Differential Diagnosis for Rash after Subcutaneous Injection

• Single most likely diagnosis
  • Local allergic reaction: This is the most common cause of a rash after a subcutaneous injection, often due to an allergic response to the medication or the preservative used in the formulation. The reaction typically presents as redness, swelling, and itching at the injection site.
• Other Likely diagnoses
  • Infection (cellulitis or abscess): Bacterial infection can occur at the site of injection, especially if sterile technique is not followed. Symptoms include redness, swelling, warmth, and tenderness.
  • Irritant reaction: Some medications can cause a non-allergic irritant reaction when injected subcutaneously, leading to inflammation and rash at the injection site.
  • Urticaria: Hives can occur as an allergic reaction to the medication or another component of the injection.
• Do Not Miss diagnoses
  • Anaphylaxis: Although rare, anaphylaxis is a life-threatening allergic reaction that can occur after an injection. It requires immediate recognition and treatment.
  • Serum sickness: A systemic immune complex-mediated hypersensitivity reaction that can occur after certain injections, characterized by rash, fever, and arthritis.
  • Necrotizing fasciitis: A severe infection that can rapidly progress and is life-threatening, presenting with severe pain, swelling, and necrosis of the skin and underlying tissues.
• Rare diagnoses
  • Scleroderma-like reaction: Some medications can cause a scleroderma-like reaction, characterized by thickening and hardening of the skin at the injection site.
  • Panniculitis: Inflammation of the subcutaneous fat, which can occur as a reaction to certain medications or infections.
  • Erythema multiforme: A skin condition characterized by target lesions, which can be triggered by certain medications or infections.