Simply, a sign is an indication of some fact or quality; and, in everyday English, 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—such as elevated blood pressure, or a clubbing of the fingers (which may be a sign of lung disease, or many other things), or arcus senilis.
Thus, by this definition, signs are different from symptoms: the "subjective" experiences, such as the fatigue, that patients might report to their examining physician. The term sign is also not to be confused with the term indication, which denotes a valid reason for using some treatment.
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.
The art of interpreting clinical signs was originally called semiotics in English. This term, then spelt semeiotics (derived from the Greek adjective σημειοτικός: 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:
…nor is there any thing to be relied upon in Physick, but an exact knowledge of medicinal phisiology (founded on observation, not principles), semeiotics, method of curing, and tried (not excogitated, not commanding) medicines…
A number of medical signs are named after the doctors who first described them.
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:
…a sign is an objective symptom of a disease; a symptom is a subjective sign of disease.
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".
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 progignṓskein, προγιγνώσκειν, "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.
Anamnestic signs (from anamnēstikós, ἀναμνηστικός, "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.
Diagnostic signs (from diagnōstikós, διαγνωστικός, "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 pathognomonikós, παθογνωμονικός, "skilled in diagnosis", derived from páthos, πάθος, "suffering, disease", and gnṓmon, γνώμον, "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. Singular pathognonomic signs are relatively uncommon.
[Thus] a symptom is a phenomenon, caused by an illness and observable directly in experience. We may speak of it as a manifestation of illness. When the observer reflects on that phenomenon and uses it as a base for further inferences, then that symptom is transformed into a sign. As a sign it points beyond itself — perhaps to the present illness, or to the past or to the future. That to which a sign points is part of its meaning, which may be rich and complex, or scanty, or any gradation in between.
In medicine, then, a sign is thus a phenomenon from which we may get a message, a message that tells us something about the patient or the disease. A phenomenon or observation that does not convey a message is not a sign. The distinction between signs and symptom rests on the meaning, and this is not perceived but inferred. (King, 1982, p.81)
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”.
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 Corvisart’s translation of Auenbrugger's work in 1808.
the 1819 introduction of the technique of auscultation, following the 1819 publication of René Théophile Hyacinthe Laënnec's (1781-1826) findings on the use of his modified stethoscope.
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 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.
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:
The belief that a symptom is a subjective report of the patient, while a sign is something that the physician elicits, is a 20th-century product that contravenes the usage of two thousand years of medicine. In practice, now as always, the physician makes his judgments from the information that he gathers. The modern usage of signs and symptoms emphasizes merely the source of the information, which is not really too important. Far more important is the use that the information serves. If the data, however derived, lead to some inferences and go beyond themselves, those data are signs. If, however, the data remain as mere observations without interpretation, they are symptoms, regardless of their source. Symptoms become signs when they lead to an interpretation. The distinction between information and inference underlies all medical thinking and should be preserved.
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.
^ John Locke (1632-1704), also used the term semeiotics in Book 4, Chapter XXI ("Of the Division of the Sciences") of his (1690) work, "An Essay Concerning Human Understanding". In part 1, he explains how science can be divided into three parts:
All that can fall within the compass of human understanding, being either, first, the nature of things, as they are in themselves, their relations, and their manner of operation: or, secondly, that which man himself ought to do, as a rational and voluntary agent, for the attainment of any end, especially happiness: or, thirdly, the ways and means whereby the knowledge of both the one and the other of these is attained and communicated; I think science may be divided properly into these three sorts. (Locke, 1823/1963, p.174).
Locke then elaborates further, in 4.XXI.4, upon the nature of this third category, labelling it, in Greek, as Σημειωικη (Semeiotike), and explaining it to mean, in his usage, "the doctrine of signs" (Locke, 1823/1963, p.175).
^ Perhaps the most famous prognostic sign is the facies Hippocratica.
[If the patient's facial] appearance may be described thus: the nose sharp, the eyes sunken, the temples fallen in, the ears cold and drawn in and their lobes distorted, the skin of the face hard, stretched and dry, and the colour of the face pale or dusky.… and if there is no improvement within [a prescribed period of time], it must be realized that this sign portends death.(Chadwick & Mann, 1978, p.170-171)
^ 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”.
^ See, for example, Jewson, 1974; Jewson, 1976; King, 1982; and Tsouyopoulos, 1988.
^ The process through which “the physician can assess the state of the underlying lung by sensing the character of vibrations by gentle taps on the chest wall […something which…] greatly facilitated the diagnosis of pneumonia and other respiratory diseases” (Weatherall, 1994, p.46)
^ The process of listening to sounds of the heart, lungs, etc.
^ 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. Laënnec's 1819 publication was Forbes translated into English in four editions between 1821 and 1834 by Sir John Forbes (1787-1861).
^ 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).
^ King (1982, p.83) 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.
Jewson, N.D., "Medical Knowledge and the Patronage System in 18th Century England", Sociology, Vol.8, No.3, (1974), pp.369-385.
Jewson, N.D., "The Disappearance of the Sick Man from Medical Cosmology, 1770-1870", Sociology, Vol.10, No.2, (1976), pp.225-244.
King, L.S., Medical Thinking: A Historical Preface, Princeton University Press, (Princeton), 1982.
Locke, J., The Works of John Locke, A New Edition, Corrected, In Ten Volumes, Vol. III, T. Tegg, (London), 1823. (facsimile reprint by Scientia, (Aalen), 1963.)
Stubbe, H. (Henry Stubbes), The Plus Ultra reduced to a Non Plus: Or, A Specimen of some Animadversions upon the Plus Ultra of Mr. Glanvill, wherein sundry Errors of some Virtuosi are discovered, the Credit of the Aristotelians in part Re-advanced; and Enquiries made...., (London), 1670.
Tsouyopoulos, N., "The Mind-Body Problem in Medicine (The Crisis of Medical Anthropology and its Historical Preconditions)", History and Philosophy of the Life Sciences, Supplement to Vol.10, (1988), pp.55-74.
Weatherall, D., Science and the Quiet Art: Medical Research & Patient Care, Oxford University Press, (Oxford), 1994.