True Serum Sickness, Pearls for Clinical Diagnosis

Background: True serum sickness is a type 3 hypersensitivity reaction against foreign antibodies, resulting in vasculitis and an acute clinical presentation. Historically reported with anti-venin, currently anti-thymocyte globulin in the context of transplant rejection prophylaxis remains one of the most common causes. The classic clinical triad of fevers, arthralgias, and rash is not consistently present, and the rash is often difficult to distinguish from typical drug reactions. However, certain unique findings can assist with diagnosis. Case Presentation: We present a case of true serum sickness secondary to anti-thymocyte globulin featuring key exam and laboratory findings that enabled differentiation from other possible overlapping clinical entities, particularly drug reactions. Conclusions: Marked temporomandibular jaw pain is an important early clue to the diagnosis. Linear serpiginous erythema along the plantar margin may be a specific feature when rash is present. To our knowledge, neither have been reported in similar clinical entities including serum-sickness-like reaction. Serum complement levels and direct immunofluorescence (if skin biopsy done) are useful for distinguishing true serum sickness from primary differentials serum sickness-like reaction and drug rash with eosinophilia and systemic symptoms.