**6. Disease evaluation**

To assess the severity of psoriasis and PsA (baseline/in response to treatment), a number of tools are now available, of which the Psoriasis Activity and Severity Index (PASI) is the most frequently used (Fredriksson & Pettersson, 1978). The PASI combines assessments of the extent of body surface involvement in four anatomical regions (head, trunk, arms and legs) and the severity of desquamation, erythema and plaque induration (thickness) in each region, yielding an overall score of 0 (no psoriasis) to 72 (severe psoriasis) (Fredriksson & Pettersson, 1978). PASI 75 is defined as a 75% reduction in PASI compared with baseline, instead a PASI score of >10 is defined as moderate to severe disease, necessitating systemic therapy (PUVA, UVB 311, MTX, cyclosporin A or biological drugs) (Pathirana et al., 2009)

An additional tool to assess the psoriasis severity is the physician's global assessment (PGA). The PGA takes into account the involvement of the body surface area, induration, scaling and erythema and grades the patient's psoriasis overall, relative to baseline, as 1 (clear), 2 (excellent), 3 (good), 4 (fair), 5 (poor) or 6 (worse) (Pathirana et al., 2009).

In trials investigating patients with PsA, the American College of Rheumatology Criteria (ACR) are most commonly used. The ACR clinical response criteria are defined as percentage reduction [20% (ACR 20), 50% (ACR50) and 70% (ACR 70)] in tender and swollen joint counts and in 3 of the, remaining, 5 ACR core items (patient and physician global assessments, pain, disability and an acute phase reactant) (Montecucco, 2006; Radtke et al., 2009).

A supplementary tool to evaluate clinical remission in psoriatic patients is the Disease Activity Score (DAS) comprising the number of swollen and tender joints, the erythrocyte sedimentation rate and the general health of the patient (measured on a visual analogue scale) (Montecucco, 2006). The DAS measures 44 swollen joints, whereas the modified DAS 28 measures only 28 swollen and tender joints (Prevoo et al., 1995). The effect of psoriasis on the patient's quality of life is measured by the 10-item Dermatology Life Quality Index (DLQI) questionnaire. DLQI scores range from 0 (not at all) to 30 (very much) (Pathirana et al., 2009).
