Editing Pathognomonic (section)

== 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|positive predictive value]] and high [[sensitivity and specificity|specificity]]<!--not always 100% but always close--> but does not need to have high [[sensitivity and specificity|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]].{{citation needed|date=December 2021}}

Singular pathognomonic signs are relatively uncommon. Examples of pathognomonic findings include [[Koplik's spots]] inside the [[mouth]] in [[measles]], the palmar [[xanthoma]]ta 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|date=December 2021}}

As opposed to [[symptom]]s (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 anemia]]s (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).{{citation needed|date=December 2021}}

In contrast, a test with very high [[sensitivity and specificity|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]].<ref>{{cite journal | vauthors = Lynch HT, Lynch JF, Lynch PM, Attard T | title = Hereditary colorectal cancer syndromes: molecular genetics, genetic counseling, diagnosis and management | journal = Familial Cancer | volume = 7 | issue = 1 | pages = 27–39 | year = 2007 | pmid = 17999161 | doi = 10.1007/s10689-007-9165-5 | s2cid = 20103607 }}</ref><ref>{{cite journal | vauthors = Lynch HT, Lanspa SJ | title = Colorectal cancer survival advantage in MUTYH-associated polyposis and Lynch syndrome families | journal = Journal of the National Cancer Institute | volume = 102 | issue = 22 | pages = 1687–9 | date = November 2010 | pmid =  21044965| doi = 10.1093/jnci/djq439 }}</ref>