Diagnosis is the process of determining the nature and the cause of a person’s symptoms, within the framework of existing medical knowledge. The diagnosis may be obvious to all, such as a simple physical injury – a broken bone, for instance – or it may be a complicated disorder with many and variable manifestations, such as tuberculosis.

Despite the scientific ingredient of much diagnostic work it is still regarded as something of an ‘art form’. The doctor’s approach must remain attentively personal, for several reasons. Knowledge of the personality, occupation, environment and relationships of the person seeking medical advice may contribute valuable information to the search for a diagnosis. For example, certain disorders run in families, whereas others are hazards of a particular job; such background knowledge will set the doctor on a particular diagnostic pathway. Knowledge of the personality of a patient is also important. Mental depression and anxiety, for instance, may stimulate overactivity of the thyroid gland, or the opposite may occur. So the good diagnostician must be alert to all of these features if he or she is to ascribe the right cause to the patient’s symptoms.

Practical diagnosis broadly follows a set routine, with three components.

The first, the history, consists of taking the story of the present illness, together with that of previous health problems. This is almost always the most important part of the consultation, and usually leaves the doctor with one or more tentative diagnoses. The second, the examination, is the physical process of looking, listening, feeling and using specialized instruments to study the body, based on the information provided by the patient.

Finally come investigations and tests, designed to give accurate information about the functions of the body in health and illness. Depending on their nature these may be carried out quickly and simply at the doctor’s surgery, or ordered and arranged at a separate clinic or hospital. These cannot totally replace the history and examination, but the investigations and tests may be of great value in confirming or refuting a doctor’s provisional diagnosis.

The ‘art’ of diagnosis

Diagnosis can be rationalized scientifically as a logical step-by-step procedure in which the presence of certain symptoms and signs exclude some disorders and indicate alternatives; whereas the absence of other symptoms and signs does the same. However, most experienced doctors, like detectives trying to identify a criminal in a group of suspects, develop an ‘eye’ or a ‘hunch’, without mentally following the complete diagnostic procedure. The skill lies in eliciting from patients a true account of their symptoms and supporting information, because some people hide their complaints among decoy symptoms for fear of the consequences; whereas others drop in ‘just for a chat’ and their actual complaints require much wheedling out. And of course experience plays a large part. Such diagnostic talents reach apocryphal proportions in the case of the ‘great’ physicians and surgeons of previous decades, who reportedly could pin-point a hospital patient’s ailment by a glance in his direction during a ward round – and thereby amaze junior colleagues who had been sweating over the problem for days. Medicine, like other professions, has its share of acknowledged showmen.