Looking for Disease

The diagnostic method is the process by which a doctor identifies the disease or disorder that is causing symptoms. The doctor may use various tests to confirm or eliminate a particular diagnosis or to help to decide between several possible diagnoses. The process may include finding an underlying agent, such as a virus or bacteria. Screening tests are used to detect early signs of disease in large groups of people or are used individually to look for risk factors that predispose a person to an illness, such as cancer, in the future.

There are several reasons why you might visit a doctor. You may have a pain or other sensation that is causing you concern; you may have noticed a change in your body’s appearance; or you may simply be anxious about your general physical or mental state. At other times, you may need to see your doctor if a medical examination is required for work-related or insurance purposes.

If you have a health problem, your doctor will base his or her diagnosis on your symptoms, medical history, a physical examination, and possibly some diagnostic tests.

Why tests are done

Your doctor begins the diagnostic process by asking questions about your general health and any specific symptoms. The next step is to take a complete medical history by asking you questions about past illnesses, family history of disease, lifestyle, and occupation. If you have symptoms or signs of a common disease or disorder, your doctor may be able to arrive at an immediate diagnosis after examining you. If he or she is unable to arrive at a diagnosis, you may be asked to have some tests. At certain stages of your life, you may be asked to undergo screening tests. Screening uses one or more of the same tests as diagnosis but the aim of screening is to detect diseases such as cancer before symptoms develop.

Examining the throat

The first step in the diagnostic process is often a physical examination. If you have a sore throat, the doctor will inspect it for signs of infection.

Types of test

Medical tests can be simple enough to be carried out in your doctor’s surgery but some require sophisticated equipment in a hospital or laboratory. Tests may be performed on body fluids, such as urine or blood, or on tissue, such as cells from the cervix. Viewing tests involve the use of instruments such as endoscopes, viewing tubes through which a doctor can look directly into the body, usually through a natural opening such as the mouth or nose. Imaging tests, such as MRI, CT scanning, and ultrasound, create images of internal body structures to detect abnormalities. Certain tests, such as mammography (X-rays of the breasts), can be used for both diagnosis and screening.

You and your doctor will balance the medical value and reliability of an appropriate medical test against any health risks and discomfort.

Bone scan of the chest

Radionuclide scanning is often used to detect cancer that has spread to the bones, visible here as light areas along the spine.

Highly magnified viruses

Tiny organisms, such as viruses, can be seen only with an electron microscope. This image shows a highly magnified cluster of hepatitis A viruses.


Some tests, such as diagnostic tests on tissue samples, are extremely accurate, but most are less reliable. There may be inaccuracies in the test equipment, the procedure, or in the interpretation of results, which, together with human errors, can lead to false results. A false positive result indicates the presence of disease in a person who is in fact healthy, causing anxiety and necessitating further tests. A false negative result indicates that a person is free of the disease when he or she actually has it. This may delay diagnosis until symptoms develop, at which stage treatment may be less effective. Since most tests are not totally reliable, all results must be carefully assessed in the context of a person’s medical history and examination. Some tests may carry health risks. A risky or painful test may still be justified medically and be acceptable to the person having it if it yields information that could be lifesaving.


Screening is used to identify people who already have a disease but have not yet developed symptoms, and only if treatment is available for an early stage of the disease. For example, breast cancer screening is routine because early detection gives the best chance of a cure. Screening tests may also look for risk factors or a genetic predisposition to developing a disease. For example, a test might look for high blood cholesterol, associated with coronary heart disease, or for the genes associated with breast cancer. Screening may also be used to detect healthy carriers of abnormal genes, which may be passed on to children, and these tests may be offered to couples who are planning to have children. Successful, reliable, and cost-effective screening relies on careful selection of the population. Screening must be targeted at groups that are at increased risk. For example, breast cancer screening usually starts in women who are over the age of 50, when the tests will be more effective in detecting disease.

Cervical smear test result

A smear test is used to detect abnormal cervical cells that might become cancerous. This common screening test enables treatment to be given at an early stage. The cells shown here are healthy.

Trends in disease detection

Technological advances are making diagnosis safer and less invasive, and the use of computers is making the analysis, storage, and retrieval of test results quicker and more reliable.

Various reliable home tests, such as tests for blood sugar levels (necessary in diabetes) and blood pressure monitors, are available. Used under medical supervision, these tests can help you to monitor your long-term illnesses and the effects of treatments, enabling you to take control of your health.

Visiting your Doctor

Visualizing Internal Organs

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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