Differential Diagnosis for Itchy Red Patch after Subq Injection

• Single most likely diagnosis
  • Allergic reaction to the injected substance: This is the most likely diagnosis because an itchy red patch is a common presentation of an allergic reaction, which can occur after the introduction of a foreign substance into the body, such as a subcutaneous injection.
• Other Likely diagnoses
  • Infection at the injection site: Bacterial or fungal infections can cause redness, itching, and swelling at the site of a subcutaneous injection, especially if the injection technique was not sterile.
  • Irritant contact dermatitis: The skin's reaction to an irritant, such as a component of the injected substance or the vehicle used for the injection, can cause an itchy red patch.
  • Delayed hypersensitivity reaction: A type IV hypersensitivity reaction, which is cell-mediated, can occur hours to days after exposure to an antigen, such as a component of the vaccine or drug injected.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Anaphylaxis: Although anaphylaxis typically presents with more systemic symptoms, a localized itchy red patch can be an early sign, and missing this diagnosis could be fatal.
  • Necrotizing fasciitis: A severe infection that can start with minor symptoms, such as redness and itching at the injection site, but can rapidly progress to a life-threatening condition.
• Rare diagnoses
  • Erythema nodosum: A type of skin inflammation that can occur in response to various stimuli, including infections and medications, and presents with painful red nodules, which could be itchy.
  • Serum sickness: A systemic immune complex-mediated hypersensitivity reaction that can occur after the administration of certain medications or antiserum, and can present with a variety of symptoms including a rash.