Developing a Case-Specific Differential Diagnosis

This chapter was reproduced from the Emergency Medicine Clerkship Primer, 2008, Chapter 9, with the permission of the Editor, for ease of viewing on mobile devices.


By this time in your medical school training, you should be familiar with the traditional concept of a differential diagnosis in which a list of possible diagnoses is generated and ruled in or out until a final diagnosis is identified. Like other fields of medicine, developing a differential diagnosis list is essential to the care of emergency patients, but the process of developing a differential diagnosis in emergency medicine is distinctive.


In emergency medicine, we not only focus on the likely diagnosis but must also think about the potentially life-threatening diagnoses or other problems that could endanger the patient if delayed or missed.

In the traditional model, the patient’s signs and symptoms are categorized into a problem list (e.g., headache, chest pain, back pain). The differential diagnosis is generated from the patients’ problem list. In emergency medicine, we not only focus on the likely diagnosis but must also think about the potentially life-threatening diagnoses or other problems that could endanger the patient if delayed or missed. We may actually never make the final diagnosis, but we can at least exclude life-threatening conditions.

Medical Decisionmaking

Several cognitive strategies are used in the medical decisionmaking process. These strategies include hypotheticodeductive, algorithmic, pattern recognition, rule-out-worst-case scenario, exhaustive, and event driven strategies. Each of these strategies has advantages and disadvantages. To avoid error, it is helpful for clinicians to understand which strategies they are using and what the limitations are of the particular strategy.

Hypotheticodeductive Decisionmaking

Hypotheticodeductive is the most common decisionmaking strategy. A preliminary diagnosis is made on the basis of a series of inferences. This strategy is most like the old fashioned detective who gathers clues about a particular suspect. The working diagnosis is tested and refined as new data are discovered. It is important for this hypothesis to be tested and verified. Failure to change course as contradictory information is gathered can lead to misdiagnosis because of premature closure. An advantage of the hypotheticodeductive method is that it is flexible.

Case Study

A 45-year-old man presents in the winter months with a complaint of headache and nausea. His complaints started yesterday, and appear worse in the morning and improve throughout the day. As a medical detective, if you do not consider carbon monoxide toxicity and attribute his complaints to a nonspecific illness, you will miss a critical diagnosis.

Algorithmic Decisionmaking

In the algorithmic method, a series of steps is followed to simplify the decisionmaking process. An example could be chest pain pathways or pulmonary embolism diagnostic algorithms. This type of model is easy to teach and can improve the care for certain patients; however, algo-rithms are often too inflexible to cover all situations. The one-size-fits-all approach to patients can lead to problems if a patient presentation does not fit the algorithm.

A good clinician needs to be mindful of the concept of premature closure.

Pattern Recognition

In pattern recognition, a series of signs and symptoms are clumped together into a known grouping. Pattern recognition is often used by seasoned clinicians with extensive clinical experience. Pattern recognition is subject to premature closure and anchoring bias, in which physicians continue to stick with the original diagnosis despite conflicting data. Anchoring bias refers to the tendency to rely too heavily or to “anchor” on one piece of information during the decisonmaking process. Failure to incorporate new data is known as confirmation bias. Confirmation bias can be compared to closed-mindedness. This pattern of decisionmaking refers to the tendency to add weight or value to facts that confirm or support one’s beliefs, while ignoring or undervaluing the relevance of contradictory information.

Rule-Out-Worst-Case Scenario

The rule-out-worst-case-scenario method is designed to eliminate the life-threatening diagnosis for a given clinical presentation rather than to focus on the likely diagnosis. This style may, at times, lead to extensive workups and excessive use of resources. Less-experienced clinicians of-ten use this method because it is least likely to lead to catastrophic results. However, the diagnosis of less common and noncritical diseases will often be delayed or missed completely when using the rule-out-worst-case-scenario method.

Case Study

Consider a 71-year-old male who presents to the emergency department with a traumatic right flank pain. The most important condition to consider in this presentation is an abdominal aortic aneurysm. Although the patient may be much more likely to have a kidney stone, failure to consider abdominal aortic aneurysm in your differential could have potentially catastrophic consequences if missed.

Exhaustive Decisionmaking

In the exhaustive method, all the possible data are gathered in an indiscriminate pattern and then sorted through. This method also can lead to excessive workups and is very time-consuming—the typical “shot gun approach.”

Event-Driven Decisionmaking

In event-driven decisionmaking, clinicians respond to the clinical scenario and treat the symptoms with limited thought as to the underlying cause. An intervention is made, and the situation is reassessed. The clinician may at times back into the diagnosis on the basis of response to therapy. This strategy is often used for a critically ill patient such as one in acute respiratory failure. The event-driven strategy is often combined with the rule-out-worst-case-scenario method. The event-driven method is particularly well suited to the emergency department environment; however, it tends to be reactive rather than proactive.


Eliminating life-threatening conditions from the differential is more important than making the correct diagnosis of a benign condition

Putting Decisionmaking to Work

Ruling out potential life-threatening presentations is a high priority in emergency medicine. Eliminating life-threatening conditions from the differential is more important than making the correct diagnosis of a benign condition, see the accompanying case. The important point is that if a particular illness, condition, or injury is not considered, sooner or later it may be missed.

A good clinician needs to be mindful of the concept of premature closure. This occurs when an incorrect diagnosis is made at an early stage of the patient encounter. In these cases, the differential diagnosis is too narrow, and the healthcare provider fails to consider other possibilities that could account for the patient’s presenting complaint. As a result, an incorrect diagnosis is pursued, and the true underlying condition may be missed. Keep an open mind and a broad differential. As new information is acquired, or if the patient’s condition changes, the differential should be reassessed and adjusted. If the new data do not support the leading diagnosis, other conditions need to be considered. Premature closure and failure to continually reassess the differential diagnosis can lead to catastrophic results.

When creating a differential diagnosis, it is extremely important to create a list that includes all of the likely diagnoses as well as all of the potentially life-threatening conditions, even if they are uncommon or less likely. There is an old saying that “you cannot make a diagnosis that you do not think about.” Taken one step further, if you do not think of a particular diagnosis, you will miss it. The case studies in this chapter describe such examples.

So how do you begin to build a differential diagnosis? As a junior learner with limited clinical experience, a good differential is more often drawn from your fund of medical knowledge and less from your clinical experience. For this reason, because emergency medicine is in many ways a com-plaint-driven specialty, it is helpful to review both common and potentially serious causes of routinely encountered chief complaints. Starting your rotation with a solid fund of medical knowledge will allow you to develop more in-depth differentials from day one.

In the end, developing a differential diagnosis in emergency medicine is an active process incorporating a variety of decisionmaking skills. Data gathering, hypothesis testing, and treatment often occur simultaneously. Clinicians need to be particularly careful to avoid premature closure on cases to avoid error. The true art of developing and working through your differential diagnosis is often made by the balance of ruling out the life-threatening conditions and correctly diagnosing the likely conditions. Remember that not every diagnosis will be made in the emergency department. The importance of developing a case-specific differential is that your assessment of the case will have a profound impact on your diagnostic testing and management.

Suggested Reading

  • Sandu H, Carpenter C, Freeman K, et al. Clinical decision making: opening the black box of cognitive reasoning. Ann Emerg Med. 2006;48:713–719.
    • This article describes the decisionmaking process and the different cognitive strategies used to make decisions.