FormalPara Key Summary Points

Disease severity is an important outcome measure in clinical trials of treatments for plaque psoriasis.

The most widely used clinical measure to assess disease severity of plaque psoriasis is the Psoriasis Area and Severity Index (PASI).

In determining the PASI score, area of involvement of the body region is estimated and given a number; however, for areas < 10% a non-granular score of 1 is applied regardless of the actual extent of affected area.

To address this limitation of the PASI for affected areas < 10%, we describe the methodology for the PASI-high discrimination (PASI-HD), a modification to the PASI.


Psoriasis Area and Severity Index (PASI) is the standard measure to assess severity of plaque psoriasis in clinical trials [1]. A limitation of PASI is that it is less accurate when the affected body surface area (BSA) is < 10%. Topical treatments are the most commonly used for patients with mild to moderate psoriasis (involving < 10% BSA) or to treat disease flaring from and within areas with < 10% involvement [2]. With innovations in topical treatments for psoriasis, a more precise estimate of PASI is necessary to evaluate these results. The aim of this commentary is to introduce the PASI-high discrimination (PASI-HD), a measure designed to be used in clinical trials to provide greater precision in estimating disease severity and evaluate treatment effectiveness. PASI-HD remains true to the PASI measure by maintaining the anatomical regions of the PASI. PASI-HD provides a higher level of discrimination and a more accurate evaluation of treatment effectiveness in patients in whom the areas of affected body surface within the anatomical regions measured by PASI are < 10%. The method used to calculate PASI-HD follows.

PASI is determined by assessing severity in four anatomical regions (head/neck, upper extremities, trunk, and lower extremities) [1]. Each of the three primary clinical signs of psoriasis—erythema, induration, and desquamation—is ranked as an average severity across each region using a scale of 0 (none) to 4 (maximum) and added together generating an overall severity score for each body region. Next, the area of involvement in the body region is determined and assigned area scores based upon the proportion of the anatomical region involved with psoriasis. Area scores are multiplied by severity scores for a region-proportionate score. In body regions where the area of psoriasis is < 10%, using the traditional PASI, an area score of 1 is given. This is the point of differentiation of PASI-HD versus PASI. Our modification for PASI-HD generates a linear score when the area of involvement is < 10% of the anatomical region; scores of 1–9% become 0.1–0.9 (in place of 1 with PASI). The affected area can be estimated using the size of the patient’s hand as a relative standard. One palmar surface of the hand and fingers is approximately 1% of total BSA. Smaller area of psoriasis can be estimated using the thumb (0.1%), thumb nail (0.02%), and fifth-finger nail (0.01%), as shown in Fig. 1. The measures were developed consistently with the origination of the PASI measure, which determined that the area from the proximal crease of the palm/wrist to the tips of the fingers is roughly 1% of the BSA. To create the higher level of discrimination used in the PASI-HD, the 0.1% area of the thumb was determined by approximating that ten thumb units (from the base of the thumb to the tip of the thumb) fit across the palmar surface (1%). Similarly, five thumbnails fit within each thumb unit (0.1%), resulting in an area measure of 0.02%. Finally, the fifth fingernail fits within the thumbnail (0.02%) approximately two times, resulting in an area measure of 0.01%.

Fig. 1
figure 1

Depiction of locations on the hand to use in estimating the area of psoriasis involvement when < 10%

Representation of the relationship between PASI-HD and PASI is illustrated in Fig. 2. The scoring method is identical for areas of psoriasis with ≥ 10% involvement. However, if the area involved falls below 10% with treatment, a reduction in the severity score to reflect clinical improvement can be obtained only with the PASI-HD. Similarly, PASI-HD more precisely captures disease flaring from and within areas with < 10% involvement (e.g., after treatment withdrawal).

Fig. 2
figure 2

Illustration of the relationship between PASI-HD and PASI. The severity scores are the same for PASI-HD and PASI when the affected area is ≥ 10%. Below 10%, more precise determination of a severity score can be obtained with PASI-HD, but the PASI remains static when the area of involvement declines further

Other alternatives to the PASI have been suggested by researchers. Physician Global Assessment (PGA) is an outcome measure to assess severity, but provides an average score of disease severity across anatomical regions [3]. The previously described PGAxBSA [4] is similar to PASI-HD in that both are discriminating and linear from 100% to 0%; however, PASI-HD preserves the anatomical component of the PASI measure by assessing the severity and percentage area affected score in the four body regions. Preserving the anatomical regions and body surface area weighting, the areas can be individually assessed allowing a more precise score.

We propose that the PASI-HD provides higher discrimination of effects of treatment in areas with < 10% involvement than the traditional PASI while maintaining the anatomical component of the PASI. Because most patients with psoriasis have mild to moderate disease, this higher level of discrimination with the PASI-HD allows for a more accurate evaluation of treatment effectiveness as a function of BSA in these patients.