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This site is intended for US Healthcare Professionals only.


Psoriasis (PsO) is a chronic autoimmune skin disease that accelerates skin cell growth, thus causing patches of thick red skin and silvery scales to emerge. These patches and scales can appear anywhere on the body, but are usually found on the elbows, knees, scalp, lower back, face, palms, and soles of feet.1,2 There are five main types of PsO: plaque, guttate, inverse, pustular, and erythrodermic.3


Researchers estimate that at least 10% of people inherit at least one gene linked to PsO, with epigenetic factors leading to the development of disease in only 2% to 3% of the population. External triggers can include stress and injury, such as sunburn or scratches. These external triggers, together with genetic factors, are thought to trigger the development of psoriasis.2 In plaque PsO, immune response involves myeloid dendritic cells, which secrete tumor necrosis factor (TNF) α, IL-12 and IL-23—with the latter two promoting type 1 (TH1) and type 17 (TH17) response. TH1-type (TNF-α and IFN-γ) and TH17-type (IL-17, IL-21, and IL-22) cytokines induce keratinocyte proliferation in the epidermis, further contributing to the inflammatory cascade of disease.4

Although the TNF-α–IL-23–TH17 inflammatory pathway is characteristic of plaque PsO, alternate pathways have been identified for different types of PsO. For example, dermal expansion of T cells may be triggered by streptococcal superantigens in guttate PsO. While pustular PsO involves IL-17 signaling, it also exhibits increased expression of IL-1β, IL-36α, and IL-36γ transcripts.4

Signs & Symptoms

PsO is most often characterized by raised, scaly, red patches of skin that appear on the scalp, elbows, lower back, palms, legs, knees, and soles of feet. However, signs and symptoms will vary depending on the type of PsO.5,6

  • Plaque psoriasis is the most common type of PsO, which presents with plaques of thick, red skin that may be covered in silvery scales.
  • Guttate psoriasis is most often triggered by upper respiratory/strep infections and appears as small, dot-like lesions on the trunk, limbs, and scalp.
  • Inverse psoriasis appears as smooth, very red patches in areas where skin touches skin such as under the arm.
  • Pustular psoriasis most often appears on the hands and feet, and causes white pustules surrounded by red skin.
  • Erythrodermic psoriasis is a severe form of psoriasis in which red, fiery skin develops over most of the body. Potentially life threatening, erythrodermic PsO requires immediate care.

Patients can experience PsO to varying degrees—from mild to severe. Severity may be measured by the extent of the body affected or the impact on the patient’s quality of life. Patients with mild PsO, for example, may experience minimal impact with less than 3% of skin coverage; patients with moderate or severe disease that covers more than 10% of the body will experience more significant impact.

Risk Factors

Risk factors for the onset and exacerbation of psoriasis are many. The main risk factor appears to be a genetic predisposition to the disease, although it is possible to develop psoriasis even when there is no hereditary link. Other risk factors include:7

  • Stress
  • Injury to the skin (aka Koebner phenomenon)
  • Illness such as an ear or respiratory infection
  • Cold weather, which can cause flare-ups
  • Other triggers include some foods, allergies, and some environmental factors

Disease &
Clinical Trial Tools

Broad global measures of PsO disease activity and the impact they have on a patient's quality of life are used to gauge the severity of a patient's disease and their response to treatment. However, in clinical trials, more objective, validated instruments are usually required. Several such tools have been created to assess the severity of skin lesions since they have a profound effect on patients' lives, and there is growing recognition by the healthcare community of the need to measure the quality-of-life impact of the disease along with the severity of the lesions. Below are several assessment tools currently in use.8

Psoriasis Area and Severity Index (PASI)

The Psoriasis Area and Severity Index (PASI) is a commonly used clinical tool for assessing psoriatic skin changes. The PASI evaluates both the severity of lesions as well as the involved body surface area.9

To calculate the PASI, the body is divided into four areas: head and neck, upper limbs, trunk, and lower limbs. Within each region, the severity of erythema, induration, and desquamation is then assessed on a 5-point visual analog scale (VAS) and the extent of body area involved is assessed on a 6-point VAS.9,10

The PASI score varies from 0 to 72, with higher scores reflecting more severe manifestations of disease.9,10 Although the PASI has been criticized for being complex,9,10 a 75% improvement in PASI (PASI 75) has been well established as a benchmark of success in clinical trials.11

Download PASI

Body Surface Area (BSA)9,10

A commonly used measure of PsO severity, Body Surface Area (BSA) assesses the extent of body surface involvement. Most commonly, psoriatic lesions are estimated using the “rule of nine” which divides the body into segments that each represent 9% of the body surface area, as seen in the chart.

