DDx

Differential diagnosis, abbreviated as ddx or DDx, is a structured approach used in medicine to identify the range of possible causes that could explain a patient’s symptoms, signs, and test results. The goal is to consider multiple plausible conditions and systematically narrow the list to determine the most likely diagnosis while recognizing serious conditions that require urgent treatment.

The process usually begins with thorough history-taking and a physical examination to generate an initial broad differential. A problem list is created and prioritized by likelihood and by potential harm if missed. Clinicians select targeted diagnostic tests and imaging to differentiate among remaining possibilities. Results are interpreted in the context of pretest probability, and the differential is refined as new information becomes available. The final diagnosis may rely on test results, clinical response to therapy, or, in some cases, ongoing observation.

Practitioners distinguish between more likely conditions (rule-in candidates) and those that must be excluded (rule-out candidates). Diagnostic reasoning often uses Bayesian thinking, pattern recognition, and guidelines, while also addressing cognitive biases that can influence judgment. The differential is a dynamic construct that is updated as information evolves or new symptoms appear.

Common strategies include organ-system frameworks and the use of memory aids, though many clinicians customize the differential to the individual patient. Attention to red flags and urgent conditions is essential to ensure timely management when necessary.

Examples illustrate the approach: for chest pain, the differential may include myocardial infarction, angina, pulmonary embolism, aortic dissection, pneumothorax, or noncardiac causes such as reflux or musculoskeletal pain. For headache, possibilities range from migraine and tension-type headache to intracranial hemorrhage or tumor, depending on clinical features.

Limitations of ddx include time constraints, incomplete data, and cognitive biases. A well-documented differential supports clinical reasoning, guides testing and management, and facilitates communication with patients and other clinicians.