Diagnosis of exclusion

Generally, most diagnoses are made from positive findings. A specific pattern of chest pain is used to diagnose a heart attack and certain signs elicited on examination of the chest will identify a pleural effusion. Investigations take this further and can confirm or refute diagnoses beyond reasonable doubt.

However, some diseases don't have specific symptoms and signs or their symptoms and signs can easily be found in another disease. Moreover, they don't have any investigations which can positively identify them. As such, the only way to diagnose such a disease would be through excluding all other feasible diagnoses. This can be done by testing for all other possibilities and showing that they are all negative leaving you with your diagnosis of exclusion.