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.

The needle is then withdrawn from the vein, and the tourniquet removed if it hasn't been removed earlier.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.

Preparation for blood drawing is minimal. You may be instructed to refrain from eating or drinking anything except water for about eight hours before blood is drawn. These so-called fasting specimens guard against interference from the elements in food or liquids that may cause inaccurate test results. Blood glucose and triglycerides (a constituent of fats) are examples of tests that should ordinarily be done on fasting specimens. For many, if not most, tests, however, nonfasting (random) specimens are fine.

The amount of blood that will be drawn depends on the total amount needed for a particular test, as well as the amount needed in each tube to mix with an anticoagulant or preservative to achieve the desired effect. While one or more tubes of whole blood may be drawn, usually only a fraction of it is needed, but that fraction may come from the whole blood, plasma, or serum. To put this in perspective, your body contains about 100 ounces, or 3 quarts, of circulating blood. A typical tube contains only about 1/3 of an ounce. A drop of blood, serum, or plasma is often enough to do one test, and sometimes many, on an automated instrument. This drop is equal to about 2/1000 of an ounce—a trivial amount compared to the total amount available in your body.

Finger sticks. If an even smaller amount of blood is needed (to check for anemia if you are planning to donate blood, for example) or if your veins are too small or too fragile for a venipuncture, blood can be obtained by sticking a finger with a small, sharp blade. This is frequently done in children. An earlobe or a heel (especially in newborns or infants) can also be used.

For a finger stick, the nurse or technician will wash your skin with alcohol and then make a quick prick with the blade designed to obtain blood from capillaries—hair-sized vessels that connect the smallest of veins to the smallest of arteries. He or she will then gently squeeze your finger to produce drops of blood that are gathered into micropipettes (tiny glass or plastic straws) or very tiny tubes. Because capillaries are so small, they usually produce only enough blood for a few tests, and the blood flow quickly ceases. There are no precautions to take after the test; you may not even need a bandage.

Urine. Urine for analysis can be obtained in several different ways. The most common method is a random (also called a "spot" specimen that is used for the standard chemical and microscopic urinalysis. It simply requires you to urinate into a cup, jar, bottle, or tube. The container must be thoroughly clean and dry, but it needn't be sterile. If the specimen is not directly examined, or sent within a few hours to a laboratory, it should be refrigerated.

A clean catch specimen requires that you thoroughly clean your external genital area with a mild soap and water and then dry off the area before urinating into a clean, dry container. This is because skin naturally contains bacteria that could obscure bacteria from your urinary tract or falsely indicate an infection. Your doctor will commonly ask for a clean catch specimen if a kidney or bladder infection is suspected.

If a sterile urine specimen is needed to identify a specific bacterium, a doctor or nurse will obtain it by catheterization. While you are lying down, a catheter—a thin, flexible tube—is inserted into the outer opening of your urethra, the tube through which urine from your bladder leaves your body. Urine is then drained into a sterile specimen container. This technique is often used for patients who cannot urinate voluntarily. If that is the case, the catheter may be left in place.

Sometimes your doctor may request a timed test, which measures the quantity of a substance excreted in the urine over a period of time—typically 24 hours, but occasionally two, six, or 12 hours. For a 24-hour urine test, you begin by voiding and discarding the first specimen. This is because the substance being measured has to be estimated over an exact period of time. Including the first specimen, which has been building up in your bladder over an unspecified amount of time, will throw off the measurement of the substance (such as a hormone) being tested. After discarding the first specimen, you collect the rest of the urine you void over the specified time period in a clean plastic jug, which may contain a preservative (for specific instructions, see chapter 12. In the laboratory, the total volume of urine is measured and an aliquot, or sample of the total volume, is used for the analysis.

Sputum and Other Specimens. Sputum (phlegm) is the product of a deep cough and can be collected directly into a clean, widemouthed plastic or glass specimen container. Sometimes you may be asked to cough directly into a Petri dish. This round, shallow glass or plastic dish with a cover contains a gel-like substance, or medium, in which bacteria will grow.

Other body fluids (cerebrospinal, pleural, abdominal, or joint) are obtained by aspiration. After a local anesthetic is injected or applied to your skin, a fine needle is injected into the appropriate body cavity or joint, and a small amount of fluid is aspirated (withdrawn). The fluid can then be examined for its microscopic cells or chemical constituents, or cultured for infectious agents.

