Medical Diagnosis through Semiotics
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Medical Diagnosis through Semiotics

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which we do not suspect. 
Personal Bias 
Our interpretations tend to be biased [15, 20] and may go astray because experience wills our perspective [19, 21]. Just as Moby Dick has different meanings to Ahab and Starbuck, so too may a patient's cough suggest asthma to an allergist and esophageal reflux to a gastroenterologist: To a carpenter everything tends to look like a nail. With the flush of excitement that comes when we think we know the meaning of the sign, we might well stop a moment to reflect on the advice of Francis Bacon: ".whatever (the) mind seizes and dwells upon with peculiar satisfaction is to be held in suspicion, and that so much the more care is to be taken in dealing with such questions to keep the understanding even and clear" [22]. Our interpretations are by nature subjective and prejudicial, objectivity the goal. 
As in all forms of symbolic communication, the medical sign is not an autonomous messenger; the meaning that we construe must conform with its contexts [21, 23-25]. Diarrhea in an elderly bedridden patient spells (among other possibilities) fecal impaction, in a healthy youngster, the "summer complaint," a benign, unspecified transitory gastroenteritis. Ventricular premature beats have a grave import in patients with congestive heart failure but rarely so in the healthy. A low cholesterol level is a good sign, but not if the patient is malnourished\u2014it all depends. Facts presented in a void are devoid of meaning. To ignore context is to invite misunderstanding and chaos in diagnosis and all forms of interpretation. 
Once named, a disease (disorder or condition) becomes a sign. The physician and patient must now join hands to interpret and comprehend the disease\u2014to tell the story as it were\u2014in terms of how it is expressed in and experienced by the patient [26]. The same pneumonias are not the same; the patient may be immunosuppressed. My headache is not your headache. A minor hangnail could have been a devastating consequence for a concert pianist. Understanding must be expanded to account for the circumstances surrounding the patient: family and employer sentiments, third-party regulations, societal and ethical restraints and other\u2014often conflicting\u2014contingencies [27]. In the end, many, many contexts have converged to bring greater and greater specificity and particularity of meaning to our interpretation. Contexts serve to reduce the ambiguity of the sign [24] and crystallize diagnosis. 
	The Interpretive Process 
I have been talking about the elements of interpretation and now the process itself. At times in medical care no interpretation is necessary. A splinter under the nail and scoliosis of the spine speak for themselves; in terms of disease alone, sign and meaning coincide. In some instances we derive meaning by following "if-it's-this-it's-that" rules or algorithms, or by assessing a datum (sign) as it relates to other data within a physiologic framework\u2014as in working through fluid and electrolyte problems. The latter are two traditional types of diagnostic reasoning as described by Kassirer [28]; the semiotic equivalent of the third type, probabilistic reasoning, follows. 
More often we proximate meaning; rather than certitude, we deal with what is most likely [28-30]. Inferring from the signs what might be wrong, we test for truth and discriminate among competing diagnostic possibilities by examining for the presence of a mosaic of harmonious signs that we might expect to find if our conjecture (or one of them) was tenable. In the patient whose head nods with each heartbeat, we selectively inquire of the presence of leg cramps with exertion and look for a rib notching on the chest film and a delayed weak femoral pulse as further supporting signs of coarctation of the aorta, as opposed to aortic insufficiency. The more supporting signs that we can amass, the greater the particularity of description [31] and the less uncertain we are of that which is being portrayed. 
The corroborative signs are mustered selectively through communication and informed participatory interpretation, by the physician conducting a pointed, exploratory question-and-answer conversation with the patient and other sign sources. After selectively questioning the patient and suspecting angina pectoris, we elect to do a graded exercise test. In choosing this particular test, we are asking a leading question: Do the ST segments of the electrocardiogram become depressed with exercise and, if so, to what degree? The test results\u2014the ST changes\u2014is a yes or no, loud or soft answer to our question and a supporting or detracting sign. In a literary sense, we have narrowed the gap of uncertainty between writer and reader [32] by teaming up with the patient and other sign authors to rewrite and clarify the text; we have become both the creator of the information as well as its interpreter. 
	Quantifying Medical Decisions 
Turning for a moment to medicine proper, clinical epidemiology, decision analysis, and evidence-based medicine have provided quantitative methods to guide us in the retrieval and critical appraisal and use of medical information in making clinical decisions [15, 16, 33-39]. These advances constitute some of the most important additions to the medical canon of the last quarter century. 
Touching but lightly on some of these developments, the numeric diagnostic strength of an ever-increasing number of signs is now available [16] and is expressed as the sign's sensitivity and specificity and likelihood ratio, the latter being the odds that a given sign would occur (or be absent) in a patient with, as opposed to a patient without, the target disorder. If we are 50% sure in our minds that our patient with chest pain has coronary disease and the likelihood ratio of the electrocardiographic changes occurring on the graded exercise test is 10, we can read off a simple nomogram that the probability of disease has been increased to 90%; our interpretation has been numerically authenticated. 
Semiotics emphasizes that the sign must be interpreted in terms of its contexts; Bayes theorem that the likelihood of a test correctly predicting the presence or absence of disease depends not merely on the strength of the test but on the likelihood of the patient having the condition in the first place. Both are making the same point; the before-test likelihood of disease (prevalence) is a numeric expression of the effects of risk factors and other contexts. Even though two patients have similar chest pains as presenting signs, positive electrocardiographic changes on a graded exercise test would be far more likely to indicate coronary disease in an overweight hypertensive 60-year-old male cigarette smoker than in a 24-year-old female jogger with mitral valve prolapse. The contexts of the sign and, therefore, the probability of disease differ [40]. 
Decision Analysis 
The interpretation of a sign is usually followed by some appropriate action [41], which in medicine translates into doing what is best for the patient. Just what is best, however, may be difficult to say. We would have no problem recommending hip replacement for a degenerated hip to an otherwise vigorously healthy star athlete. Surgery would likely be successful and, if so, of immense value to the patient. On the other hand, in a 70-year-old sedentary accountant with a history of a previous pulmonary embolus after an automobile accident and with increasing symptoms of prostatism, determining whether it would be best to operate would be far from clear. Using decision analysis, the most propitious course of action can be determined by weighing and comparing consequences; the act with the most highly valued outcome is the winner. The values of the differing outcomes are derived by multiplying the probabilities of the outcomes taking place by their assigned numeric utilities or net benefits (relative preference) for the patient [42]. We can also establish ascending threshold levels of probability of disease at which,