What additional laboratory tests are recommended for a patient with psoriasis and lumbar pain, beyond Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP)?

Laboratory Testing for Psoriasis Patients with Lumbar Pain

For a patient with psoriasis presenting with lumbar pain, HLA-B27 testing and radiographic imaging of the sacroiliac joints and spine should be performed in addition to ESR and CRP to evaluate for psoriatic arthritis with axial involvement. 1

Core Laboratory Evaluation

When evaluating a patient with psoriasis who presents with lumbar pain, the following laboratory tests should be ordered:

1. Inflammatory markers (already mentioned in question)

   • ESR (Erythrocyte Sedimentation Rate)
   • CRP (C-Reactive Protein)

2. Essential additional tests

   • HLA-B27 - Critical for assessing risk of axial involvement 1
   • Complete blood count (CBC) - To evaluate for anemia, leukocytosis, and thrombocytosis 1, 2
   • Comprehensive metabolic panel - Including:
     • Liver enzymes (ALT, AST, ALP)
     • Renal function (BUN, creatinine)
     • Serum albumin (marker of inflammation and nutritional status) 1

3. Immunological tests

   • Rheumatoid factor (RF) - To help differentiate from rheumatoid arthritis (typically negative in PsA) 1, 3
   • Anti-CCP antibodies - More specific for rheumatoid arthritis (typically negative in PsA) 3

Imaging Studies

Imaging is crucial for patients with psoriasis and lumbar pain:

• Plain radiographs of the sacroiliac joints and lumbar spine - To detect structural changes 1
• MRI of the sacroiliac joints and/or spine - To detect early inflammatory changes and monitor disease activity 1
  • STIR sequences are sufficient to detect inflammation without contrast medium 1

Additional Considerations

• Fecal calprotectin - Consider if there are gastrointestinal symptoms to rule out inflammatory bowel disease, which can be associated with psoriatic arthritis 1
• Infectious disease screening - Rule out infectious causes if acute flare is suspected:
  • Stool cultures
  • C. difficile toxin test 1
• YKL-40 (Chitinase-3-like protein 1) - A newer inflammatory biomarker that may be more sensitive than CRP in detecting systemic inflammation in psoriasis 4

Interpretation of Results

• Elevated inflammatory markers (ESR, CRP) attributable to psoriatic arthritis are considered indicators of severe disease and poor prognosis 1
• However, it's important to note that ESR and CRP are elevated in only about 50% of patients with psoriatic arthritis 3, 5
• Elevated inflammatory markers in obese patients may reflect adiposity-related inflammation rather than disease activity 6

Clinical Pearls and Pitfalls

• Pearl: HLA-B27 positivity is associated with axial involvement in psoriatic arthritis and should guide treatment decisions
• Pitfall: Normal ESR and CRP do not exclude active psoriatic arthritis with axial involvement
• Pearl: TNF-α inhibitors appear more effective than IL-12/23 inhibitors in normalizing CRP levels 5
• Pitfall: Obesity can elevate inflammatory markers independently of disease activity, potentially confounding interpretation 6

Remember that the diagnosis of psoriatic arthritis with axial involvement is based on a combination of clinical, laboratory, and imaging findings rather than any single test.