What is the diagnostic workup for sarcoidosis and when is a Positron Emission Tomography (PET) scan indicated?

Diagnostic Workup for Sarcoidosis and Indications for PET Scan

The diagnostic workup for sarcoidosis should include chest radiography, laboratory tests, and targeted biopsies, while PET scans are primarily indicated for suspected cardiac involvement, assessment of extrapulmonary disease, or evaluation of treatment response in refractory cases. 1, 2

Initial Diagnostic Evaluation

Clinical Assessment

• Evaluate for highly probable clinical features:
  • Löfgren's syndrome (bilateral hilar adenopathy with erythema nodosum and/or periarticular arthritis)
  • Characteristic skin lesions (lupus pernio, erythema nodosum)
  • Ocular manifestations (uveitis, optic neuritis)
  • Seventh cranial nerve paralysis 1

First-line Imaging

• Chest radiography: Initial imaging test showing:

  • Bilateral hilar adenopathy (highly specific feature)
  • Upper lobe or diffuse infiltrates
  • Peribronchial thickening 1, 2

• Chest CT scan: More sensitive than chest X-ray for:

  • Early-stage disease detection
  • Identification of perilymphatic nodules
  • Assessment of lymph node involvement
  • Evaluation of parenchymal changes 2, 3

Laboratory Testing

• Serum calcium and vitamin D levels (25-OH and 1,25-OH)
• Liver function tests (alkaline phosphatase often elevated)
• Renal function tests (creatinine)
• Complete blood count (may show normocytic anemia)
• Angiotensin-converting enzyme (ACE) level (elevated in ~60% of cases) 1, 2

Tissue Diagnosis

• Biopsy of the most accessible involved organ showing non-caseating granulomas
• Bronchoalveolar lavage (BAL) showing lymphocytosis or elevated CD4:CD8 ratio
• Special stains and cultures to exclude infectious causes 1, 2

Indications for PET Scan in Sarcoidosis

Primary Indications

1. Suspected cardiac involvement:

   • When cardiac MRI is unavailable or contraindicated
   • For assessment of active myocardial inflammation
   • To guide myocardial biopsy 2, 4, 5

2. Evaluation of extrapulmonary disease:

   • When conventional imaging is inconclusive
   • To determine the full extent of organ involvement
   • To identify suitable biopsy sites 2, 6, 7

3. Treatment monitoring:

   • Assessment of response to immunosuppressive therapy
   • Identification of steroid-resistant disease
   • Evaluation of persistent symptoms despite treatment 4, 7

Specific Clinical Scenarios for PET Scan

• Persistent disabling symptoms without serological signs of inflammatory activity
• Patients with radiologic signs of fibrosis but suspected active inflammation
• Unexplained deterioration in clinical status
• Guiding decisions on immunosuppressive therapy escalation 4, 7

Interpretation of PET Findings

• Positive findings: Increased FDG uptake in:

  • Hilar/mediastinal lymph nodes
  • Lung parenchyma
  • Heart (patchy or diffuse myocardial uptake)
  • Other organs (liver, spleen, bones, parotid glands) 6, 5

• Follow-up PET scans:

  • Decreased SUVmax and fewer involved segments indicate good response
  • Increased or persistent uptake suggests steroid-resistant disease
  • May guide decisions on therapy intensification 4

Common Pitfalls to Avoid

• Over-reliance on ACE levels for diagnosis (limited sensitivity and specificity)
• Failure to exclude other granulomatous diseases (tuberculosis, fungal infections)
• Incomplete evaluation of extrapulmonary involvement
• Misinterpretation of PET findings (physiologic uptake vs. disease activity)
• Using PET as first-line imaging rather than for specific indications 2

Special Considerations

• PET involves radiation exposure and is costly - use selectively
• Proper patient preparation is essential (fasting, dietary restrictions)
• Cardiac PET protocols differ from standard oncologic protocols
• Consider alternative imaging (MRI) for cardiac and neurological involvement 5, 7