Diagnosis of psoriasis is primarily on clinical grounds, based on examination of plaques or skin lesions with the characteristic silvery scales. Pathology, serology or imaging tests are usually not required (Cohen et al., 2012; Boehncke & Schön, 2015; Diani et al., 2015).
Psoriasis severity depends on a number of clinical factors and the disease is associated with a range of comorbidities and risk factors (Lubrano et al., 2015).
The severity of psoriasis is generally determined using body surface area (BSA) measurements. Psoriasis is commonly classified as mild, moderate or severe depending on the proportion of BSA affected, as described in the table below (National Psoriasis Foundation, 2019).
Table 1. Classification of disease severity by BSA affected (National Psoriasis Foundation, 2019).
Other measures used to assess severity can be used to assess response to treatment - see below.
However, if a patient’s psoriasis that covers only a small area, but is very visible (such as on the face), or debilitating (for example if the palms of the hands and soles of the feet are involved), then the psoriasis could be disabling and thus still be considered a severe case (National Psoriasis Foundation, 2019). Therefore, it is important to measure impact on the patient’s quality of life as well as BSA.
Subjective measures that assess impairment in quality of life (QoL) or difficulties performing activities of daily living, as well as those identifying the presence of cardio-metabolic comorbidities are important in estimating the impact psoriasis is having on the individual (Lubrano et al., 2015).
Scroll down to see specific QoL assessment tools in the patient related outcomes section below.
Response to treatment
A number of tools in existence to assess the severity of psoriasis can also be used to measure response to treatment.
Table 2. Tools to classify disease severity and assess treatment response.
Psoriasis Area and Severity Index
One of the most commonly used tools is the Psoriasis Area and Severity Index (PASI), which gives a score from 0 to 72 based on the severity of the lesions (average redness, thickness and scaling of the lesions each graded on a 0–4 scale) and the body surface area (BSA) affected.
The gold standard for assessment of psoriasis in the clinic is the Psoriasis Area and Severity Index (PASI), which measures psoriatic disease by the severity (thickness, redness, and scaling) and the extent of the plaque coverage.
The following process, is required to generate a PASI score (Feldman & Krueger, 2005; Bonifati & Berardesca, 2007; Palfreeman et al., 2013):
An online calculator for PASI scores is available here.
Measuring response to treatment using PASI
Response to treatment is measured as improvement in PASI score from baseline (Nast et al., 2012).
The future of PASI assessment
European S3 Guidelines on the systemic treatment of psoriasis propose a 75% or more improvement in Psoriasis Area and Severity Index (PASI) from baseline (PASI 75) and a Dermatology Life Quality Index (DLQI) of 0 or 1 as the primary treatment goals. Clinicians have historically considered a PASI 75 response to be a clinically meaningful improvement in patients with severe psoriasis (Mrowietz et al., 2011; Nast et al., 2015). However, achieving a PASI 90 response has a dramatic impact on the QoL of psoriasis patients (Torii et al., 2012) and the greatest enhancements are seen in patients reaching PASI 100 (Reich et al., 2015a). In addition to improvements in symptom severity and quality of life, the number of 100% symptom-free days have been shown to be significantly higher for patients achieving PASI 100 following modern biologic therapy (Strober et al., 2016a).
Find out the importance of aiming for PASI 100 in your patients from Professor Andrew Blauvelt.
Investigator's Global Assessment/Physician Global Assessment scale
The 5-point Investigator's Global Assessment (IGA) scale is a modified tool developed with input from regulatory authorities and clinical trial investigators for evaluating plaque psoriasis severity in clinical trials (Langley, 2015a).
The IGA has been well validated as a measure of disease severity and meets the need for a clinically meaningful measure of success for psoriasis treatment studies (Langley, 2015a). As the Physician Global Assessment (PGA) simpler than the more detailed PASI assessment, it is better suited for the clinic (Robinson, 2012).
The PGA is a 7-point scale ranging from clear (0) to severe (7) and in most versions of the PGA, the individual elements of psoriasis plaque morphology or degree of BSA involvement are not quantified (Bonifati & Berardesca, 2007). In clinical trials treatment success is often defined as achieving a static PGA (sPGA) score of 0 or 1 (Feldman, 2015a).
Table 3. Physician Global Assessment (PG) (Bonifati & Berardesca 2007).
Combining Physician Global Assessment and Body Surface Area Assessments
Despite PGA being considered more practical than PASI, it is limited by the lack of assessment of body surface involvement. In contrast, BSA doesn’t measure the quality or morphology of lesions so unsuitable as a sole measure. Therefore, a gap remains in the need for a practical, uniform, validated, and standardised outcome measure that can be used both in clinical practice and in clinical trials. Some feel this unmet need could be solved with the product of PGA and BSA (PGA x BSA); the tool has been previously reported to be a simple and sensitive measure of psoriasis severity, and is increasingly being used to simply measure psoriasis severity (Au et al., 2013; Buzney et al., 2015; Chiesa Fuxench et al., 2015; Sorensen et al., 2015; Duffin et al., 2017).
In an effort to evaluate PGA x BSA as an alternative to PASI, pooled data from the randomised PRISTINE and PRESTA trials were analysed which included PASI, PGA, and BSA measurements at Weeks 0, 12, and 24. The results revealed PGA x BSA has a high agreement with PASI in measuring psoriasis severity (Spearman correlation coefficient 0.78–0.90, p<0.0001) and could be a valuable tool for both clinical practice and clinical trials (Walsh et al., 2018).
Patient reported outcome tools
Patient reported outcome (PRO) tools such as the psoriasis symptom and sign diary (PSSD) and DLQI are now redefining treatment goals from the patient’s perspective.
Dermatology Life Quality Index
Health related quality of life (HRQoL) is a subjective measure of a patient's perception of their symptoms, and in psoriasis the dermatology life quality index (DLQI) can help quantify this. The DLQI is the most widely used measure for assessing QoL related to skin disease in psoriasis trials. This tool consists of ten questions covering six domains:
symptoms and feelings
work and school
issues with psoriasis treatment
The response options range from not affected at all (0) to very much affected (3) to give an overall range of 0–30 where lower scores represent better QoL (Bonifati & Berardesca, 2007).
The relationship between treatment efficacy and HRQoL
A recent systematic review and meta-analysis explored the relationship between treatment efficacy (measured by PASI score) and HRQoL, demonstrating that PASI 90 responders could achieve a superior HRQoL compared to PASI 75 responders (Puig et al., 2017). This meta-analysis suggested that moving from PASI 75 to PASI 90 would result in a 12% greater improvement in DLQI score and significantly higher numbers of patients achieving a DLQI score of 0/1, meaning no effect on the patient’s quality of life (Puig et al., 2017).
Psoriasis Symptom and Sign Diary (PSSD)
A symptom-free status on PSSD was shown to be associated with greater improvements in HRQoL than a PASI 100 response for patients with moderate-to-severe psoriasis (Bissonnette et al., 2018a).
While clinician-determined PASI assessments and patient-reported PSSD outcomes were highly correlated, there were discrepancies between PASI 100 response rates and patient-assessed achievement of symptom- and sign-free status (Gordon et al., 2018a). In addition, data showed that, in comparison to the PASI 100 response rate, patient-reported outcomes from the PSSD were more aligned to the DLQI score (Bissonnette et al., 2018a). These findings emphasise the importance of using PRO measurement tools, such as the PSSD, as an additional tool to assess the true impact of psoriasis on patients, and to determine an accurate response to therapy.