In medicine, differential diagnosis (sometimes abbreviated DDx or ΔΔ) is the systematic method physicians use to identify the disease causing a patient's symptoms.
Before a medical condition can be treated, it must be identified. In the process of listening to a patient's complaints, examining the patient, and taking the patient's personal, family and social history, the physician makes a mental list of the most likely causes, or differential diagnoses. The doctor asks additional questions and performs tests to eliminate possibilities until he or she is satisfied that the single most likely cause has been identified.

In popular culture
The term and methodology have been recently popularized by the television series House, MD, where Dr. Gregory House, a certified diagnostician, uses differential diagnosis to solve medical mysteries.