In medicine, a differential diagnosis is a diagnosis which examines all of the possible causes for a set of symptoms in order to arrive at a diagnosis. For example, if a patient presents with a runny nose, doctors might consider causes like hay fever and colds in the differential diagnosis in an attempt to arrive at the right diagnosis. Many doctors use this system of diagnosis in their practices, and some doctors known as diagnosticians actually specialize in differential diagnosis. Fans of House, MD may be familiar with the mechanics of a differential diagnosis.

To perform a differential diagnosis, the physician starts by reviewing the patient's case. He or she interviews the patient to collect symptoms, and also gathers a family, personal, and social history which provides a picture of the patient's background. Typically, examinations and tests are also included, to get specific information about the patient's current condition. In a particularly puzzling case, the doctor might interview friends and coworkers or examine the patient's natural environment to look for causes.

Once the physician collects all of the evidence, he or she considers possible causes for the patient's condition. Before any cause can be dismissed, the physician must come up with a good reason. Multiple causes are not ruled out in a differential diagnosis, as it is possible for patients to be sick with more than one thing at once. Once the physician has considered all the possible factors, he or she comes up with a working diagnosis and begins treatment.

The patient's condition under treatment is monitored, and if the patient fails to improve or gets worse, the doctor returns to the drawing board. The failure of treatment is incorporated into the differential diagnosis, and the doctor begins again. In a sense, differential diagnosis is a process of elimination, but it can get extremely involved and very complex, since many diseases and common afflictions look very similar, and it is easy for a physician to be misled.

Working out a differential diagnosis can be like assembling the pieces of a puzzle, especially when a patient has conflict symptoms or a complex history. It is an important part of the practice of internal medicine, a medical specialty which focuses on the diagnosis and treatment of nonsurgical conditions in patients. Internists are sometimes called in to assist other medical specialists with a differential diagnosis, since they are able to draw upon a wide body of knowledge and experience.