# Pathognomonic

Pathognomonic (synonym pathognomic) 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. The absence of a pathognomonic sign does not rule out the disease. Labelling a sign or symptom "pathognomonic" represents a marked intensification of a "diagnostic" sign or symptom.
The word is an adjective of Greek origin derived from πάθος pathos 'disease' and γνώμων gnomon 'indicator' (from γιγνώσκω gignosko 'I know, I recognize').

## Practical use

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 positive predictive value and 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, or succinylacetone for Tyrosinemia Type I.
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. An example is the hypersegmented neutrophil, which is seen only in megaloblastic anemias (not a single disease, but a set of closely related disease states). 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).
In contrast, a test with very high sensitivity rarely misses a condition, so a negative result should be reassuring (the disease tested for is absent). A sign or symptom with very high sensitivity is often termed sine qua non. An example of such test is a genetic test to find an underlying mutation in certain types of hereditary colon cancer.

## Examples

| Disease | Sign |
| Cytomegalovirus infection | Owl's eye appearance of inclusion bodies |
| Lyme disease | Erythema chronicum migrans |
| Inclusion body myositis | Filamentous material seen in inclusion bodies under electron microscopy |
| Hypocalcemia | Trousseau sign and Chvostek sign |
| Tetanus or Strychnine poisoning | Risus sardonicus |
| Measles | Koplik's spots |
| Wilson's disease | Kayser–Fleischer ring |
| Diphtheria | Pseudomembrane on tonsils, pharynx and nasal cavity |
| Chronic hemorrhagic pancreatitis | Grey-Turner's sign (ecchymosis in flank area) |
| Cholera | Rice-watery stool |
| Enteric fever | Rose spots in abdomen |
| Meningitis | Kernig's sign and Brudzinski's sign |
| Angina pectoris | Levine's sign (hand clutching of chest) |
| Patent ductus arteriosus | Machine-like murmur |
| Parkinson's disease | Pill-rolling tremors |
| Whipple's disease | Oculo-masticatory myorhythmia |
| Acute myeloid leukemia | Auer rod |
| Multiple sclerosis | Bilateral internuclear ophthalmoplegia |
| Pericarditis | Pericardial friction rub |
| Rheumatic fever | Aschoff bodies |
| Rabies | Hydrophobia and negri bodies |
| Gout | Tophi |
| Acute tubular necrosis | Muddy brown casts |
| Granulosa cell tumour | Call-Exner bodies |
| Malakoplakia | Michaelis–Gutmann bodies |
| Narcolepsy (with cataplexy) | Cataplexy |
| Endodermal sinus tumor | Schiller–Duval body |
| Parkinson's disease | Rigidity with tremor |
| Atrial flutter | Flutter waves |
| Sickle cell disease | Vaso-occlusive crises |
| Lightning injury | Lichtenberg figure on skin |