistency: Reduces inter-observer variability and potential human errors.
• Patient Adherence: Helps patients adhere to regular monitoring schedules.