Pathognomonic signs are significant indicators of disease that a doctor may use to make a definitive diagnosis. They are so characteristic of particular medical conditions that when a doctor sees them, the chances are extremely high that a patient has a given condition. Not having those signs doesn’t necessarily mean a patient does not have the disease, as they do not appear in all cases. Such indicators can also sometimes appear with other conditions, in which case the doctor may need to use a differential diagnosis to find out what is going on.
These go beyond symptoms, which are indicators of disease. Something like a fever is a symptom. It means the patient is unwell, and that something is causing a fever reaction. Fevers can be seen in a wide variety of diseases, however, and do not usually indicate a specific condition. On the other hand, a distinctive grinning facial spasm is a pathognomonic sign for tetanus, also known as lockjaw. Likewise, a bulls eye-shaped rash may be considered a pathognomonic sign for Lyme disease.
If a patient displays pathognomonic signs of disease, the doctor may want to take a history, and could collect some other information to confirm the diagnosis. In the example above, if a patient presents with spasm of the facial muscles, the doctor might check for injuries that could have caused tetanus. Bloodwork can also be helpful, and may help the doctor rule out strychnine, which can also cause facial spasm, although it usually causes other muscles in the body to tense as well.
Identification of pathognomonic signs can be especially important with highly contagious disease. Measles, for example, can be readily diagnosed with Koplik’s spots, distinctive lesions inside the mouth. If a doctor sees them, the patient can promptly be isolated to prevent transmission. In addition, the doctor can alert parents and care providers so they can warn people who may have come into contact with the patient. This allows people to respond quickly to a measles outbreak to limit the spread of the disease.
Medical texts may include discussions, sometimes with illustrations, of pathognomonic signs. These allow trainee medical personnel to learn about what to look for when making a rapid and definitive diagnosis. Texts can also talk about potential sources of confusion and mixups, and ways to avoid them. This reduces the risk of false diagnosis based on misread signs. Some pathognomonic signs are topics of dispute, and doctors may argue about whether they are truly so characteristic of a specific disease that they can be used for an authoritative diagnosis.