What is “Differential Diagnosis”?

There is an old adage in medicine that says “When you hear hoof beats, think horses. Don’t think zebras.” This phrase can be applied to every patient and case; your eyes don’t tell the whole story. It is usually good advice to confirm the obvious, rather than spending time and resources investigating rare things. Consider the virtue of the “differential diagnosis”.

In medicine, the differential diagnosis is a process by which the consideration of diagnosing one disease is distinguished from other conditions presenting with similar features. The “differential” is the logical exercise to eliminate other possible diagnoses based on the taking of a careful patient history, expert observation, a physical exam and laboratory tests. The latter, the use of the clinical laboratory, is essential in modern practices. The lab is used to “rule in” one diagnosis and/ or to “rule out” other possibilities. Importantly, the laboratory is objective, reproducible and sensitive, taking your clinical exam to a new level of confidence.

The differential diagnosis is also critical in dentistry. Or at least it should be. For example, not all cases of periodontitis can be discerned from probing and clinical observation. Tests to investigate the true cause of inflammation/ periodontal disease sometimes reveal surprises. Most common is a test to identify the specific profile of pathogenic bacteria. But what do you do when your expected culprits are not detected- think zebras? Maybe, but less dramatically think Candida or viruses, like Herpes simplex. The differential use of one or more reliable lab tests will likely get the real answer.

The exercise of the using the differential diagnosis includes the regular use of the laboratory. That is the modern way to practice and the best expression of clinical expertise.

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Ronald C. McGlennen, MD