An alternative method to estimate BSA, the number of a patient’s hand areas affected may be assessed with the assumption that hand surface area is representative of 1% of the total body surface area. Research has shown, however, that BSA is often overestimated—possibly due to the fact that one hand is actually representative of less than 1% of total body surface area.

It is important to note that regardless of method, BSA only measures the extent of lesions and not associated quality or morphology as the PASI does.

Download BSA

Psoriasis Symptoms and Signs Diary (PSSD)12,13

The Psoriasis Symptoms and Signs Diary (PSSD) is a patient-reported outcome measure that assesses the severity of PsO symptoms and signs. Patients rate their individual signs and symptoms on a scale of 0 to 10, in which 0 represents no signs or symptoms and 10 represents the worst possible expression of signs or symptoms.

A symptom summary score and a sign summary score, each ranging from 0-100, are derived from the average scores of the five symptoms and the six signs, respectively. Higher scores indicate more severe disease.

Download PSSD

sPGA Explained

A clinical measure of PsO severity, the static Physician’s Global Assessment (sPGA) has many variations including 5-, 6-, and 7-point scoring systems. Physicians assess the average induration, erythema, and scaling of all PsO lesions.14,15 These category scores determine the overall sPGA score, as seen in the chart.

The extent of the involved body surface area is not incorporated into sPGA. As a result, a patient with one small plaque should be scored the same as a patient with many similar plaques. In practice, however, physicians may indirectly incorporate body surface area into their assessments—introducing variability to the scoring system.14

Download sPGA Explained


  1. CDC Psoriasis. Accessed June 22, 2020.
  2. National Psoriasis Foundation. Published April 16, 2020. Accessed May 28, 2020.
  3. National Psoriasis Foundation. Accessed August 21, 2020.
  4. Rendon A, Schäkel K. Psoriasis pathogenesis and treatment. Int J Mol Sci. 2019;20(6):1475.
  5. Psoriasis. National Institute of Arthritis and Musculoskeletal and Skin Diseases. Published March 2017.
    Accessed May 15, 2020.
  6. National Psoriasis Foundation. Published March 8, 2020. Accessed May 28, 2020.
  7. Mayo Clinic. Accessed June 22, 2020.
  8. Feldman SR, Krueger GG. Psoriasis assessment tools in clinical trials. Ann Rheum Dis. 2005;64 Suppl 2(Suppl 2):ii65-ii73.
  9. Fink C, Uhlmann L, Klose C, Haenssle HA. Automated, computer-guided PASI measurements by digital image analysis versus conventional physicians’ PASI calculations: study protocol for a comparative, single-centre, observational study. BMJ Open. 2018;8:e018461.
  10. Bozek A, Reich A. The reliability of three psoriasis assessment tools: psoriasis area and severity index, body surface area and physician global assessment. Adv Clin Exp Med. 2017;26(5):851-856.
  11. Feldman SR, Krueger GG. Psoriasis assessment tools in clinical trials. Ann Rheum Dis. 2005;64 Suppl 2(Suppl 2):ii65-ii73.
  12. Papp KA, Blauvelt A, Kimball AB, et al. Patient-reported symptoms and signs of moderate-to-severe psoriasis treated with guselkumab or adalimumab: results from the randomized VOYAGE 1 trial. JEADV. 2018;32:1515-1522.
  13. Armstrong A, Puig L, Langley R, et al. Validation of psychometric properties and development of response criteria for the psoriasis symptoms and signs diary (PSSD): results from a phase 3 clinical trial. J Dermatol Treat. 2019;30(1):27-34.
  14. Chow C, Simpson MJ, Luger TA, et al. Comparison of three methods for measuring psoriasis severity in clinical studies (part 1 of 2): change during therapy in psoriasis area and severity index, static physician’s global assessment and lattice system physician’s global assessment. JEADV. 2015;29:1406-1414.
  15. Simpson MJ, et al. Comparison of three methods for measuring psoriasis severity in clinical studies (part 2 of 2): use of quality of life to assess construct validity of the Lattice System Physician’s Global Assessment, Psoriasis Area and Severity Index and Static Physician’s Global Assessment. JEADV. 2015;29:1415-1420.