Medical sign
A sign is an indication of some fact or quality; and a medical sign is an objective indication of some medical fact or quality that is detected by a physician during a physical examination of a patient.
There is a strong implication that the signs have no meaning for a patient, and may not even be noticed by them; yet they are full of meaning for the physician, and are often significant in assisting a physician to identify the disease(s) responsible for the patient's symptoms.
Examples include elevated blood pressure, a clubbing of the fingers (which may be a sign of lung disease, or many other things), and arcus senilis.
The term sign is not to be confused with the term indication, which denotes a valid reason for using some treatment.

# Signs and semiotics
The art of interpreting clinical signs was originally called semiotics in English. This term, then spelt semeiotics (derived from the Greek adjective : semeiotikos, "to do with signs"), was first used in English in 1670 by Henry Stubbes (1631-1676), to denote the branch of medical science relating to the interpretation of signs:

# Eponymous signs
A number of medical signs are named after the doctors who first described them.

# Signs versus symptoms
Signs are different from symptoms: the "subjective" experiences, such as the fatigue, that patients might report to their examining physician.
For convenience, signs are commonly distinguished from symptoms as follows: a symptom is something abnormal, that is relevant to disease, experienced by a patient, whilst a sign is something abnormal, that is relevant to disease, discovered by the physician during his examination of the patient:
According to King, it is an essential feature of a sign that there is both a sign and a thing signified. And, because "the essence of a sign is to convey information", it can only be a sign if it has meaning. Therefore, "a sign ceases to be a sign when you cannot read it".
A slightly different definition views signs as any indication of a disease that can be objectively observed (i.e. by someone who isn't the patient), whereas a symptom is merely any manifestation of a disease that is apparent to the patient (i.e. reasons why diseases are bad).  From this definition, it can be said that an asymptomatic patient is uninhibited by disease.  With this set of definitions, there is some overlap--certain things may qualify as both a sign and a symptom (e.g. a bloody nose).

# Types of signs
Medical signs may be classified by the type of inference that may be made from their presence, for example:
- Prognostic signs (from progignoskein, , "to know beforehand"): signs that indicate the outcome of the current bodily state of the patient (i.e., rather than indicating the name of the disease). Prognostic signs always point to the future. Perhaps the most famous prognostic sign is the facies Hippocratica.
- Anamnestic signs (from anamnestikos, , "able to recall to mind"): signs that (taking into account the current state of a patient's body), indicate the past existence of a certain disease or condition. Anamnestic signs always point to the past. (Whenever we see a man walking with a particular gait, with one arm paralysed in a particular way, we say This man has had a stroke; and, if we see a woman in her late 50s with one arm distorted in a particular way, we say She had polio as a child.)
- Diagnostic signs (from diagnostikos, , "able to distinguish"): signs that lead to the recognition and identification of a disease (i.e., they indicate the name of the disease).
- Pathognomonic signs (from pathognomonikos, , "skilled in diagnosis", derived from pathos, , "suffering, disease", and gnomon, , "judge, indicator"): the particular signs whose presence means, beyond any doubt, that a particular disease is present. They represent a marked intensification of a diagnostic sign. (An example would be the palmar xanthomata seen on the hands of people suffering from hyperlipoproteinaemia.) Singular pathognonomic signs are relatively uncommon.

# Technological development creating signs detectable only by physicians
Prior to the nineteenth century there was little difference between physician and patient. Most medical practice was conducted as a joint co-operative interaction between the physician and the patient as equals. Whilst each noticed much the same things, the physician had a more informed interpretation of those things: the physicians knew what the findings meant and the layman did not.

