Differential Diagnosis for Patient with Hives

The patient presents with hives on the chest, abdomen, legs, and elbow creases, and is currently taking Bactrim. Here's a differential diagnosis organized into categories:

• Single Most Likely Diagnosis

  • Allergic reaction to Bactrim: This is the most likely diagnosis given the temporal relationship between the initiation of Bactrim and the onset of hives. Bactrim, a combination of sulfamethoxazole and trimethoprim, is known to cause allergic reactions, including urticaria (hives).

• Other Likely Diagnoses

  • Urticarial vasculitis: This condition involves inflammation of the blood vessels and can present with hives, but it tends to be more painful and can leave bruises. It could be considered if the hives are atypical or if there are other systemic symptoms.
  • Insect bites or stings: If the patient has been exposed to insects, this could be a possible cause of the hives, especially if they are localized to areas that are more exposed.
  • Contact dermatitis: If the patient has come into contact with an allergen or irritant, this could cause hives in the affected areas.

• Do Not Miss Diagnoses

  • Anaphylaxis: Although the patient's symptoms are currently limited to hives, anaphylaxis is a life-threatening condition that can progress rapidly. It's crucial to monitor for signs of anaphylaxis, such as difficulty breathing, rapid heartbeat, or a drop in blood pressure.
  • Serum sickness: This is a reaction to certain medications or antiserum and can cause hives, among other symptoms. It's less common but can be serious.
  • Stevens-Johnson syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): These are severe skin conditions usually caused by medications, including antibiotics like Bactrim. They can start with symptoms similar to hives but progress to widespread skin necrosis and detachment.

• Rare Diagnoses

  • Autoimmune disorders (e.g., lupus, rheumatoid arthritis): These conditions can cause a wide range of symptoms, including hives, but would typically be accompanied by other systemic symptoms.
  • Mastocytosis: A condition characterized by an accumulation of mast cells in the skin, which can cause hives and other skin symptoms.
  • Erythema multiforme: An immune-mediated disorder that can cause skin lesions, including hives, often in response to infections or medications.