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.
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