Differential Diagnosis for Hard Spot from Injection Site

• Single most likely diagnosis
  • Localized reaction or inflammation: This is the most common cause of a hard spot at an injection site, often due to the body's immune response to the injected substance.
• Other Likely diagnoses
  • Abscess formation: An infection at the injection site can lead to the formation of an abscess, which may feel hard and swollen.
  • Hematoma: Bleeding into the tissue at the injection site can cause a hard, swollen area.
  • Granuloma: A granuloma is a small area of inflammation that can form in response to a foreign substance, such as a vaccine or medication, and can feel hard to the touch.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Necrotizing fasciitis: A rare but life-threatening infection that can cause severe pain, swelling, and hardness at the injection site.
  • Gas gangrene: A bacterial infection that can cause gas to form in the tissues, leading to hardness, swelling, and severe pain.
  • Septic arthritis (if the injection was given near a joint): An infection in the joint can cause severe pain, swelling, and hardness.
• Rare diagnoses
  • Lipodystrophy: A rare condition characterized by abnormal fat distribution, which can cause hard, swollen areas at injection sites.
  • Scleroderma: A rare autoimmune disorder that can cause hard, thickened skin and tissues.
  • Foreign body reaction: A rare reaction to a foreign substance, such as a piece of glass or metal, that can cause a hard, swollen area at the injection site.