I was talking with a colleague recently about how a certain test session behavior could almost be considered a "pathognomonic sign" of a specific disorder. That conversation got me to thinking I could use a little refresher on the terminology for signs, symptoms, and related terms.
Sign: In medicine, a sign is something that can be objectively seen by the examiner. E.g., something a doctor could observe during an examination, or in a lab result. A rash (assuming the doctor sees it) is a medical sign. In psychology, pressured speech during an interview is a sign. Symptom: A symptom is something that the individual self-reports. Most symptoms are subjective -- that is, only the individual can report on their experience. A headache is a symptom. Fatigue is a symptom. In psychology, "racing thoughts" is a symptom. Symptoms can also be objective. An objective symptom is something that could theoretically be observed by the examiner, but which is or was not. Reports of a fever, rash, or cough would all be symptoms if the individual reported them but they were not present on examination. If the doctor saw these things, these symptoms would become signs. Combinations of Signs & Symptoms: Most signs and symptoms are not very specific - each sign or symptom can be caused by a lot of different diseases or disorders. Most signs and symptoms are also not very sensitive - the absence of the sign does not necessarily mean the absence of the disorder. However, certain combinations of signs and symptoms usually point towards, or exclude, various diseases and disorders. The DSM-V is essentially a list of signs and symptoms of various disorders, with cut off thresholds for how many signs and symptoms you need to declare the presence of a disorder.
(Need a refresher on sensitive vs. specific? Me too. Sensitivity is the ability of the test to detect people who truly have the disease or disorder. Specificity is the ability of the test to correctly identify people who do not have the disease or disorder.)
Pathognomonic Sign: While most signs are nonspecific, some signs are highly specific. Some are so specific they are diagnostic: When that sign is present, the disease is present. There is no need for any further differential diagnosis, because there are no other disorders with those signs. These signs are called pathognomonic signs. A classic example is Koplik's spots in the mouth for measles. Assuming intact cognitive functioning and normal academic exposure, many psychologists would consider confirmed academic deficits in reading to be a pathognomonic sign of a specific learning disorder in reading. Note pathognomonic signs are not necessarily sensitive. You can have the disorder without the sign being present. Sine Qua Non: Some signs are highly specific. Some signs are so specific that their absence means the absence of the disease. This type of sign is a sine qua non. An example would be a definitive lab test for certain types of cancer- if your test shows you don't have the specific marker, you do not have that type of cancer . In the DSM-V, some diagnostic criteria effectively function as a sine qua non. If the individual does not meet criteria A, for example, of many disorders, they do not have that disorder. A child who does not have some social communication problems does cannot have autism, for example. Social communication problems are a sign qua non of autism. Indication: An indication is a reason for a certain test or treatment. An indication is not the same as a diagnosis. For example, psychotic symptoms are an indication for use of an anti-psychotic medication, while schizophrenia is one diagnosis that includes psychotic symptoms. Irritability is another possible indication for use of an anti-psychotic medication. In testing, the signs, symptoms, and demographic details you gather during the intake may all be indications for specific tests. Parent report of failure to make developmentally-appropriate friendships may be an indication for use of the ADOS-2. Further information on signs, symptoms, and diagnoses may all be indications for the treatments you recommend. Counter-Indication: The opposition of an indication is a counter-indication, or a reason not to proceed with a specific test or treatment. If you use a flexible-battery approach or focus on brief assessments, you may gain information during the intake that counter-indicates the use of a test. You may also gain information during the evaluation that counter-indicates certain treatment approaches or techniques. A quick example would be "talk therapy" for a child with a specific language disorder. Useful Link:
Kraft N.H. & Keeley J.W. (2015). Sign versus Symptom - includes a discussion of the biases that may prevent psychologists from seeing clinical signs, including "selective attention, top‐down processing, overshadowing, misattribution of behaviors, and lack of diagnostic knowledge." Unfortunately behind a paywall.