Investigations

Investigation should be directed at finding an underlying cause:

• careful history - especially attention to drugs
• complete blood count with differential – ESR and CRP
• evaluation for streptococcal infection (e.g., throat culture for group A streptococci, anti-streptolysin O titre, and polymerase chain reaction assay)
• consider possible sarcoidosis or tuberculosis
  • chest X-ray, purified protein derivative test
• viral titres may be indicated:
  • viral causes of erythema nodosum include:
    • herpes simplex virus, Epstein-Barr virus, hepatitis B and C viruses, human immunodeficiency virus
• stool culture may be indicated in some patients
  • stool culture and evaluation for ova and parasites in patients with diarrhoea or gastrointestinal symptoms
  • consider evaluation for inflammatory bowel disease
• excisional biopsy – if clinical diagnosis is in doubt
  • key histologic findings are septal panniculitis, actinic (Miescher's) radial granulomas, lymphocytic infiltrate with neutrophils, absence of vasculitis, and no organisms

Reference:

1. Perez-Garza D et al. Erythema Nodosum: A Practical Approach and Diagnostic Algorithm. Am J Clin Dermatol. 2021 May;22(3):367-378