Feces specimens need to be collected directly into a clean, dry cardboard or plastic container.

Common Clinical Laboratory Tests

Clinical laboratories are typically divided into several specialty laboratories, which may include hematology, clinical chemistry, immunology and serology, microbiology, and the blood bank. The tests described in this table are arranged according to the laboratory in which they are usually performed (indicated in boldface).

Hematology: This laboratory examines the formed or cellular elements of blood, which include red blood cells, white blood cells, and platelets (cells necessary for clotting); the amounts of clotting factors; and the types and amounts of hemoglobin, the red pigment in red blood cells. Such disorders as anemias, hemophilia and other blood-clotting disorders, and leukemias are first diagnosed and then monitored in this laboratory. Organizationally, the urinalysis laboratory is often grouped with hematology. Urine is examined both chemically and microscopically.

Test	What It Shows/What It's Used For
Complete blood count (CBC) (includes white blood cell count, red blood cell count, hematocrit, red blood cell morphology and indices) and differential (the proportion of the various types of white blood cells in the blood)	Identifies anemias, some cancers such as leukemias and lymphomas; evaluates blood loss and response to infection. Sometimes only a part of the CBC is performed (eg, white blood cell count or hematocrit). Often used as a general screening test before surgery or as a part of a routine medical checkup.
Platelet count, fibrinogen	Evaluates, diagnoses, and monitors bleeding and coagulation (clotting) disorders.
Prothrombin time (PT), partial thromboplastin time (PTT), and specific clotting factor assays	Monitors anticoagulation therapy (PT, PTT); evaluates bleeding and coagulation disorders such as hemophilia.
Reticulocyte count	Assesses red blood cell production.
Routine urinalysis (includes color; pH; specific gravity; turbidity; chemical analysis for occult blood, protein, ketones, and glucose; and microscopic examination of sediment for red blood cells, white blood cells, bacteria, crystals, and casts)	Indicates kidney and bladder infections and other diseases, certain metabolic and systemic diseases, dehydration, and urinary tract bleeding.

Clinical chemistry: This laboratory is concerned primarily with measuring the amounts or concentrations of various chemical constituents of blood, and less often with simply identifying their presence. The scope of clinical chemistry is very broad, and many different tests can be done to assess various substances found in blood or urine. The major ones are listed below.

Enzymes: Levels of enzyme activity in blood help determine which organs are damaged or diseased and to what extent. When organs or tissues are damaged, enzymes leak out into the blood. The following are examples of enzymes produced by various organs:

Test	What It Shows/What It's Used For
Heart
Creatine kinase (CK)	Early marker for acute myocardial infarction (heart attack). Also present in skeletal muscle. CK-NB is a form of CK that is mostly found in the heart muscle and provides more specific information about heart damage.
Lactate dehydrogenase (LDH)	Later marker for acute myocardial infarction. Present in all organs and also released into blood in disorders of liver, kidneys, red blood cells, and muscle. Isoenzymes, or forms of the enzyme that are specific for different organs, can help pinpoint the source of LDH elevations.
Liver
Alanine aminotransferase (AAT, SGOT) and aspartate aminotransferase (AST, SGPT)	Elevated in many types of liver disorders including hepatitis. May also be abnormal with damage to several other organs or tissues.
Alkaline phosphatase	Elevated in obstructive liver disease, in which excretion of bile by the liver is impaired. The causes include gallstones, tumors, and some forms and stages of hepatitis. Also elevated in bone disease, including Paget's disease, vitamin D deficiency (rickets), hyperparathyroidism, and cancer that has metastasized to the bone. Because their bones are growing, healthy children have higher values than adults.
Pancreas
Amylase and lipase	Elevated in inflammation of the pancreas (pancreatitis), and less often in cancer of the pancreas.

Hormones: Hormone levels in the blood are used to evaluate the function of various endocrine glands and can indicate hyper- (over) and hypo- (under) activity.

Test	What It Shows/What It's Used For
Cortisol	Adrenal gland function.
Catecholamines	Adrenal gland: elevated with uncommon tumor of adrenal gland that can cause hypertension.
Thyroxine (T4), TSH, T4 indices	Thyroid gland function.
ACTH, FSH, LH, GH (growth hormone), TSH	Pituitary gland function; directly relates to function of adrenal glands, sex glands, and thyroid gland.
Parathormone	Parathyroid gland function.

