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Differential diagnosis (sometimes abbreviated DDx or or ΔΔ) is the systematic method used to identify the condition, syndrome or disorder causing a patient's signs and symptoms.

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 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).[How to reference and link to summary or text]

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.[How to reference and link to summary or text]

The Core Process of Differential Diagnosis

The patient presents with symptoms A and B. The diagnosistician creates a list of diseases, disorders and syndromes that include symptoms A and B. Consider there are only three "disease processes" that feature both these symptoms. Let them be called Conditions 1, 2 and 3.

  • Condition 1: A, B, C
  • Condition 2: A, B, C, D
  • Condition 3: A, B, E

The diagnostician now tests for the presence of symptom C. A positive result would support a diagnosis of Condition 1 or 2, and would rule out the possibility of Condition 3. If the client tested positive for C, a test for D could be used to differentiate between Condition 1 and Condition 2. If the client tested negative for C, a test for E would confirm the diagnosis of Condition 3.

The Practical Application of Differential Diagnosis

Before a psychologicaln or psychiatric condition can be treated, by one of the various psychological therapies and/or by psychotropic medications, a mental illness must first be identified correctly.

The diagnosing physician begins by observing the patient during an extended interview process. The patient is observed for signs and is asked for a list of symptoms. A detailed history is taken and questions are asked about the patient's present condition and circumstances. A thorough assessment of all possible supports and stressors and supports in the patient environment is made. When it is possible, a patient's family member might help in gathering the raw materials of a differential diagnosis of mental illness.

The term differential diagnosis refers a process of elimination that progressively weeds out the impossible and less probable, so a remaining list is created of the possible and the more probable.

All the raw materials have been elicited, recorded and evaluated. All the possible psychological and psychiatric conditions that might be suggested by the presenting constellation of signs and symptoms are considered. Then further questions and observations will attempt to narrow down the possibilities.

Eventually, the therapist will be reasonably satisfied that the single most likely cause has been identified. Once a provisional or working diagnosis is reached, a course of therapy is prescribed. If the patient's condition does not improve, the diagnosis and therapy must be reassessed. A change of diagnosis might be a wise step or perhaps a change in therapies will seem in order.


Differential Diagnosis in Mental Illness

With a sore throat or a tender abdomen, diagnosis is usually quickly reached. Symptoms elicited and recorded. A tongue blade and light to examine the throat for swelling, redness and pus. A manual palpation of the abdomen for guarding, rebound tenderness, auscultation for bowel sounds. A throat culture taken. A CBC and UA to test for the presence of infections. Soon pallitative measures, curative medications or a surgical consult will be decided upon.

But with mental conditions, whether psychological or psychiatric, there are very few diagnosistic exams or tests to rule in or rule out a given diagnosis. This will be less likely true in the decades to come what with all the current neuropsychiatric research, but for now, differential diagnosis of mental illness remains more art than science.

Because of this, the skill level of the diagnostician is a vital component of psychiatric diagnosis. So too is the process of differential diagnosis. One thing a differential diagnosis takes into account is how likely a particular diagnosis is. For instance, in a person under 65, one puts Alzheimer's further down on the list of possibilities and more closely pursues other possible sources of dementia and memory loss. Again, when a person presents with serious depression with no obvious causation, one still investigates for more common depressive triggers before ordering tests to rule out pancreatic cancer.

One of the most confusing tasks in psychiatric differential diagnosis is that of determing the presence of personality disorders. For instance, in determining whether the narcissistic personality disorder is present, the psychiatrist must first identify whether, among others, psychopathy, other personality disorders, bipolar disorder, Asperger's Syndrome, ADHD and/or addictions might be either the primary cause of the symptomatology, not present at all, or might exist as comorbid diagnoses.

With mental illness, psychological or psychiatric, differential diagnosis will, more often than in medicine, use the response to medications to rule in or rule out a given diagnosis. If a person presents with depression, antidepressants might help, but if that patient has Bipolar I and is presenting with depression, those same antidepressants might trigger an agitated, rageful or manic episode. This would immediately rule out simple depression and would strongly suggest bipolar illness. It would also indicate the urgent need for an immediate change in prescribed medications.

A person presenting with a great deal of nervousness, anxiety and feelings of dread might seem to need an anxiolytic (a tranquilizer). However, if that person is given diazepam, and then reports an overwhelming depression the next day, perhaps even with suicidal ideation, then what you have is not primarily anxiety, but a depression, masked by anxiety, that mandates treatment for depression, not anxiety.

A patient presenting with complaints of sleeping her life away and retreat from an outside life may suggest many things, but the diagnosis of another atypical depression might be missed. The right medication will have her actively engaged in life in little time. Gross obesity and lack of appetite and thinness must likewise be investigated from many directions to determine if an eating disorder exists or if depression, anxiety, a medication or a medical condition might be at fault.

The goal in treatment of all the various mental illnesses and conditions is to return the individual to maximum functionality. The goal is for the person to have stable working relationships within the family, wider social networks and at work. A good working diagnosis ensures that the best possible psychotherapy approach can be selected and that the proper medications can augment and reinforce that psychotherapy.

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 [1], Lyme disease [2] or Ventilator Associated Pneumonia [3]. Others such as Iliad, QMR and DiagnosisPro [4] 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.


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