Doctors diagnose by elimination, treating each possible disease as a hypothesis
When a patient arrives with a cough, a clinician does not jump straight to an answer. They draw up a list of conditions that could produce the same signs, rank them by likelihood and danger, then test to knock candidates out. That structured guessing game is called differential diagnosis.
A differential diagnosis, often shortened to DDx, separates one condition from others with similar clinical features. Each candidate on the list is itself called a differential: acute bronchitis might be considered for a cough even when the patient turns out to have a common cold. The aim is to pin down the specific disease, or at least to make sure nothing immediately life-threatening is missed. Because candidates are treated as hypotheses to be confirmed or rejected, the process borrows from the hypothetico-deductive method.
The procedure is often described in four stages. The clinician gathers the patient's history and current signs and symptoms, lists possible causes, which need not be written down, and prioritises them by weighing probability against the risk of each diagnosis, a judgement that is subjective rather than objective. Finally, tests are used to rule conditions out. Sometimes the picture stays unclear, and the clinician treats empirically, a step nicknamed the educated best guess. Evidence such as symptoms and history gradually lowers the probability of candidates until only one remains plausible, whether in a doctor's head or in computer-assisted diagnostic software.
Experience changes the approach. Beginners tend to work through every possible explanation systematically, while seasoned clinicians lean on pattern recognition to spare patients delays, risks and needless tests; the best combine that experience with evidence from clinical research. In an emergency there may be no time for weighing probabilities, so a protocol such as ABC, airway, breathing and circulation, takes over, with fuller reasoning later. A pathognomonic sign, one that almost guarantees a condition, can shortcut the process, as can the absence of a sine qua non feature, which nearly rules it out.
Psychiatry uses the method as well, because symptoms overlap between conditions. Someone diagnosed with bipolar disorder may also have ADHD, major depressive disorder, PTSD, anxiety disorders or borderline personality disorder considered as differentials.
Source: Differential diagnosis
