Erythema nodosum (looking for a cause)

Laboratory tests

• FBC
• CRP
• Antistreptolysin O antibodies (ASO)
• Throat swab (culture for group A streptococcus)
• Serum β-hCG (woman of childbearing potential)if needednon-French guidelineAny woman of childbearing potential: pregnancy is among the identifiable causes of erythema nodosum ^[1].
• IGRA test (if tuberculosis contact, origin from an endemic area or abnormal chest X-ray)if neededTuberculosis-endemic area or abnormal chest X-ray: IGRA or tuberculin skin test, with sputum examination for bacilli ^[1]. Reimbursed only in case of documented contact (investigation around an index case) ^[2].
• Stool culture (if diarrhoea or abdominal pain)if needednon-French guidelineDiarrhoea, abdominal pain or other gastrointestinal symptom: stool examination and culture ^[1] (consider yersiniosis, salmonellosis, Campylobacter, IBD).
• Stool parasitology, to be done 3 times (if diarrhoea or abdominal pain)if needednon-French guidelineDiarrhoea, abdominal pain or other gastrointestinal sign: an American Family Physician review (2007) and DermNet (New Zealand, 2019) pair stool culture with a stool parasitology examination ^[3,4]; amoebiasis and giardiasis are among the infectious causes of erythema nodosum ^[3].
• Not a laboratory test: chest X-ray.