The Severity of Your Psoriasis

How It's Determined and Why It Matters

Not only are there different types of psoriasis, but there are different degrees of severity—mild, moderate, and severe, with shades of gray in between. It is important that each case is characterized correctly to ensure that it is appropriately treated. Given the unpleasant symptoms of psoriasis, you might be eager to try the most aggressive options. But overtreating a mild case, for example, can actually prove ineffective and cause side effects.

To determine the severity of your psoriasis, a healthcare provider will assess three things:

  • The percentage of skin affected
  • The characteristics of the lesions (referred to as plaques)
  • The impact of the disease on your daily life

Living with Plaque Psoriasis

Levels of Psoriasis Based on Body Surface Area

Arguably, the most important factor in assessing the severity of psoriasis is the amount of skin involved. This will not only direct which treatment options are appropriate, but it can also provide insights into the emotional effects of the disease. In the simplest possible terms, the more skin involved, the greater the physical and psychological impact.

To this end, many healthcare providers will base the severity of psoriasis on the body surface area (BSA) of skin affected, denoted by percentages. (By way of reference, a hand has a BSA of roughly 1%.)

Based on the BSA, severity can be broadly classified as follows:

Severity Level BSA (Percentage of Body Affected)
Mild psoriasis Less than 3%
Moderate psoriasis 3% to 10%
Severe psoriasis More than 10%

As straightforward as this system seems, it has its limitations. This is especially true when the disease hovers between mild and moderate or moderate and severe. While is possible to use clinical judgment to direct treatment, most specialists prefer a more insightful approach to diagnosis.

PASI Scores

To many specialists, using BSA as the sole determinant of disease severity is akin to looking at psoriasis through a peephole. It allows for a wide range of interpretations based on the broadest of measurements.

For example:

  • Might someone with 3% of affected skin be treated the same as someone with 10%, even though they both have moderate psoriasis?
  • What if there is a lot of affected skin with only mild flaking or a smaller area of skin with cracking and bleeding?
  • How might the decision be affected if the plaques are on the face versus hidden under a sleeve on a forearm?

In the end, all of these concerns should factor into the clinical decision. It is for this reason that a tool called the Psoriasis Area and Severity Index (PASI) was created. PASI uses BSA and plaque characteristics to render a more qualified assessment of the disease severity. This is done in two steps.

Step 1: Scoring the BSA

The calculations used in a PASI survey are broken down into four body parts:

  • Head (total BSA of 10%)
  • Trunk (total BSA for 30%)
  • Arms (total BSA of 20%)
  • Legs (total BSA of 40%)

A score of 0 to 6 is assigned for the percentage of skin involved in each of the four body parts (for a maximum score of 24):

  • 0: 0% involvement
  • 1: Less than 10% involvement
  • 2: 10% to 29% involvement
  • 3: 30% to 49% involvement
  • 4: 50% to 69% involvement
  • 5: 70% to 89% involvement
  • 6: 90% to 100% involvement

Step 2: Calculating PASI

After the BSA score is tallied, the healthcare provider will assess three different disease characteristics on each of the four body parts.

  • Erythema (redness)
  • Induration (thickness)
  • Desquamation (scaling)

Each of the symptoms is assigned a score of 0 to 4 for a maximum score of 12 per body part. This is then added to your BSA for the final PASI score.

While it is possible to get a maximum score of 72 (24 for the BSA plus 36 for the symptoms), there are rarely PASI scores of over 40.

Quality of Life Assessments

Generally speaking, a higher PASI score correlates to a lower quality of life. But, that's not always the case. The simple fact is that people respond to diseases differently, and psoriasis is no exception. Even though two people may have the same PASI score, one might have a greater perception of pain and experience more emotional distress and social discomfort than the other.

To this end, some healthcare providers will perform a subjective survey to quantify the impact of psoriasis on your day-to-day life. These may include:

  • Psoriasis Index of Quality of Life (PSORIQoL): Evaluates 25 different factors, including sleep, emotions, and social interactions
  • Psoriasis Life Stress Inventory (PLSI): Asks how stressful 18 daily tasks, like getting a haircut or going to public places, are for you to perform
  • Psoriasis Disability Index (PDI): Assesses how psoriasis affects your work, leisure time, and personal relationships

How This Guides Treatment

By evaluating how psoriasis affects you physically and emotionally, your healthcare provider can make a qualified decision regarding your treatment.

With mild psoriasis, for example, your practitioner may prescribe moisturizing creams and topical medications or recommend that you quit smoking or lose weight to reduce your risk of flares.

With moderate psoriasis, additional treatments may be incorporated, including immunosuppressant drugs like methotrexate or retinoids.

With severe psoriasis, additional interventions may be prescribed, including phototherapy and injectable biologic drugs like Humira (adalimumab) and Enbrel (etanercept).

Quality-of-life tests can also inform treatment decisions, particularly if the disease is causing anxiety or depression. In the end, treating psoriasis to address its physical symptoms is just as important as treating it to ease its emotional ones.

A Word From Verywell

PASI scores, as well as quality-of-life surveys, are also valuable in tracking your response to treatment. By keeping track of your condition, your healthcare provider can ensure the optimal response to treatment and the best quality of life.

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

By Christina Donnelly
Christina Donnelly is a freelance writer and editor who has extensively covered health and science content. She currently works at Anthem Health as a content lead.