Your PASI score reflects the overall severity of psoriasis by combining the extent of body surface area involvement with the intensity of three clinical signs — redness, thickness, and scaling — across four body regions. A PASI score of 0 indicates complete clearance, while a score of 1 to 5 generally corresponds to mild psoriasis. Scores between 5 and 10 suggest mild-to-moderate disease, and scores above 10 are typically classified as moderate-to-severe psoriasis warranting systemic therapy or biologic treatment.

Scores above 20 indicate severe disease with significant skin involvement. In clinical practice, the PASI score is most valuable when tracked over time to measure treatment response. The standard treatment endpoints are PASI 75 (75% improvement from baseline), PASI 90 (90% improvement), and PASI 100 (complete clearance). Modern biologic therapies frequently achieve PASI 90 or PASI 100 responses.

It is important to note that the PASI score may not fully capture disease burden. Low PASI scores can still reflect significant impairment if psoriasis affects highly visible or functionally important areas such as the face, palms, soles, genitals, or nails. The PASI score should be interpreted alongside quality of life measures like the Dermatology Life Quality Index (DLQI) for a complete picture of disease impact.

The PASI score should be calculated when assessing psoriasis severity for treatment decisions, particularly when considering systemic therapies, biologics, or entry into clinical trials. Most clinical guidelines and insurance authorization processes require a documented PASI score (often along with body surface area and DLQI) to qualify patients for biologic treatment, typically requiring a PASI of 10 or greater.

PASI is also essential for monitoring treatment response over time. It should be calculated at baseline before starting a new therapy and at regular follow-up intervals (commonly at weeks 12, 16, 24, and 52) to determine whether treatment goals such as PASI 75 or PASI 90 have been achieved. If treatment targets are not met, the PASI score provides objective evidence to support therapy changes.

The PASI score has notable inter-rater variability, particularly in grading the severity of erythema, induration, and scaling, which are subjective assessments. Different clinicians may assign different severity grades for the same lesion, leading to inconsistent scores. Training and standardization can reduce but not eliminate this variability.

The PASI scoring system gives disproportionate weight to the lower limbs (40% weighting) relative to the head (10% weighting), which means that extensive scalp or facial psoriasis may produce a deceptively low PASI score. Similarly, the area scoring uses broad percentage ranges (e.g., 1-9% and 10-29%), which reduces sensitivity to small changes in affected area, particularly in mild disease.

PASI does not assess nail psoriasis, psoriatic arthritis symptoms, or the psychological impact of the disease. A comprehensive psoriasis assessment should include additional tools such as the Nail Psoriasis Severity Index (NAPSI), joint assessment, and quality of life questionnaires. The minimum clinically important difference for PASI has not been definitively established, making it difficult to interpret small score changes.