Differential diagnosis

A differential diagnosis (sometimes abbreviated DDx, ddx, DD, D/Dx, or ΔΔ) is a systematic method used to identify unknowns. This method, essentially a process of elimination is used by physicians, psychiatrists and other trained medical professionals to diagnose the specific disease in a patient. The process is also is used by taxonomists to identify organisms.

In medicine
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:


 * 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.

Not all medical diagnoses are differential ones: some diagnoses merely name a set of medical sign and symptoms that may have more than one possible cause, and some diagnoses are based on intuition or estimations of probability. 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, 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.

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 is also used more loosely, to refer 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 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.

Process
Differential diagnosis has four steps. First, the doctor should gather all information about the patient and create a symptoms list. The list can be in writing or in the doctor's head, as long as he or she makes a list. Second, the doctor should make a list of all possible causes of the symptoms. Again, this can be in writing or in the doctor's head but it must be done. Third, the doctor should prioritize the list by placing the most urgently dangerous possible cause of the symptoms at the top of the list. Fourth, the doctor 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 means to use tests and other scientific methods before crossing off a possible cause of the problem. 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, Neoplastic, Degenerative/Deficiency, Idiopathic/Intoxication, Congenital, Autoimmune/Allergic, Traumatic, Endocrine

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 other disciplines
Differential diagnostic processes are also is used by biological taxonomists to identify and classify organisms, living and extinct.