A large number of tests are widely available. Many tests are specialized for a particular disorder or group of related disorders (which are usually described with the appropriate disorders in this book). Other tests are commonly used for a wide range of disorders.
Tests are done for a variety of reasons, including
Sometimes a test is used for more than one purpose. A blood test may show that a person has too few red blood cells (anemia). The same test may be repeated after treatment to determine whether the number of red blood cells has returned to normal. Sometimes a disorder can be treated at the same time a screening or diagnostic test is done. For example, when colonoscopy (examination of the inside of the large intestine with a flexible viewing tube) detects growths (polyps), they can be removed before colonoscopy is completed.
There are different types of medical tests but the lines that separate them often 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. Tests are usually one of the six following types.
The most commonly analyzed fluids are
Less often, sweat, saliva, and fluid from the digestive tract (such as gastric juices) are analyzed. Sometimes the fluids analyzed are present only if a disorder is present, as when fluid collects in the abdomen, causing ascites, or in the space between the two-layered membrane covering the lungs and lining the chest wall (pleura), causing pleural effusion.
These tests provide a picture of the inside of the body—in its entirety or only of certain parts (see Overview of Imaging Tests). Ordinary x-rays are the most common imaging tests. Others include ultrasonography, radioisotope (nuclear) scanning, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and angiography.
A viewing tube (endoscope) is used to directly observe the inside of body organs or spaces (cavities). Most often, a flexible endoscope is used, but in some cases, a rigid one is more useful. The tip of the endoscope is usually equipped with a light and a camera, so the examiner watches the images on a television monitor rather than looking directly 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, such as the following:
Nose: To examine the voice box (laryngoscopy) or the lungs (bronchoscopy)
Mouth: To examine the esophagus (esophagoscopy), stomach (gastroscopy), and small intestine (upper gastrointestinal endoscopy)
Anus: To examine the large intestine, rectum, and anus (coloscopy)
Urethra: To examine the bladder (cystoscopy)
Vagina: To examine the uterus (hysteroscopy)
However, sometimes an opening in the body must be created. A small cut (incision) is made through the skin and the layers of tissue beneath the skin, so that the endoscope can be passed into a body cavity. Such incisions are used to view the inside of the following:
Often, body functions are measured by recording and analyzing the activity of various organs. For example, electrical activity of the heart is measured with electrocardiography (ECG), and electrical activity of the brain is measured with electroencephalography (EEG). The lungs’ ability to hold air, to move air in and out, and to exchange oxygen and carbon dioxide is measured with pulmonary function tests.
Usually, cells from skin, blood, or bone marrow are analyzed. Cells are examined to check for abnormalities of chromosomes, genes (including DNA), or both. Genetic testing may be done in the following:
Fetuses: To determine whether they have a genetic disorder
Children and young adults: To determine whether they have a disorder or are at risk of developing a disorder
Adults: Sometimes to help determine the likelihood that their relatives, such as children or grandchildren, will develop certain disorders
Every test has some risk. The risk may be the possibility of injury during the test, or it may be the need for further testing if the result is abnormal. Further testing is often more expensive, dangerous, or both. Doctors weigh the risk of a test against the usefulness of the information it will provide.
Normal test values are expressed as a range, which is based on the average values in a healthy population. That is, 95% of healthy people have values within this range. However, average values are slightly different for women and men and may vary by age. For some tests, these values also vary among laboratories. Thus, when doctors get a laboratory test result, the laboratory also gives them its own normal range for that test. The table below lists some typical normal results. However, because values vary by laboratory, people should consult their doctor about the significance of their own test results rather than refer to this table.