A large number of tests are widely available. Many tests are specialized for a particular disease or group of related diseases. Generally, specialized tests are described with the appropriate diseases in this book. However, other tests are used commonly for a wide range of diseases.

Tests are performed for a variety of reasons, including screening, diagnosing a disease, evaluating the severity of a disease so that treatment can be planned, and monitoring the response to treatment. Sometimes, a test may be used for more than one purpose. A blood test may reveal that a person has too few red blood cells (anemia), and then the same test may be repeated after treatment to check that the number of red blood cells has returned to normal. In some instances, a condition can be treated at the same time a screening or diagnostic test is performed. For example, when colonoscopy, a test in which a flexible viewing tube is used to examine the inside of the large intestine, reveals growths (polyps), they can be removed before colonoscopy is completed.

Types of Tests

Medical tests generally fall into one of six categories: analysis of body fluids, imaging tests, endoscopy, measurement of body functions, biopsy, and analysis of genetic material in cells. In many instances, the lines that separate the categories become blurred. For example, endoscopy of the stomach enables the examiner to view the inside of the stomach as well as obtain tissue samples for examination in a laboratory.

Analysis of body fluids: most often consists of tests of the blood, urine, and fluid that surrounds the spinal cord and brain (cerebrospinal fluid). Less often, fluids such as sweat and saliva and fluid from the digestive tract (for example, gastric juices) are analyzed. Sometimes, the fluids analyzed are present only if a disease is present, such as when fluid collects in the abdomen (ascites) or in the space between the two membranes covering the lungs (pleural effusion).

Imaging: consists of tests that provide a picture of the inside of the body, either in its entirety or only of certain parts. Ordinary x-rays are the most common imaging tests, but others include ultrasound, radioisotope (nuclear) scans, computed tomography (CT) scans, magnetic resonance imaging (MRI) scans, and positron emission tomography (PET) scans.

Endoscopy: is the use of a viewing tube to directly observe the inside of body organs or spaces (cavities). Most often, the endoscope is flexible, although a few types are rigid. The tip of the endoscope is usually equipped with a light and a camera, so images can be seen on a television monitor while the examiner is observing through the endoscope. Tools are often passed through a channel in the endoscope. One type of tool is used to cut and remove tissue samples.

Endoscopy usually consists of passing the viewing tube through an existing body opening. For example, esophagogastroduodenoscopy (EGD) is accomplished by passing a viewing tube through the mouth. Colonoscopy involves passing a viewing tube through the anus. However, sometimes an opening in the body must be created. This can be done by making a small cut (incision) through the skin and through layers of tissue beneath the skin, so that the endoscope can be passed into a body cavity. For example, in arthroscopy, an endoscope is passed through an incision to view a joint, such as the knee or shoulder.

Measurement of body functions: often involves recording and analyzing the activity of various body organs. For example, electrical activity of the heart is measured with electrocardiography (ECG), and electrical activity of the brain is measured with electroencephalography (EEG).

Biopsy: involves removing tissue samples and examining them, usually with a microscope. The examination often focuses on finding abnormal cells that might provide evidence of inflammation or of a disease, such as cancer. Tissues that are commonly examined include skin, breast, lung, liver, kidney, and bone.

Analysis of genetic material: usually involves testing cells from skin, blood, or bone marrow. Genetic testing consists of an examination for abnormalities of chromosomes, genes, or both. Examination of genes includes analysis of DNA. Fetuses may undergo genetic testing to determine whether they have a genetic disorder. Children and young adults often undergo genetic testing to determine whether they themselves have a disease or are at risk of a disease. Adults sometimes undergo genetic testing to help determine the likelihood that their relatives, such as children or grandchildren, will develop certain diseases.

Risks and Results

Every test has some risk. The risk may be only the need for further testing if the result is abnormal, or it may be the possibility of injury during the test. Doctors weigh the risk of a test against the usefulness of the information it will provide.

Normal test result values are expressed as a range, which is based on the average values in a healthy population; 95% of healthy people have values within this range, but average values are slightly different for women and men and also may vary by age. These values also vary somewhat among laboratories.

Last full review/revision February 2003