diagnosis, systems of Medical diagnosis is concerned with the determination of the nature of a diseased condition; a process usually undertaken by a doctor. It requires the identification of the disease by careful investigation of its symptoms and history.

First we must define what is meant by the term ‘disease’. To many a broken leg may not be regarded as a disease, whilst unquestionably cancer of the lung or diabetes would be so described. For the purpose of this article, the definition of disease will follow that of the Oxford English Dictionary: ‘a condition of the body, or of some part or organ of the body, in which its functions are disturbed or deranged’. It may also be defined as a departure from the state of health, especially when caused by structural change.

The process of diagnosis in medicine now often requires a team of doctors, nurses, and technicians. It involves: (i) the careful assessment of the history of the symptoms of the disorder; (ii) clinical examination to determine whether there are any signs of abnormalities of the body; (iii) investigation of the blood or other body fluids to reveal any abnormalities indicative of the nature of the disorder; (iv) the use of special techniques for visualization, such as radiological examination or endoscopy.

The history is obtained by first asking the patient to tell the story — to give an outline of the symptoms of their condition — without asking leading questions. Thereafter the doctor may ask specific questions in order to clarify the sequence of events and to elicit any features which might be diagnostic; for example, has there been any blood in the stools or urine, any breathlessness? In some illnesses, for example migraine or epilepsy, the diagnosis may be evident from the history alone. The clinical history is always vitally important, since it often suggests the direction in which further investigations should be pursued; on the other hand the patient's account may not always be relevant to the diseased state. All alert doctors are aware of the patient who gives a long history of headaches, or tiredness, or of something else and then, when leaving, turns back from the door to say ‘And by the way, doctor …’ Only then may the significant symptoms emerge. Often, the symptoms presented by the patient may cover up the real nature of their disorder, particularly in the case of psychiatric problems, or when the main difficulty may lie in family relationships. Experienced doctors know that the clinical history more often than not provides the main clues to diagnosis.

After the clinical history has been recorded, the doctor turns to physical examination. This involves four steps: observation, palpation, percussion, and auscultation. Careful observation detects whether there are any abnormalities such as pallor of the skin (indicating anaemia), jaundice (indicating possible liver disease), or the bluish-purple colour of cyanosis (indicating hypoxia from lung or heart conditions). Where a clinical history of breathlessness has been elicited and cardiac disease suspected, any abnormal pulsations or distension of the veins in the neck will be looked for. Any unusual lumps or moles which may have been enlarging are noted. There might be visible evidence of thyroid disease, as indicated by tremor and staring eyes (thyrotoxicosis) or sluggishness of demeanour (thyroid underactivity or myxoedema). The pulse is examined to detect any abnormality of cardiac rhythm, and the blood pressure measured to establish whether there is hypertension.

The ‘systems’ of the body are then examined — first usually the heart and lungs, then the abdomen, where palpation may be of great importance in revealing tumours or other palpable masses. Percussion is helpful in detecting fluid in the chest, and is equally important in examining the abdomen: the tapping finger detects resonance where there is air in the lungs or gas in the guts, but dullness where there is an abnormal collection of fluid.

Examination of the nervous system takes a little more time. Power and sensation are tested in the limbs, and the tendon reflexes are examined. After a stroke, for example, there may be loss of power in one side of the body and the reflexes will be unusually brisk. The optic fundi (the retina and optic nerves in the eyes) are then examined with an ophthalmoscope: they provide a window not only on the brain but also on the state of the body's blood vessels: brain damage can be reflected in swelling, and conditions such as hypertension and diabetes also show characteristic changes.

At the first interview certain simple tests may have been carried out already. The urine is examined for the presence of sugar (indicating diabetes), albumin (indicating kidney disease), or bile pigment (indicating liver or gall bladder disease). The next stage involves the various pathology laboratories. In the haematology laboratory the blood is examined: the numbers of the white and red blood cells are measured, using an automatic cell counter. Diseases such as anaemia due to iron deficiency, pernicious anaemia, and leukaemia are diagnosed in this way. In patients with a history of bleeding, studies of blood clotting may be indicated. The microbiology laboratory is extremely important in cases of fever, sepsis, diarrhoea, pneumonia, or sexually transmitted disease. Isolating a specific microorganism from, for example, stool, urine, or blood, and determining its susceptibility to different antibiotics, often not only provides the diagnosis but also indicates the correct treatment. In the clinical chemistry laboratory, the blood electrolyte levels (concentrations of the different ions) are measured; measurement of chemical substances related to liver function is also usually carried out as a routine. The measurement of the blood glucose level is important in the diagnosis and management of diabetes. The levels of immune globulins and of certain antibodies in the blood may indicate a problem in the immune system. Some tests may be useful in screening for disorders which have not yet produced symptoms. The finding of a raised prostate specific antigen (PSA) may indicate cancer of the prostate, often permitting treatment in the early stages when the condition is most likely to be amenable to treatment.

