Diagnosis, the process of determining the nature of a disease or disorder and distinguishing it from other possible conditions. The term comes from the Greek gnosis, meaning knowledge.

The diagnostic process is the method by which health professionals select one disease over another, identifying one as the most likely cause of a person’s symptoms. Symptoms that appear early in the course of a disease are often more vague and undifferentiated than those that arise as the disease progresses, making this the most difficult time to make an accurate diagnosis.

Reaching an accurate conclusion depends on the timing and the sequence of the symptoms, past medical history and risk factors for certain diseases, and a recent exposure to disease.

The physician, in making a diagnosis, also relies on various other clues such as physical signs, nonverbal signals of distress, and the results of selected laboratory and radiological and other imaging tests. From the large number of facts obtained, a list of possible diagnoses can be determined, which are referred to as the differential diagnosis.

The physician organizes the list with the most likely diagnosis given first. Additional information is identified, and appropriate tests are selected that will narrow the list or confirm one of the possible diseases.