What are the key diagnostic criteria for psoriasis in adults, and how can I differentiate it from other skin conditions?

The diagnosis of psoriasis in adults is primarily based on clinical findings ¹. When assessing a person with suspected psoriasis, it is important to evaluate the disease severity, the impact on physical, psychological, and social wellbeing, and to screen for psoriatic arthritis and other comorbidities ².

Key aspects of assessment include recording the results of a static Physician's Global Assessment (classified as clear, nearly clear, mild, moderate, severe, or very severe), the patient's own assessment of severity, and the body surface area affected ². It is also crucial to note any involvement of high-impact and difficult-to-treat sites such as the face, scalp, palms, soles, flexures, and genitals, as well as any nail involvement ². Systemic upset, such as fever and malaise, can be present in unstable forms like erythroderma or generalised pustular psoriasis ². In specialist settings, the Psoriasis Area and Severity Index (PASI) is used to assess severity ². Adults with any type of psoriasis should be offered annual assessment for psoriatic arthritis, particularly within the first 10 years of psoriasis onset, using validated tools like the Psoriasis Epidemiological Screening Tool (PEST) ² (Helliwell and Taylor, 2005).

Psoriasis is not an infectious condition ¹.

Differentiating psoriasis from other skin conditions is usually achieved through clinical assessment ¹. However, in rare cases, a skin biopsy may be necessary to distinguish psoriasis from other inflammatory skin conditions ¹ (Johnson and Armstrong, 2013). Diagnostic uncertainty is a key reason for referral to a specialist dermatology service ¹,². Furthermore, suspected generalised pustular psoriasis or erythrodermic psoriasis are medical emergencies requiring immediate same-day specialist dermatology assessment and ongoing management ¹,². Referral to a rheumatologist is indicated if psoriatic arthritis is suspected, as early diagnosis can reduce the risk of erosive joint damage ¹. Severe or atypical psoriasis is also considered an HIV indicator condition ².