Pathognomonic (synonym pathognomic[1]) is a term, often used in medicine, that means "characteristic for a particular disease".
The word is an adjective of Greek origin derived from πάθος pathos 'disease' and γνώμων gnomon 'indicator' (from γιγνώσκω gignosko 'I know, I recognize').
Examples of pathognomonic findings include Koplik's spots inside the mouth in measles, the palmar xanthomata seen on the hands of people suffering from hyperlipoproteinemia, Negri bodies within brain tissue infected with rabies, or a tetrad of rash, arthralgia, abdominal pain and kidney disease in a child with Henoch–Schönlein purpura, or succinylacetone for Tyrosinemia Type I.
[citation needed] As opposed to symptoms (reported subjectively by the patient and not measured) and signs (observed by the physician at the bedside on physical exam, without need for a report) a larger number of medical test results are pathognomonic.
More often a test result is "pathognomonic" only because there has been a consensus to define the disease state in terms of the test result (such as diabetes mellitus being defined in terms of chronic fasting blood glucose levels).