Pathognomonic (often misspelled as pathognomic) is an adjective of Greek origin (παθογνωμονικό [σύμπτωμα]), often used in medicine, which means diagnostic for a particular disease. A pathognomonic sign is a particular sign whose presence means, beyond any doubt, that a particular disease is present. It is derived from the Greek páthos (πάθος, disease) and gnōmon (γνώμον, “judge”). Labelling a sign or symptom “pathognomonic” represents a marked intensification of a “diagnostic” sign or symptom.

While some findings may be classic, typical or highly suggestive in a certain condition, they may not occur uniquely in this condition and therefore may not directly imply a specific diagnosis. A pathognomonic finding on the other hand allows immediate diagnosing, since there are no other conditions in the differential diagnosis. A pathognomonic sign or symptom can sometimes be absent in a certain disease, since the term only implies that when it is present, the doctor instantly knows the patient’s illness.

Singular pathognomonic signs are relatively uncommon. 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, or a tetrad of rash, arthralgia, abdominal pain and kidney disease in a child with Henoch-Schönlein purpura.