Differential diagnosis is the process by which a physician utiltizes the scientific method and the skills learned in medical school, internship, and residency to take a history, examine the patient, and do the appropriate testing to determine the nature and extent of a disease process in a patient.
The use of a differential diagnosis is meant to be a dynamic process to determine what disease entities are suggested by presentation, and, after listing the most likely causes, the physician must then utilize other appropriate testing to "rule in" and "rule out" the causes listed on his or her differential, thereby eliminating the less likely causes and focusing on the likely cause(s), thus arriving at a working diagnosis.
It is more systematic than the old-fashioned method of diagnosis by gestalt (impression).
The lack of improvement during prescribed therapy for the working diagnosis necessitates reassessment.