A differential diagnosis (sometimes abbreviated DDx, ddx, 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 or other clinicians 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.
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:
- more clearly understand the condition or circumstance
- assess reasonable prognosis
- eliminate any imminently life-threatening conditions
- plan treatment or intervention for the condition or circumstance
- enable the patient and the family to integrate the condition or circumstance into their lives, until the condition or circumstance may be ameliorated, if possible.
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).Template:Fact
The method of differential diagnosis is based on the idea that one begins by first considering the statistically more probable diagnoses. Medical students are taught the adage, "When you hear hoofbeats in Texas, think horses, not zebras"; only after more probable diagnoses are ruled out should the somewhat less likely ones be considered.
It used to be that 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 there will be unexpected findings that will be further studied, though such findings may not be supported by further investigation. They are a valuable tool, but not infallible; it still often takes a physicians' 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, or 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.Template:Fact
Machine differential diagnosis
Machine differential diagnosis is the use of computer software to partly or fully make a differential diagnosis. It may be regarded as an application of artificial intelligence.
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.
In Popular Culture
The central characters on the American television program House, M.D. are diagnosticians, and rely heavily on the differential diagnosis method of identifying and treating medical conditions.
- "differential diagnosis." Merriam-Webster's Medical Dictionary. Merriam-Webster, Inc. 12 Aug. 2007. Dictionary.com.
- "differential diagnosis." The American Heritage Stedman's Medical Dictionary. Houghton Mifflin Company. 12 Aug. 2007. Dictionary.com.
- "differential diagnosis." Google search of the online Merck Manual.
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