In many disease conditions the final achievement of an accurate diagnosis lies with the histopathologist. In disorders of the female reproductive tract, for example, where screening techniques offer the hope of early diagnosis, it is the pathologist or trained technician who will make the diagnosis, on the basis of microscopic examination of a sample of tissue in a cervical smear. There are a number of biopsy techniques (taking a sample of living tissue) which are often vital in clinching a diagnosis suggested by the clinician. Liver or kidney biopsies, taken by a needle through the skin, provide the pathologist with material that is often diagnostic. Jejunal biopsy — obtaining a small knuckle of the lining membrane of the intestine — provides material from which the pathologist may make an almost instant diagnosis.

The pathologist is also of vital importance in establishing the diagnosis in conditions such as cancer of the breast, when the surgeon operates to obtain tissue for immediate histological examination before deciding on what operation to carry out.

In the modern era, imaging techniques have virtually made the human body transparent, from the first use of X-rays at the end of the nineteenth century, to the various imaging techniques now available. Ultrasound has been particularly helpful in pelvic disorders. The use of radioactive tracers makes it possible to outline a wide range of conditions, the take-up of the tracer being detected by a radioactive scanner. Whilst conventional radiological examinations are still an important part of diagnosis, particularly for example in injuries involving fractures of limbs, scanning techniques using computerized axial tomography (CT scanning) have greatly increased the accuracy of diagnosis, particularly in the detection of tumours or of abnormalities of the different organs of the body. In such instances, however, it is often necessary to obtain tissue either by needle biopsy or at surgery before the diagnosis can be certain.

Abnormalities of the heart and blood vessels are best shown by the introduction of a contrast medium (radio-opaque substance) via the bloodstream into the relevant area, followed by X-ray examination (angiography), a technique now routinely used in the diagnosis of the common condition of coronary heart disease. Ultrasonic techniques are also valuable in cardiac diagnosis.

A more recent development has been the introduction of magnetic resonance imaging (MRI), a remarkable scanning technique which has made it possible to diagnose abnormalities of the skeletal system and within the central nervous system with great accuracy. The earliest abnormalities in the brain in multiple sclerosis may be revealed in this way. MRI is also being exploited for the determination of brain and liver function, as well as providing evidence of metabolic disorders.

These and other contemporary body imaging techniques have made it possible to reveal not only structural defects but also disordered function.

Likewise in the modern era, the practice of endoscopy has revolutionized diagnosis, particularly in common conditions such as stomach ulcer or cancer of the colon. Endoscopy has made it possible to obtain specimens from stomach or colon, or from the bronchial tree in the lungs, which can be examined by histological techniques, allowing an accurate diagnosis to be made.

The number of techniques that may be used in the investigation of human disease has increased enormously in recent years. Many of these tests are extremely costly, so that the expense of obtaining a precise diagnosis has to be balanced against how precise the diagnosis needs to be for effective decision-making and treatment. Neither are they all entirely free of risk to the patient's well-being, whether by causing some physical complication, or through the mental stress of a ‘false-positive’ finding which proves to be a false alarm.

In the utilization of medical diagnostic technologies, nothing can replace the importance of the clinical history and physical examination, since these determine the diagnostic pathway that should be followed. Doctors, aware of the risk of litigation should the diagnostic process not prove accurate, tend to carry out too many investigations; the patient then becomes locked in to a series of complicated and expensive procedures that may be neither necessary nor in their best interest. As in all things in medicine, medical diagnosis requires prudence, and more than a modicum of common sense.

Christopher Booth

See also biopsy; endoscopy; imaging techniques; radiology; X-rays.