Differential diagnosis

The process of determining the most likely cause, or causes, of the patient's distress, is forming the differential diagnosis and is an important step is medical diagnosis. When a clinician evaluates a patient, there can be a long list of patient-reported symptoms, information gained from medical history taking, the results of generalized and specialized physical examination, laboratory tests, diagnostic imaging, and many other sources of information, such as knowing about infections or toxins local to an area.

These often group into syndromes, or, "a group of symptoms and signs of disordered function related to one another by means of some anatomic, physiologic, or biochemical peculiarity." Clinicians hypothesize the most likely causes, and then test those hypotheses against the available information. It may well be that no specific diagnosis can be reached, but examining the hypotheses leads to the collection of additional information.

It is entirely possible that more than one disease process, or comorbidity, can be present. The interactions of multiple diseases can make it challenging to find the causes. One disease may result from another, or their joint presence can be coincidental.