Diagnosis of exclusion

In medicine, we describe a condition as a diagnosis of exclusion when there’s no definitive test to prove it exists, or that a particular person has it. Instead, we diagnose it as a collection of particular symptoms, while we’ve done tests to rule out other conditions that can cause a similar presentation.

Irritable bowel syndrome is a good example – gut symptoms are very real to patients, but (as yet) we have no specific test. Instead, we rule out Crohn’s disease, ulcerative colitis and coeliac disease, which also cause gut symptoms.

It’s also useful for conditions such as Alzheimer’s disease, where the clinical suspicion fits with what we know of the disease. There is a test available but it’s so invasive – requiring a sample of brain tissue – that it’s not an acceptable risk. So we rule out other causes for memory loss such as an infection or an electrolyte problem, and then diagnose Alzheimer’s based on the symptoms.