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 to assess cardiovascular, respiratory, gastrointestinal, genitourinary, nervous and musculoskeletal functions. Since diseases typically present with recognizable signs and symptoms, the clinician may make a diagnosis of the disease based on the clinical history and the examination and then initiate treatment. Sometimes this may not be possible, given that many clinical symptoms and signs are not specific to any one disease.
However, a range of diagnostic services is also available to the clinician in modern health care systems. These include imaging techniques, physiological function tests, radiographic examinations (X-rays) and pathology laboratory investigations that can be applied to confirm, reject or distinguish between the various provisional diagnoses. The clinician may only be able to make a provisional diagnosis or a shortlist of possible diagnoses and then request additional investigations that rely on the diagnostic services available at the surgery, clinic or hospital.