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Nature and Investigation of Diseases

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  INTRODUCTION Disease can be defined as any abnormality or failure of the body to function properly and this may require medical treatment ( Figure 1.1 ). The scientific study of diseases is called  pathology . Every disease has a distinct set of features that include a cause, associated clinical symptoms and a characteristic progression, with associated morphological and functional changes in the patient. The presence of an abnormality on its own, however, does not necessarily indicate disease since the affected individual must also suffer from ill health.  Health  can be defined as an absence of signs and symptoms associated with any disease. This definition has limitations in that there are circumstances where individuals believe they are ill even though detectable indications of disease are not present. Conversely, there are individuals who believe they are healthy but on detailed examination are found to suffer from a serious disease. For this reason, the World Health Organizatio
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Characteristic Features of Diseases

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  CHARACTERISTIC FEATURES OF DISEASES Every disease has a number of characteristic features. These features allow diseases to be categorized and allow a better understanding of the disease, its diagnosis and management. A correct diagnosis should mean that appropriate treatment is given. ETIOLOGY Etiology refers to the cause of a disease. Etiological agents can be  endogenous , in other words originating from within the body, or  exogenous , coming from outside the body. Endogenous agents include genetic defects and endocrine disorders, while exogenous agents include microorganisms such as viruses ( Figure 1.2 ), bacteria ( Figure 1.3 ) and fungi that cause infections, chemicals, physical trauma and radiation. Many diseases are said to be predictable and arise as a direct consequence of exposure to the causative agent. Other diseases are considered probable in that they may be a consequence of the causative agent but the development of illness is not inevitable. An individual can be in
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Classification of Diseases

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  CLASSIFICATION OF DISEASES Some diseases share common features and can be grouped together in a classification system. One way of classifying diseases is on the basis of their cause ( Table 1.2 ) .  This is by no means perfect as some diseases have multiple causes and there is likely to be an overlap between the different categories. Infectious diseases  are caused when microorganisms such as viruses, bacteria,fungi, protozoa and helminths enter and spread within the body. Immunological diseases   occur in circumstances inwhich the immune system can cause damage to the body’s own tissues. In autoimmune conditions, for example autoimmune thyroiditis, antibodies are produced that attack the body’s own tissues. Alternatively, there are diseases associated with immunodeficiency that increase the susceptibility of the patient to infectious agents. This occurs in severe combined immunodeficiency (SCID) and in AIDS. Endocrine diseases  arise from the over- or underproduction of hormones orf
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Epidemiology of Disease

  EPIDEMIOLOGY OF DISEASE Epidemiology is the study of how diseases spread in populations in relation to their causal factors. Consequently, epidemiology is largely concerned with the collection and interpretation of data about diseases in groups of people rather than in individuals. The types of data collected in epidemiological studies provide information about the etiology of the diseases, whether there is a need for screening or the introduction of other preventative measures and whether health care facilities are appropriate. The  prevalence  of a disease refers to the proportion of people in a population affected at a specific time. The  incidence rate  is the number of new cases of a disease in a population occurring within a specified period of time. Epidemiological studies can often provide information about the cause(s) of diseases. Thus if a disease has a high incidence in a particular region or population, then the disease may have a genetic origin or it may be caused by en
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Investigating Diseases

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  INVESTIGATING DISEASES For the majority of diseases, the clinical outcome is likely to be improved if treatment is started at an early stage. Consequently the proper investigation of disease is necessary to ensure a rapid and accurate diagnosis and to allow appropriate treatment to be initiated as soon as possible. The procedure for investigating a disease is outlined in  Figure 1.8 . It starts with the affected person presenting symptoms and visiting his or her physician when feeling unwell or after a period of ill health. The examination usually begins with the clinician asking the patient about his or her current and past medical histories, current and previous medications, use of alcohol and tobacco, any family history of disease and possibly occupational history. This is usually followed by a clinical examination to look for signs of any abnormality. This may involve visual examinations of the skin, eyes, tongue, throat, nails and hair to detect abnormalities together with tests
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Types of Pathology Laboratories

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  TYPES OF PATHOLOGY LABORATORIES The function of hospital pathology laboratories ( Table 1.3 ) is to make scientific investigations of disease. The typical pathology service offered by hospitals has six main branches: medical microbiology, immunology, clinical biochemistry, hematology, histopathology and clinical genetics. Types of pathology laboratories Medical microbiology Immunology Clinical biochemistry Hematology Histopathology Clinical genetics Medical microbiology  is concerned primarily with the detection and identifica-tion of pathogenic microorganisms. For clinical purposes, these consist of viruses, bacteria, protozoa, fungi and helminths (worms). Microorganisms are detected directly in specimens obtained from the patient or on swabs (for example throat, nasal) that are cultured in growth medium to increase the number of microorganisms and allow their easier detection. The presence of microorganisms may also be determined indirectly by detecting antibodies produced by the p
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Role of Hospital Laboratory Tests

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  ROLE OF HOSPITAL LABORATORY TESTS Tests performed by the pathology laboratory can assist clinicians in investigating disease. The tests may only give a subjective assessment, such as when a pathologist assesses the types of cells obtained from a fine needle aspirate of a suspected breast tumor when investigating breast cancer. However, tests may provide quantitative information, such as the concentration of thyroid hormones in the serum, that can then be compared with a  normal  value. Unfortunately, the term normal is often difficult to define in clinical terms. To alleviate this problem,  reference ranges  have been widely adopted. Numerical reference limits are based on the mean value plus or minus two standard deviations against which test results can be compared. The uses of reference ranges are explored more thoroughly later. The term  normal range  is still used synonymously with reference range. In general, the roles of laboratory tests include: •        the diagnosis, to ide
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