A differential diagnosis is a list of possible diseases, disorders, or other conditions under consideration based on the signs, symptoms, and data available.

A differential diagnosis is used by clinicians to direct further examination so that the most effective and appropriate treatment is given. The goal is to narrow the differential diagnosis as much as possible and ultimately arrive at one definite diagnosis.

For example, the differential diagnosis of right lower quadrant abdominal pain includes the following:

To narrow the differential diagnosis, several pieces of historical, clinical, and laboratory information would be taken into consideration. The patient's gender will (obviously) rule in or out the last six diagnoses; furthermore, these diagnoses would remain in the differential diagnosis of RLQ abdominal pain in a female of reproductive age, but ectopic pregnancy would be removed from the list in a post-menopausal woman. In a woman with a history of a hysterectomy, uterine fibroids would be ruled out; in a woman with a history of a hysterosalpingo-oophorectomy, ovarian cysts and torsion would be ruled out as well. In the gastrointestinal tract, appendicitis is usually the first on the list for this symptom; rebound tenderness (pain after the removal of applied pressure) is a common sign of a peritoneal infection and might strengthen any of the diagnoses that could lead to these conditions (the first seven in the list fall into this category). Diverticulitis is much more common in adults than in children, as is a colonic neoplasm.

Several radiological examinations would be of use in making the proper diagnosis: a CT scan may demonstrate a swollen appendix causing a peritoneal reaction (thickening of the bowel wall, visible peritoneal fluid) and would hasten the patient's trip to the operating room. Cysts and fibroids would be visible on both a CT scan and an ultrasound examination. Crohn's disease usually has a characteristic appearance in an upper GI series. Colorectal cancer may be detected using an air-contrast barium enema; it would also show up on a CT scan and, depending on its size and location, during colonoscopy.

Forming a differential diagnosis is a clinical process of deduction; committing lists of "differentials" to memory is a major component of medical training.

For a little bit of fun with differential diagnoses, check out eMedicine's differential diagnosis generator at http://www.emedicine.com/diagnosis.shtml - hypochondriacs beware!