The term “differential diagnosis” is often used incorrectly by both native and non-native English writers. “Running a differential diagnosis” is the process of distinguishing between medical conditions on a differential list, based on a patient’s symptoms. The physician first builds a differential diagnosis list of all of the possible diseases that could cause the patient’s symptoms. The physician then conducts specific physical exams, orders specific laboratory tests and medical images. The findings from those exams, tests, and images are used to select the most likely diagnosis.
For example, the differential diagnosis for body aches and fever includes viral, bacterial, fungal, autoimmune, chemical, cancerous, and mechanical conditions. The physician may discuss the differential diagnosis with other medical professionals, perform physical exams, run laboratory tests, and use chest x-rays or other imaging procedures to help discern which diagnoses may match the data. After this, the physician is able to decide that some diseases on the list are more likely, others are less likely, and some are completely ruled out. If a bacterial infection seems likely, the physician will then make a new differential diagnosis list of possible bacterial pathogens and will systematically test for these.
“Differential diagnosis” is always singular, because it describes a single list of multiple diagnoses. The term can be useful in case reports and articles about medical decision-making or specific laboratory tests, especially when describing the process of diagnosing a specific disease.