Differential Diagnosis for Complete Hive Rash Except on Palms, Feet, or Face

Single Most Likely Diagnosis

• Urticaria (Hives): This is the most common cause of a hive-like rash and can be triggered by allergic reactions, infections, or autoimmune disorders. The fact that the rash spares the palms, feet, and face is consistent with typical presentations of urticaria.

Other Likely Diagnoses

• Insect Bite or Sting Reaction: Reactions to insect bites or stings can cause hives and typically spare the face, palms, and soles. The distribution and appearance of the rash can help differentiate this from other causes.
• Drug Reaction: Certain medications can cause urticarial reactions. The history of recent medication use is crucial in diagnosing drug-induced urticaria.
• Viral Exanthem: Some viral infections can cause rashes that resemble hives. The presence of systemic symptoms like fever and the absence of rash on the palms, soles, and face might suggest a viral etiology.

Do Not Miss Diagnoses

• Anaphylaxis: Although anaphylaxis typically presents with more systemic symptoms, including respiratory distress, cardiovascular instability, and often involvement of the face, lips, and tongue, it can sometimes present atypically. Early recognition is crucial as anaphylaxis is life-threatening.
• Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis (SJS/TEN): These are severe skin conditions usually triggered by medications. While they often involve the mucous membranes and can progress to blistering and peeling of the skin, early stages might resemble a hive-like rash. SJS/TEN can be fatal if not promptly recognized and treated.

Rare Diagnoses

• Mastocytosis: A condition characterized by the accumulation of mast cells in the skin, which can cause hives, flushing, and other symptoms. It's less common and might be considered if the rash is persistent and accompanied by other systemic symptoms.
• Erythema Multiforme: An immune-mediated disorder that can cause target-like lesions, often in response to infections or medications. While it typically involves the extremities and can include mucous membranes, atypical presentations might mimic a hive-like rash.
• Autoimmune Urticaria: A form of chronic urticaria caused by autoantibodies against the patient's own mast cells or the IgE receptor. It's a diagnosis of exclusion and might be considered in cases of chronic, unexplained hives.