A differential diagnosis (sometimes abbreviated DDx, ddx, DD, or ΔΔ) is a systematic method used to identify unknowns. This method, essentially a process of elimination, is used by taxonomists to identify living organisms, and by physicians, physician assistants, and other trained medical professionals to diagnose the specific disease in a patient.
Not all medical diagnoses are differential ones: some diagnoses merely name a set of signs and symptoms that may have more than one possible cause, and some diagnoses are based on intuition or estimations of likelihood.
The term differential derives from the word difference: careful differential diagnosis involves first making a list of possible diagnoses, then attempting to remove diagnoses from the list until at most one diagnosis remains. 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.
In medicine, differential diagnosis is the process whereby a given condition or circumstance, called the presenting problem or chief complaint, is examined in terms of underlying causal factors and concurrent phenomena as discerned by appropriate disciplinary perspectives and according to several theoretical paradigms or frames of reference, and compared to known categories of pathology or exceptionality. Differential diagnosis allows the physician to:
If the patient's condition does not improve as anticipated when the treatment or therapy for the disease or disorder has been applied, the diagnosis must be reassessed.
The method of differential diagnosis was first suggested for use in the diagnosis of mental disorders by Emil Kraepelin. It is more systematic than the old-fashioned method of diagnosis by gestalt (impression).
The method of differential diagnosis is based on the idea that one begins by first 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, don't look for 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.
At one time doctors ordered only particular blood tests, but now a full blood chemistry profile is standard, which can speed up the process of diagnosis as well as uncover sub-clinical conditions. With the advent of better radiological studies like MRI and the wider use of nuclear medicine, it has become more likely that unexpected findings will emerge and will be further studied, though such findings may not be supported by further investigation. Such findings are a valuable tool but not infallible; often it still takes a physician or medical team to track down either a more common illness with a rare presentation or a rare illness with symptoms suggestive of many other conditions. Sometimes a definitive diagnosis might take years.
Differential diagnosis also refers simply to a list of the most common causes of a given symptom, to a list of disorders similar to a given disorder, or to such lists when they are annotated with advice on how to narrow the list down (the book French's Index of Differential Diagnosis ISBN 0340810475 is an example). Thus, a differential diagnosis in this sense is medical information specially organized to aid in diagnosis.
The professional Merck Manual of Diagnosis and Therapy has 11 index entries describing the topic as differential diagnosis. The topic is mentioned within the body of 125 other separate articles on various medical conditions.
Many studies demonstrate improvement of quality of care and reduction of medical errors by using such decision support systems. Some of these systems are designed for a specific medical problem such as Schizophrenia, Lyme disease or Ventilator Associated Pneumonia . Others such as Iliad, QMR, DiagnosisPro, and VisualDx  are designed to cover all major clinical and diagnostic findings to assist physicians with faster and more accurate diagnosis.
However, these tools all still require advanced medical skills, in order to rate the symptoms and choose additional tests to deduce the probabilities of different diagnoses. Thus, non-professionals still need to see a health care provider in order to get a proper diagnosis.