Principles of diagnosis

The main tools of the doctors are the medical history and the physical examination, but this holds particularly true for internal medicine. Subtle descriptions of disease (e.g. cyclic shallow and deep breathing, as in Cheyne-Stokes's respiration, or persistently deep breathing as in Kussmaul's) or physical signs (e.g. clubbing in many internal diseases) are important tools in guiding the diagnostic process. In the medical history, the "Review of Systems" serves to pick up symptoms of disease that a patient might not normally have mentioned, and the physical examination typically follows a structured fashion.

At this stage, a doctor is generally able to generate a differential diagnosis, or a list of possible diagnoses that can explain the constellation of signs and symptoms. Occam's razor dictates that, when possible, all symptoms should be presumed to be manifestations of the same disease process, but often multiple problems are identified.

In order to "narrow down" the differential diagnosis, blood tests and medical imaging are used. They can also serve screening purposes, e.g. to identify anemia in patients with unrelated complaints. Commonly performed screening tests, especially in older patients, are an X-ray of the chest, a full blood count, basic electrolytes, renal function and blood urea nitrogen.

At this stage, the physician will often have already arrived at a diagnosis, or maximally a list of a few items. Specific tests for the presumed disease are often required, such as a biopsy for cancer, microbiological culture etc.