## Advances in the 19th century
However, the patient was gradually removed from the medical interaction due to significant technological advances such as:
- the 1808 introduction of the percussion technique:
The techniques, which had been first described by the Viennese physician Leopold Auenbrugger (1722-1809) in 1761, became far more widely known following the publication of Corvisarts translation of Auenbrugger's work in 1808.
- the 1819 introduction of the technique of auscultation following the 1819 publication of Rene Theophile Hyacinthe Laennec's (1781-1826) findings on the use of his modified stethoscope. (He had invented a very crude form of stethoscope in 1816; but it was his subsequent modification of that later stethoscope that was the subject of his 1819 publication. Laennec's 1819 publication was Forbes translated into English in four editions between 1821 and 1834 by Sir John Forbes.)
- The 1846 introduction by surgeon John Hutchinson (1811-1861) of the spirometer, an apparatus for assessing the mechanical properties of the lungs per medium of measurements of forced exhalation and forced inhalation. (The recorded lung volumes and air flow rates are used to distinguish between restrictive disease (in which the lung volumes are decreased: e.g.,  cystic fibrosis) and obstructive diseases (in which the lung volume is normal but the air flow rate is impeded; e.g., emphysema).)
- The 1851 invention, by Hermann von Helmholtz (1821-1894), of the opthalmoscope, which allowed physicians to examine the inside of the human eye.
- the 1895 clinical use of X-rays which began almost immediately after they had been discovered that year by the German Wilhelm Conrad Rontgen (1845-1923).
- the 1896 introduction of the sphygmomanometer, designed by Italian Scipione Riva-Rocci (1863-1937), to measure blood pressure.

## Alteration of the relationship between physician and patient
The introduction of the techniques of percussion and auscultation into medical practice immediately altered the relationship between physician and patient in a very significant way, specifically because these techniques relied almost entirely upon the physician listening. (King observes that the introduction of the stethoscope did not immediately revolutionize medicine; because, although the physicians could certainly hear some thing via these techniques, they had no idea whatsoever of what those particular sounds, in those particular rhythms, in those particular combinations actually meant. In other words, although they certainly were being bombarded with noises, they were noises that signified nothing at all.)
Not only did this greatly reduce the patient's capacity to observe and contribute to the process of diagnosis, it also meant that the patient was often instructed to stop talking, and remain silent.
As these sorts of evolutionary changes continued to take place in medical practice, it was increasingly necessary to uniquely identify data that was accessible only to the physician, and to be able to differentiate those observations from others that were also available to the patient, and it just seemed natural to use "signs" for the class of physician-specific data, and "symptoms" for the class of observations available to the patient.
King proposes a more advanced notion; namely, that a sign is something that has meaning, regardless of whether it is observed by the physician or reported by the patient:

# Signs as tests
In some senses, the process of diagnosis is always a matter of assessing the likelihood that a given condition is present in the patient. In a patient who presents with haemoptysis (coughing up blood), the haemoptysis is very much more likely to be caused by respiratory disease than by the patient having broken their toe. Each question in the history taking allows the medical practitioner to narrow down their view of the cause of the symptom, testing and building up their hypotheses as they go along.
Examination, which is essentially looking for clinical signs, allows the medical practitioner to see if there is evidence in the patient's body to support their hypotheses about the disease that might be present.
A patient who has given a good story to support a diagnosis of tuberculosis might be found, on examination, to show signs that lead the practitioner away from that diagnosis and more towards sarcoidosis, for example. Examination for signs tests the practitioner's hypotheses, and each time a sign is found that supports a given diagnosis, that diagnosis becomes more likely.
Special tests (blood tests, radiology, scans, a biopsy, etc.) also allow a hypothesis to be tested. These special tests are also said to show signs in a clinical sense. Again, a test can be considered pathognonomic for a given disease, but in that case the test is generally said to be "diagnostic" of that disease rather than pathognonomic. An example would be a history of a fall from a height, followed by a lot of pain in the leg. The signs (a swollen, tender, distorted lower leg) are only very strongly suggestive of a fracture; it might not actually be broken, and even if it is, the particular kind of fracture and its degree of dislocation need to be known, so the practitioner orders an x-ray. The x-ray film shows a fractured tibia, so the film is said to be diagnostic of the fracture.

# Examples of signs