Differential Diagnosis - General Components

General Components

For additional, general, aspects, see Diagnostic procedure.

There are various methods of performing a differential diagnostic procedure, but in general, it is based on the idea that one begins by considering the most common diagnosis first: a head cold versus meningitis, for example. As a reminder, medical students are taught the adage, "When you hear hoofbeats, look for horses, not zebras," which means look for the simplest, most common explanation first. Only after the simplest diagnosis has been ruled out should the clinician consider more complex or exotic diagnoses.

Differential diagnosis has four steps. First, the physician should gather all information about the patient and create a symptoms list. The list can be in writing or in the physician's head, as long as he or she makes a list. Second, the physician should make a list of all possible causes (also termed "candidate conditions") of the symptoms. Again, this can be in writing or in the physician's head but it must be done. Third, the physician should prioritize the list by placing the most urgently dangerous possible cause of the symptoms at the top of the list. Fourth, the physician should rule out or treat the possible causes beginning with the most urgently dangerous condition and working his or her way down the list. "Rule out" practically means to use tests and other scientific methods to render a condition of clinically negligible probability of being the cause.

In some cases, there will remain no diagnosis; this suggests the physician has made an error, or that the true diagnosis is unknown to medicine. Removing diagnoses from the list is done by making observations and using tests that should have different results, depending on which diagnosis is correct.

Mnemonics are routinely taught to medical students to ensure that all possible pathological processes are considered, for example VINDICATE:

• Vascular
• Inflammatory/Infectious
• Neoplastic
• Degenerative/Deficiency
• Idiopathic/Intoxication/Iatrogenic
• Congenital
• Autoimmune/Allergic
• Traumatic
• Endocrine