Diagnosis of Erythema Nodosum
The diagnosis of erythema nodosum is usually based on clinical examination and supported by investigations to identify the underlying cause. Because erythema nodosum is a skin reaction, not a disease on its own, doctors must determine what triggered it to offer appropriate treatment.
Clinical Assessment
Doctors begin with:
A visual examination of the lesions
Noting their size, colour, symmetry, and tenderness
Asking about recent infections, medications, or travel
Reviewing systemic symptoms such as fever, joint pain, sore throat, or weight loss
The appearance of painful, red nodules on the shins in a symmetrical pattern is often enough to suspect erythema nodosum. However, identifying the cause is critical to prevent recurrence.
Laboratory Investigations
Several blood and diagnostic tests may be ordered to uncover a possible trigger:
Full blood count (FBC) – Looks for elevated white cells or inflammation
Erythrocyte sedimentation rate (ESR) / C-reactive protein (CRP) – Raised levels support the diagnosis of an inflammatory condition
Anti-streptolysin O (ASO) titre – Detects a recent streptococcal infection
HIV test – Especially important in South Africa due to the high prevalence
Tuberculosis (TB) screening, including:
Chest X-ray
Sputum test or GeneXpert
Tuberculin skin test (TST) or IGRA
Additional Tests
Depending on symptoms and suspected causes:
Throat swab, if sore throat is present
Stool cultures, if gastrointestinal symptoms are noted
Chest CT scan, in cases where sarcoidosis is suspected
Colonoscopy, if inflammatory bowel disease is suspected
Pregnancy test, if relevant
Skin Biopsy
In uncertain or atypical cases, a skin biopsy may be performed. This can confirm panniculitis (inflammation of fat tissue) and help rule out other forms of skin disease such as vasculitis, cellulitis, or erythema induratum.
Diagnosis of Erythema Nodosum In South Africa
In South Africa, TB testing is often prioritised. Erythema nodosum may be the first symptom of a previously undiagnosed TB infection. In areas where access to biopsy or lab testing is limited, clinical diagnosis supported by chest X-ray and TB symptoms is commonly used.
The diagnosis of erythema nodosum should be thorough, not just to confirm the condition but to uncover the underlying cause. This ensures faster recovery and reduces the risk of repeat episodes.
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