Besides the diseases of blood itself, such as anemia or leukemia, which result in changes in the composition or amount of various types of blood cells, there are many other conditions that can be diagnosed by examining blood samples. The same is true of urine and saliva. After the specimen is taken, it is examined by pathologists in a laboratory and may be subjected to various tests. Most specimen tests carry little or no risk. Discomfort ranges from that of a minor needle stick to draw blood to significant pain, requiring local or general anesthesia, for certain biopsies.
Diagnostic tests are an extension of a physical examination and history.
They allow the doctor to see things that are not yet visible or to confirm what the doctor or the patient already suspects. In earliest times, women knew they were pregnant when their bellies grew and they felt their babies move. They soon realized that missing a menstrual period meant the possibility of pregnancy. Then doctors learned that by examining the cervix they could see changes in the color of the cervix and character of the mucus, which signaled pregnancy before the more obvious physical signs. The famous rabbit test took them one step further, faster.