In medicine, a differential diagnosis is a diagnosis that examines all possible causes of a set of symptoms to arrive at a diagnosis. For example, if a patient presents with a runny nose, doctors may consider causes such as hay fever and colds in the differential diagnosis in an attempt to arrive at the correct diagnosis. Many doctors use this diagnostic system in their practices, and some doctors known as diagnosticians actually specialize in it.
To make a differential diagnosis, the doctor begins by reviewing the patient's case. He or she interviews the patient to collect symptoms, and also collects a family, personal, and social history that provides a picture of the patient's background. Examinations and tests are usually also included, to obtain specific information about the patient's current condition. In a particularly puzzling case, the doctor may interview friends and co-workers or examine the patient's natural environment to look for causes.
Once the doctor collects all the evidence, he or she considers the possible causes of the patient's condition. Before any cause can be ruled out, the doctor must find a good reason. Multiple causes are not ruled out in a differential diagnosis, as it is possible for patients to become ill with more than one thing at a time. Once the doctor has considered all possible factors, she arrives at a functional diagnosis and begins treatment.
The condition of the patient under treatment is monitored, and if the patient does not improve or worsens, the doctor returns to the drawing board. Treatment failure is entered into the differential diagnosis, and the physician starts over. In a sense, this type of diagnosis is a process of elimination, but it can be extremely complicated and complex, since many common diseases and conditions look very similar, and it is easy for a doctor to be misled.
Making a diagnosis using this method can be like putting the pieces of a puzzle together, especially when a patient has conflicting symptoms or a complex history. It is an important part of the practice of internal medicine, a medical specialty that focuses on the diagnosis and treatment of non-surgical conditions in patients. Internists are sometimes called upon to assist other medical specialists with a differential diagnosis, as they can draw on a broad pool of knowledge and experience.