What are the steps to diagnose and treat psoriasis?

Psoriasis Diagnosis

Psoriasis is diagnosed clinically based on the presence of characteristic erythematous, scaly plaques with silvery scale, typically on extensor surfaces (elbows, knees, scalp), without requiring laboratory investigations in most cases. 1, 2

Clinical Diagnostic Features

The diagnosis relies on recognizing specific morphologic patterns:

• Plaque characteristics: Well-demarcated, raised erythematous plaques covered with silvery-white scale, distinguishable from other conditions by the scale quality and distribution 1, 3
• Distribution patterns: Extensor surfaces (elbows, knees), scalp, lumbosacral area, and nails are classic locations; inverse psoriasis affects skin folds with smooth inflamed plaques 4, 2
• Nail involvement: Pitting, onycholysis, oil spots, and subungual hyperkeratosis support the diagnosis, particularly when associated with distal interphalangeal joint disease 5
• Koebner phenomenon: New lesions developing at sites of skin trauma is characteristic 4

Clinical Variants to Recognize

• Plaque psoriasis (90% of cases): Chronic stable plaques as described above 2, 6
• Guttate psoriasis: Small, round, drop-like lesions over trunk and proximal extremities, often following streptococcal infection 4, 7
• Pustular psoriasis: Sterile pustules on erythematous base, either localized (palms/soles) or generalized 4, 2
• Erythrodermic psoriasis: Widespread erythema covering >90% body surface area, a medical emergency 4, 7

When Laboratory/Biopsy is Needed

Laboratory investigations are rarely helpful for straightforward cases 1. However:

• Skin biopsy: Reserved only for atypical presentations that cannot be distinguished clinically from other papulosquamous disorders 7
• Screening for psoriatic arthritis: No specific lab test diagnoses PsA; rheumatoid factor is typically negative (seronegative arthritis), though ESR/CRP may be elevated with active inflammation 5

Severity Assessment

Assess two components at every visit:

• Patient-reported disability: The patient's perception of impact on quality of life and "need for treatment" 1
• Objective body surface area (BSA): Percentage of skin involved; BSA ≥10% generally defines moderate-to-severe disease requiring systemic therapy 1, 2
• PASI score: Psoriasis Area and Severity Index (0-72 scale) is used primarily in clinical trials; PASI ≥10 indicates moderate-to-severe disease 1

Screening for Psoriatic Arthritis

All patients with cutaneous psoriasis should be informed about the association with psoriatic arthritis and screened for musculoskeletal symptoms. 1

• Key symptoms to ask about: Morning joint stiffness lasting >30 minutes, joint swelling, enthesitis (heel/Achilles pain), dactylitis ("sausage digits"), inflammatory back pain 1, 5
• Timing: Skin lesions precede arthritis in 73% of cases (often by years), but 15% develop joint symptoms before skin involvement 1
• Action on positive screen: Patients with signs/symptoms suspicious for PsA require full evaluation; if comfortable with diagnosis initiate appropriate therapy, otherwise consult rheumatology 1
• Diagnostic criteria: Use CASPAR criteria for formal diagnosis, which includes peripheral joint assessment (68 joints for tenderness, 66 for swelling) 1, 5

Differential Diagnosis Considerations

Distinguish psoriasis from:

• Neurodermatitis/lichen simplex chronicus: More pruritic, lacks silvery scale, typically single lichenified plaque rather than multiple plaques 8
• Seborrheic dermatitis: Greasy yellow scale in seborrheic distribution (scalp, nasolabial folds), less well-demarcated 1
• Eczema: More pruritic, less well-demarcated, flexural rather than extensor distribution 2

Comorbidity Screening

At diagnosis, evaluate for associated conditions:

• Cardiovascular risk factors: Hypertension, hyperlipidemia, diabetes mellitus, obesity, metabolic syndrome 1, 4, 2
• Mental health: Depression and anxiety screening 5, 6
• Inflammatory bowel disease: Ask about gastrointestinal symptoms 5

Referral Indications

• Dermatology referral: Uncertain diagnosis, moderate-to-severe disease (BSA >10%), failure of topical therapy, need for systemic agents or phototherapy 1
• Rheumatology referral: Any suspicion of psoriatic arthritis based on joint symptoms 1, 5
• Ideally: Diagnosis of psoriasis should be confirmed by a dermatologist and inflammatory musculoskeletal disease by a rheumatologist 1, 5

Critical Pitfalls to Avoid

• Missing psoriatic arthritis: Failing to screen for joint symptoms can lead to irreversible joint damage; 15-30% of psoriasis patients develop PsA 1, 5
• Underdiagnosis in skin of color: Psoriasis may appear less erythematous and more violaceous in darker skin tones, leading to missed or delayed diagnosis 7
• Medication triggers: Beta-blockers, NSAIDs, lithium, antimalarials (chloroquine, hydroxychloroquine) can precipitate or severely worsen psoriasis 1, 8