special subjects
Clinical Decision Making Strategies
Hypothesis Testing
The initial differential diagnosis based on chief complaint and demographics is often large, so the clinician first generates and filters the hypothetical possibilities by obtaining the detailed history and doing a directed physical examination to support or refute suspected diagnoses. For instance, in a patient with chest pain, a history of leg pain and a swollen, tender leg detected during examination increases the probability of pulmonary embolism. When the history and physical examination form a recognizable pattern, a presumptive diagnosis is made. Diagnostic testing is used when uncertainties persist after the history and physical examination, particularly when the diseases remaining under consideration are serious or have dangerous or costly treatment. Test results further modify the probabilities of different diagnoses (post-test probability). For example, the table Hypothetical Differential Diagnosis and Pre-Test and Post-Test Probabilities for a 50-Yr-Old Hypertensive, Diabetic Cigarette Smoker With Chest Pain shows how the additional findings that the hypothetical patient had leg pain and swelling and a normal ECG and chest x-ray modify diagnostic probabilities—the probability of acute coronary syndrome, dissecting aneurysm, and pneumothorax decreases, and the probability of pulmonary embolism increases. These changes in probability may lead to additional testing (in this example, probably chest CT angiography) that further modifies post-test probability (see It may seem intuitive that the sum of probabilities of all diagnostic possibilities should equal nearly 100% and that a single diagnosis can be derived from a complex array of symptoms and signs. However, applying the principle that the best explanation for a complex situation involves a single cause (often referred to as Occam's razor) can lead clinicians astray. Rigid application of this principle discounts the possibility that a patient may have more than one active disease. For example, a dyspneic patient with known chronic obstructive pulmonary disease (COPD) may be presumed to be having an exacerbation of COPD but may also be suffering from a pulmonary embolism or heart failure. 