Diagnosis (from the Greek words dia = by and gnosis = knowledge) is the process of identifying a disease by its signs, symptoms and results of various diagnostic procedures. The conclusion reached through that process is also called a diagnosis.
Typically, someone with abnormal symptoms will consult a physician, who will then obtain a history of their present illness and examine them for signs of disease. The physician will formulate a hypothesis of likely diagnoses and in many cases will obtain further testing to confirm or clarify the diagnosis before proceeding to render treatment.
Forms of testing commonly performed are blood tests, medical imaging, EKGs, urinalysis and, occasionally, biopsy.
The list of most likely and possible diagnoses based on available information is called a differential diagnosis; the final diagnosis is only arrived at after other diseases on this list have been eliminated through further investigations or consultations.
The history of medical diagnosis began in earnest from the enlightened days of Hippocrates in ancient Greece but is far from perfect despite the enormous bounty of information made available by medical research including the sequencing of the human genome. The practice of diagnosis continues to be dominated by theories set down in the early 1900s.
Over two thousand years ago, Hippocrates recorded the association between disease and heredity. In similar fashion, Pythagoras noted the association between metabolism and heredity (allergy to Fava beans). The medical community, however, has only recently acknowledged the importance of genetics and its relevance to mainstream medicine.
The Oslerian ideal
The ideals of William Osler who innovated the practice of medicine in the early 1900’s were based on the principles of the diagnosis and treatment of disease. According to Osler, the functions of a physician were to be able to identify disease and its manifestations, understand its mechanisms, how it may be prevented and how it may be cured. For his medical students he believed that the best textbook was the patient himself – analysis of morbid anatomy and pathology were the keys. The Oslerian ideal continues today, as the basis of the Doctor’s strategy is, "What disease does this patient have and what is the best way for treatment?" The emphasis is on the classification of the disease in order to use the remedies available for its effects to be reversed or ameliorated. The human being in question is representative of a class of people with this type of disease whereas the biological individuality of this person is not given any great weight.
The successor to William Osler as Regius Professor at Oxford was Archibald Garrod. Garrod echoed the observations of his Greek counterparts of two millennia ago, ...our chemical individualities are due to our chemical merits as well as our chemical shortcomings; and it is more nearly true to say that the factors which confer upon us our predispositions to and immunities from various mishaps which are spoken of as diseases, are inherent in our very chemical structure; and even in the molecular groupings which confer upon us our individualities, and which went into the making of the chromosomes from which we sprang. Considering that the time that he formulated these ideas were the early 1900's, and the knowledge of DNA encoding genes that in turn encoded proteins responsible for bodily structure and functions not being discovered until some fifty years later it took some time before medicine could fully appreciate the fundamental importance of his concept of diagnosis.
Present-day Oslerian practice
Whereas Osler laid the founding principles by which medicine should be practiced, Garrod placed these principles in a greater context of a chemical individuality that is inherited and is subject to the mechanisms of evolutionary selection. The Oslerian ideal of medical practice continues to dominate medical philosophy today. The patient is a collective of symptoms to be characterised and analysed algorithmically in order to draw a diagnosis and subsequently produce a strategy of treatment. Medicine is about problems based solutions. In keeping with this philosophy, today's pathology reports provide a momentary snapshot of the patient's biochemical profile, highlighting the end result of the disease process.
Influence of DNA technology
Garrod's conception of biological individuality was confirmed with the advent of the sequencing of the human genome. Finally the subtle relationship between inheritance, individuality and environment became apparent via the variations detected in DNA. In each patient's DNA lies a script for how their bodies will change and become ill as well as how they will handle the assaults of the environment from the beginning of their life to its end. It is hoped that by knowing a patient's genes that the biological strengths and weaknesses in respect to these assaults will be revealed and disease processes can be predicted before they have the opportunity to manifest. Although knowledge in this area is far from complete, there are already medical interventions based on this. More importantly, the physician, forewarned with this knowledge can guide the patient towards appropriate lifestyle changes to anticipate and mitigate disease processes.
History taking is a fluid process that adapts to the information as it is presented. Almost invariably the patient presents with a complaint. Even the unconscious patient presents with the implicit complaint of being unconscious. This presenting complaint leads to the formation of hypotheses. Rather than consider the myriad of diseases that could afflict the patient, the physician narrows down the possibilities to those conditions likely to account for the presenting complaint. The history taking then proceeds to test these hypotheses, often narrowing down the diagnosis within a few questions. Sometimes the initial hypotheses are ruled out and the physician must then move on to look at other hypotheses or multiple ones. Occam's razor is then invoked to attempt to simplify the number of provisional diagnoses and it is only with some reluctance that a physician will make several provisional diagnoses to explain the symptoms elicited.
The physician then moves on to the physical examination. However, the hypothesis testing does not end at this point. Signs may confirm the provisional diagnosis or cause the physician to consider the question further and even lead to more questioning. An unexpected finding on examination may cause the physician to rethink the diagnosis.
At this point the physician usually has at least a differential diagnosis and probably a provisional diagnosis if not a firm diagnosis. Further tests are then requested, in part to confirm or disprove the diagnosis but also to document the status at that time and before treatment is started. Consultations with other physicians and specialists in the field may be sought.
Treatment itself may indicate a need for review of the diagnosis if there is a failure to respond to treatments that would normally work.
Despite all of these complexities, most consultations are relatively brief, partly because many diseases are common and pattern recognition allows the physician to recognize the diagnosis early, but also because the decision trees of most hypothesis testing are relatively short.