Differential diagnosis
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In medicine, differential diagnosis (sometimes abbreviated DDx or ΔΔ) is the systematic method physicians use to identify the disease causing a patient's symptoms.
Before a medical condition can be treated, it must be identified. The physician begins by observing the patient's symptoms, examining the patient, and taking the patient's personal and family history. Then the physician lists the most likely causes. The physician asks questions and performs tests to eliminate possibilities until he or she is satisfied that the single most likely cause has been identified.
Once a working diagnosis is reached, the physician prescribes a therapy. If the patient's condition does not improve, 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 term differential diagnosis also refers to medical information specially organized to aid in diagnosis, particularly a list of the most common causes of a given symptom, annotated with advice on how to narrow down the list. For example, this differential diagnosis of sclerotic bone lesions lists nine common causes, including infection, trauma, and drugs.
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[edit] Use of Web searches for Differential Diagnosis Generation
Most common differential diagnoses lists (and explanations)are readily available with a simple websearch. For example, one could input : "Differential diagnosis of <Whatever sign or symptom you're looking for>" into your favorite search engine.
Example :
"Differential diagnosis of Dyspnea"
"Differential diagnosis of anterior mediastinal mass"
Purists may disparage such an approach, believing that one should always either generate these lists from one's knowledge and from first prinicples, or one should memorize large sets of differential diagnosis lists. While such approaches are valuable as a learning exercise, it is also often the case that many of the differential diagnoses lists that one will find on a websearch or Pubmed search will be far more extensive, well explained and thorough than most DDx lists that one will come up with ad-hoc.
Many of these lists and related discussion articles are written by experts in that specific area. In addition, these specialists and other experts have spent considerable time and effort writing these articles and editing them to make sure that they are up to date, thorough and sufficiently extensive/exhaustive. They may come up with important albeit unusual alternative causes that most ad-hoc DDx lists might miss.
Relying on memory instead of inference can also lead to errors of omission and commission. First, one may miss key alternative diagnoses. Second, memory is fallible - particularly when one is rushed, stressed or tired. Mnemonics can be an excellent way of memorizing key items from a differential diagnosis list, but certainly can rarely be as reliable and extensive as an expert-compiled article on the subject. In today's world of "ubiquitous computing"(PDAs, etc), some consider it to be irresponsible to rely exclusively on memory except under duress. This is, however, an area of considerable controversy.
Websearches - especially in PUBMED - can be particularly effective in generating potential differential diagnoses for rare or unusual signs or symptoms. Often these DDx lists can be surprisingly short, and the conditions can be quite obscure. The PUBMED search will also often identify the most recent research and treatments, as well as the names and email addresses of experts in those disorders -(the authors of the related journal articles).
A recent study by Tang and Ng (BMJ 2006;333:1143-5) examined the effectiveness of Google searches in diagnosing extremely hard and/or unusual cases that are featured in the New England Journal of Medicine's case reports. These are cases that are challenging enough to merit publication in this leading journal. A quick search on 3 to 5 main words or phrases regarding the case found a correct diagnosis 58 % of the time. One responder (Wentz) replicated the study using the PUBMED database, and found the correct diagnosis to these challengingly difficult cases 88% of the time. (Groves T. 2007. "Can Google help you to diagnose patients' problems" sBMJ. Jan. Vol 15. p. 20-21.)
Use of rapid database searches as a tool to aid in differential diagnosis is still in its early stages of development and adoption. It will most likely meet with considerable resistance from some experts, and be the subject of valuable ongoing study and debate.
[edit] Example of a DDx
The patient presents with symptoms A and B. The physician creates a list of diseases that include symptoms A and B. There are three diseases that feature both symptoms:
- Disease 1: A, B, C
- Disease 2: A, B, C, D
- Disease 3: A, B, E
The physician can test for the presence of symptom C. This would either support 1 and 2 or support 3. If the client tested positive for C, a test for D could be used to differentiate between disease 1 and 2. If the client tested negative for C, a test for E would confirm the diagnosis of disease 3.
In modern medicine, physicians typically decide to perform tests based on weighing the likelihood of a positive result against the severity of the disease if it were to remain undiagnosed. For example, if an 18-year-old with no personal or family history of heart disease complains of chest pain, the physician is much less likely to be concerned that a heart attack occurred than if the patient were 65 years old.
While (unfortunately) it is rarely used explicitly in clinical practice, Bayes Rule provides a statistically rigorous way to use such background "conditioning" information efficiently. For example : using age, family history, past history, risk factors like smoking, obesity, etc to generate the relative likelihoods (or conditional probabilities) of various conditions. Such formal approaches tend to be used more in medical expert systems. See also : Medical Decision Support Systems, Bayesian Network and Decision Theory. (For a list of some existing medical expert systems, see : [1] )
In practice, clinical judgement, experience and rules of thumb tend to be used for most medical decisions. (See also : Amos Tversky, Daniel Kahneman and cognitive biases and list of cognitive biases ).
[edit] In popular culture
The term and methodology have been recently popularized by the television series House.