Lipids and lipoproteins: These help evaluate risk of coronary heart disease. They are also sometimes used as markers of liver disease and nutritional status.

Test	What It Shows/What It's Used For
Cholesterol	General but not absolute marker of coronary heart disease risk.
High- and low-density lipoprotein cholesterol (HDL and LDL)	Breakdown of cholesterol that provides better estimate of risk than does total cholesterol alone.
Triglycerides	With cholesterol, used to evaluate coronary heart disease risk.

Proteins: These reflect metabolic and nutritional status in a wide variety of disorders, and overproduction in some cancers.

Test	What It Shows/What It's Used For
Albumin	Reduced in some forms of liver and kidney disease, and in malnutrition.
Globulins	Elevated in some chronic infectious and inflammatory illnesses and some blood cancers. This test includes globulins or antibodies produced by the body in response to infections and allergens. Abnormal globulins can be detected in multiple myeloma and related disorders. Protein electrophoresis fractionates serum proteins into various classes, which allows for more specific diagnoses.

Electrolytes: These tests help to identify and evaluate such metabolic disorders as acidosis, alkalosis, malnutrition, dehydration, and various bone, kidney, and endocrine gland disorders. Results of these tests are nonspecific, and can be abnormal in a variety of disorders too numerous to include here (although a few are listed in table 4.2). Electrolytes are also affected by megadoses of vitamins and minerals, and by such drugs as diuretics and antacids.

Marked abnormalities in electrolytes can have important, and sometimes urgent, medical consequences and therefore require rapid intervention and treatment. In seriously ill, hospitalized patients, these tests may need to be monitored frequently, so that any abnormalities can be quickly corrected.

Electrolytes, which may vary individually or in concert with each other, are often measured in a group. The term usually refers to sodium, potassium, chloride, and bicarbonate (some labs report CO2 instead), but may also include calcium, phosphorus, and magnesium.

Blood glucose (blood sugar): The glucose tolerance test is used to assess the handling of glucose by the body. In one form of diabetes (Type 1), it reflects insulin release by the pancreas. In the other (Type 2), it reflects insulin sensitivity of various body tissues, such as liver and muscle. It is also used to assess low blood glucose, although less frequently.

Test	What It Shows/What It's Used For
Glucose, fasting	Diagnoses and monitors diabetes mellitus, evaluates and diagnoses other disorders of carbohydrate metabolism, and diagnoses hypoglycemia (low blood sugar).
Glucose tolerance test	Follow-up test that allows more specific diagnosis of diabetes mellitus after finding elevated fasting blood glucose levels.

Other metabolic products:

Test	What It Shows/What It's Used For
BUN (blood urea nitrogen) and creatinine	Measures these metabolic waste products eliminated by the kidneys. Elevated when kidney filtration function is impaired and in dehydration.
Uric acid	Measures these metabolic waste products derived from proteins. Elevated in gout, in some forms of kidney disease, and with excessive tissue destruction.

Vitamins and trace elements: Vitamin and trace element (mineral) levels can indicate deficiencies that can be responsible for anemias and nervous system and metabolic disorders, as well as excess due to industrial or environmental exposure, which can result in symptoms and signs of toxicity or poisoning. (For testing for substance abuse, see chapter 29.

Test	What It Shows/What It's Used For
Folic acid	Evaluates anemia.
Vitamin B12	Evaluates anemia and neurological symptoms.
Other vitamins (thiamine, C)	Only very rarely measured to evaluate various unexplained symptoms consistent with vitamin deficiency.
Lead	Unexplained anemia and/or neurological symptoms. Screening of infants and young children for environmental exposure.
Mercury, arsenic	Unexplained neurological symptoms; suspicion of poisoning.

Immunology and serology: This laboratory is involved with identifying antibodies (proteins produced in the body in response to an antigen, which can be an infectious agent, virus, toxin, or other foreign substance) or in the diagnosis of autoimmune diseases (antibodies against the body's own tissues) and immunodeficiency states (indicative of an underactive immune system).

Test	What It Shows/What It's Used For