Pathognomonic

Pathognomonic (often misspelled as pathognomic and sometimes as pathomnemonic) is a term, often used in medicine, that means characteristic for a particular disease. A pathognomonic sign is a particular sign whose presence means that a particular disease is present beyond any doubt. Labelling a sign or symptom "pathognomonic" represents a marked intensification of a "diagnostic" sign or symptom.

The word is an adjective of Greek origin (παθογνωμονικόν [σύμπτωμα]), derived from páthos ("πάθος" meaning "disease") and gignosko ("γιγνώσκω" meaning "to know, to recognize").

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 sign or symptom has very high specificity but does not need to have high sensitivity: for example it 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. The presence of a pathognomonic finding allows immediate diagnosis, since there are no other conditions in the differential diagnosis.

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, 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.

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