What blood tests are recommended for a patient with suspected or diagnosed psoriasis (Psoriasis Vulgaris)?

Blood Tests for Psoriasis: Evidence-Based Recommendations

Direct Answer

For diagnosis of psoriasis itself, no blood tests are required or recommended—the diagnosis is clinical, based on morphologic pattern recognition of skin lesions. 1 However, blood tests are essential for screening comorbidities, monitoring treatment safety, and evaluating for psoriatic arthritis and associated conditions.

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Diagnostic Considerations

No Blood Tests Needed for Psoriasis Diagnosis

• Psoriasis diagnosis is made clinically through visual examination and pattern recognition of characteristic skin lesions (erythematous plaques with silvery scale). 1
• Histopathological examination and blood tests are generally not valuable diagnostic tools for cutaneous psoriasis, though skin biopsy may occasionally help confirm uncertain cases. 1
• Unlike other autoimmune diseases, there are no specific serologic markers that establish the diagnosis of psoriasis. 1

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Essential Screening Blood Tests for Comorbidities

Cardiovascular and Metabolic Screening (High Priority)

All patients with psoriasis should undergo metabolic screening according to national guidelines, as they have significantly elevated cardiovascular risk. 2

Lipid Panel (Fasting)

• Perform fasting lipid panel at baseline and monitor according to national guidelines, with increased frequency for severe disease. 2
• Screen at least every 5 years, or every 2 years if risk factors present (family history, diabetes, smoking). 2
• Target values: Total cholesterol <200 mg/dL, HDL ≥50 mg/dL, LDL <100 mg/dL. 2

Glucose Screening

• Obtain fasting blood glucose and/or hemoglobin A1C according to national guidelines. 2
• Screen at least every 5 years, or every 2 years if risk factors present; target fasting glucose <100 mg/dL. 2
• Patients with prediabetes or new-onset diabetes require referral to primary care for management. 2

Complete Blood Count (CBC)

• Baseline CBC is recommended to assess for anemia and establish baseline values before initiating systemic therapy. 2
• Patients with psoriasis have increased risk of anemia of chronic disease due to systemic inflammation. 3

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Pre-Treatment Screening for Systemic Therapy

Before Starting Biologic or Systemic Agents

The strongest evidence (Grade B) supports tuberculosis screening before initiating biologic therapy, with interferon-gamma release assay (IGRA) preferred over tuberculin skin testing. 4

Mandatory Screening Tests

• Tuberculosis screening with IGRA or tuberculin skin test (PPD) is required before starting biologics. 2, 4
• Hepatitis B and C serologies should be obtained, as reactivation risk exists with immunosuppressive therapy. 2
• HIV screening is recommended, particularly given the association between syphilis and HIV, and altered treatment considerations in HIV-positive patients. 5, 6

Baseline Laboratory Panel Before Systemic Therapy

For Methotrexate:

• CBC with differential, comprehensive metabolic panel (CMP), liver function tests (LFTs), hepatitis B and C serologies, pregnancy test if applicable. 2
• Consider noninvasive fibrosis markers (FIB-4, APRI) to assess baseline liver status. 2

For Cyclosporine:

• Two separate serum creatinine measurements (some recommend 3 and average them), BUN, urinalysis, CBC, magnesium, potassium, uric acid, lipid profile, LFTs. 2
• Blood pressure measurement (must be <140/90 mmHg). 2

For Acitretin:

• Lipid panel (triglycerides and cholesterol), LFTs, pregnancy test (mandatory—Category X drug). 2
• 25-50% of patients develop hypertriglyceridemia requiring monitoring. 2

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Monitoring During Treatment

Ongoing Laboratory Surveillance

For Cyclosporine:

• Every 2 weeks for first 3 months: blood pressure, BUN, creatinine. 2
• Monthly thereafter: CBC, LFTs, lipids, magnesium, uric acid, potassium. 2

For Methotrexate:

• Monitor CBC, CMP, and LFTs regularly (frequency varies by protocol, typically every 1-3 months). 2
• Use noninvasive fibrosis assessment (vibration-controlled transient elastography preferred over liver biopsy) to monitor for hepatotoxicity. 2

For Acitretin:

• Monthly lipid panels and LFTs during treatment due to high risk of hypertriglyceridemia. 2

For Biologics:

• Evidence for routine monitoring labs is weak (Grade C or lower) beyond baseline screening. 7, 4
• Among hepatitis B carriers, monitor LFTs and viral load (Grade C evidence). 4

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Screening for Psoriatic Arthritis

Clinical Screening Questions (No Blood Test Required)

Ask all psoriasis patients at every visit about:

• Morning joint stiffness lasting ≥30 minutes. 6
• Visible or palpable joint swelling (not just pain—actual synovitis). 6
• Dactylitis (sausage-like digit swelling). 6
• Heel pain or enthesitis symptoms. 6

Laboratory Tests for PsA Evaluation

• Rheumatoid factor (RF) should be checked—RF negativity is part of CASPAR criteria (1 point). 6
• Inflammatory markers (ESR, CRP) may support diagnosis but are not required. 6
• No specific blood test diagnoses psoriatic arthritis—diagnosis is clinical using CASPAR criteria. 6

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Additional Screening Considerations

Cancer Surveillance

• Patients with extensive PUVA exposure (>200 treatments) or cyclosporine use require close skin cancer monitoring. 2
• Follow age-appropriate cancer screening per CDC/ACS guidelines (colonoscopy, mammography, etc.). 2

Renal Function

• Severe psoriasis independently increases risk of chronic kidney disease (aHR 1.90) and end-stage renal disease (aHR 2.97). 2
• Monitor creatinine and BUN, particularly in patients on nephrotoxic medications. 2

Depression Screening

• Psoriasis patients have double the odds of clinical depression (OR 1.99). 6
• Screen for depression and suicidal ideation; refer to mental health professional if positive. 2

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Common Pitfalls to Avoid

• Do not order blood tests to "diagnose" psoriasis—this wastes resources and delays treatment. 1
• Do not skip tuberculosis screening before biologics—this is the only Grade B recommendation and prevents serious complications. 4
• Do not use liver biopsy routinely for methotrexate monitoring—noninvasive methods (transient elastography, fibrosis markers) are preferred. 2
• Do not forget to screen for metabolic syndrome—psoriasis patients have 2.26 times increased odds, requiring lipid and glucose monitoring. 2
• Do not overlook psoriatic arthritis screening—73% of PsA patients have skin disease first, often years before joint symptoms, and early detection prevents irreversible damage. 6