Testing laboratories, which may be independent or an integral part of a hospital, are generally headed by pathologists --experts in identifying the causes of disease and especially the changes produced in the body by disease. These laboratories are generally grouped into two broad and overlapping categories: clinical pathology labs and anatomical, or surgical, pathology labs.

Clinical pathology is concerned with evaluating disease by analyzing blood, urine, and (less frequently) other body fluids. This branch of medical science is predicated on the fact that scores of biochemical processes are constantly going on in the body. As organs do their normal work, they are influenced by hormones, enzymes, minerals, and other chemical substances that come from food or are manufactured by the body. Organs also give off these substances as waste products. At one time or another, these chemicals travel through the bloodstream--and are sometimes found in the urine and other body fluids as well--and thus can be measured.

A level that is higher or lower than the normal range for a person's age and sex may indicate a problem, although these measurements by themselves do not always yield a specific diagnosis. They may reveal or pinpoint disorders in body function that cause a particular health problem or illness, or merely point to the organ or system that needs further study. Sometimes changes up or down, even within the normal range over time, can alert doctors to monitor that organ or system for a potential development, or enable them to monitor treatment or changes in the course of a disorder.

Anatomical pathology is concerned with identifying the causes and consequences of disease in a specific part of the body. This is done by examining samples obtained during surgery or autopsy to identify structural changes in cells, tissues, and organs. This discipline uses tools ranging from the naked eye to powerful microscopes and even molecular analysis of cell genes. Anatomical pathology examinations are among the most reliable ways to establish a diagnosis or, at least, give doctors definitive information on the type of disease suffered by the patient.

HOW TESTS ARE USED

Doctors order laboratory tests to help them establish or confirm a diagnosis, choose and monitor a type of treatment, and determine the prognosis, or outlook, for the patient. Normal laboratory test results can also be helpful in ruling out various diagnoses. Since lab tests are comparatively objective, they can be used to augment the more subjective information the doctor gets from a patient's history and physical examination.

Tests are generally ordered in a branching fashion, going from the general to the more specific, or by choosing a second test that reinforces the conclusion drawn from the first. This strategy can also be cost effective in that more general, broad-based tests are often less expensive than more complex and specific follow-up tests.

Laboratory tests are also ordered periodically to monitor the course of a disease or its response to therapy. Monitoring can help the doctor fine-tune a drug dose, for example, or determine when to stop medication. Lastly, laboratory tests are sometimes ordered as a screening mechanism to detect underlying disease in apparently healthy patients who have no symptoms (see "The Role of Screening: Tests for People Without Symptoms").

CLINICAL PATHOLOGY

Types and Sources of Specimens
There are a great many clinical laboratory tests, and they are performed on blood, urine, sputum, and other body fluids, and occasionally feces (see table 4.1). Tests can be performed on whole blood (to which an anticoagulant has been added to keep it from clotting), plasma (the fluid that remains when whole blood is centrifuged to remove the suspended red and white blood cells), or serum (the clear fluid that separates from whole blood that has clotted).

Many of the substances that are measured in blood can also be analyzed in urine or other body fluids, although the results will have different reference (normal) ranges. For example, glucose, a form of sugar, is not normally found in urine, but it is in blood, where it is about twice as concentrated as it is in cerebrospinal fluid (the fluid that surrounds the brain and spinal cord).

Besides blood, urine, sputum, and cerebrospinal fluid, other body fluids commonly examined in clinical laboratories are bronchial or pleural washings (fluids from the lungs and bronchial tubes), gastric or stomach aspirations, serous (or peritoneal) fluids from the abdominal cavity, and joint fluids. The various methods for obtaining these various body fluids are described below.

Blood .
Blood is most commonly drawn via venipuncture or finger sticks.

Venipuncture.
Blood is usually drawn from a vein on the inside of the elbow. If your doctor orders this venipuncture procedure, the nurse or technician will first wrap a tourniquet (usually a rubber hose) around your arm above your elbow to compress the blood vessels and limit the flow of blood in the veins that would normally return to the heart. You will then be asked to make a fist, which will make your vein stand out more prominently. The skin on the inside of your elbow will be cleaned with a swab or piece of cotton dampened with alcohol, and a sterile needle inserted into your vein. A coupling device attached to the needle allows blood to be drawn automatically by vacuum pressure into rubber-stoppered tubes. When a tube is filled, it can be removed and additional ones attached, depending on the amount of blood needed.

DID YOU KNOW?

Blood-drawing tubes are color-coded by stopper, indicating the type of anticoagulant or preservative they contain. For example: "Red tops" contain no anticoagulant; thus they allow the blood to clot so that serum can be drawn off. "Lavender tops" have an anticoagulant to prevent the blood from clotting. "Gray tops" contain a preservative that prevents the breakdown of glucose, a blood sugar.

The needle is then withdrawn from the vein, and the tourniquet removed if it hasn't been removed earlier. (Needles are always disposed of after one use so that there is no chance of spreading infection.) The entire procedure generally takes less than five minutes. You will be told to apply pressure to the puncture site with a piece of cotton for a few minutes. A small bandage may be placed over the site; this bandage can be removed in less than an hour. You should refrain from using your arm to carry heavy loads or do strenuous chores for about half an hour.

If for any reason blood cannot be drawn from an arm vein, the one inside your wrist or on the back of your hand can be used instead. For hospitalized patients, blood at times is obtained from the intravenous tubing used to deliver fluids directly into a patient's vein. Some tests are done on blood drawn from an artery instead of a vein, but these are rare. Because of the increased risk of bleeding, however, arterial blood is drawn by a doctor.

SOURCE:
"Chapter 4
An Overview of Diagnostic Laboratory Testing." The Patient's Guide to Medical Tests, 2nd Ed. by Faculty Members at The Yale University School of Medicine and G.S. Sharpe Communications, Inc. New York, NY: Houghton Mifflin Company, 2002.