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The needle is then withdrawn from the vein, and the tourniquet removed if it hasn't been removed earlier.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

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Table 4.1

Common Clinical Laboratory Tests

Laboratory Profiles

When you go for a routine checkup or for evaluation of a specific symptom, your doctor will often order a set list of tests called a laboratory, chemical, or biochemical profile. This standard series of tests is done on a single blood or urine specimen. The reasoning behind this is that screening tests occasionally lead to an unsuspected finding, and with modern automated instruments, as many as 30 common tests can often be done for the same cost as four or five tests ordered separately. Although laboratories differ, the most common component tests are among those described in the laboratory sections of this chapter (for a typical group of tests, see table 4.2).

The value of laboratory profiles is somewhat controversial, however, since such broad screening tests rarely lead to the diagnosis of unsuspected illnesses. Moreover, they can result in abnormal findings in the absence of disease, possibly leading to unnecessary workups. The trend among doctors now is to be very selective and to order tests based only on specific patient complaints. In fact, since March 1996, Medicare policy has been to cover only specific blood tests considered medically necessary based on symptoms, family history, or risk profiles for a specific disease. Although many private health insurers continue to pay for laboratory profiles, such organizations as the American College of Physicians (ACP) recommend that they not be done on healthy patients without specific risk factors.

A more effective variation of the general profile concept is an organ profile. This is ordered when a patient has symptoms that suggest a disease of a specific organ or organ system. Common examples would be a liver profile, a lipid profile, or a thyroid profile. In these instances, the tests comprising the profile, when considered in the context of one another, provide a much more complete picture of the condition of an organ or organ system than any single test by itself. In a sense, a complete blood count (CBC), which includes a hematocrit, red and white blood cell counts, a hemoglobin measurement, and a white blood cell differential, can be considered a hematology profile. Similarly, profiles can be customized according to a clinical problem.

Testing Technology

Automation and computerization have increased dramatically in clinical pathology laboratories, allowing lab staff to keep up with the high volume and array of tests now available. Conversion from manual to automated procedures means that tests can be done rapidly, in large batches, with very small or "micro" amounts of specimens, and by fewer staff. The computer-calculated results are highly accurate because the potential for human error that can occur with each step of multiple-step processes has been eliminated.

Many clinical chemistry tests are routinely processed on large instruments called multichannel analyzers, by only one or two technicians, generating up to 20 different test results per minute on a single patient's blood specimen. Various instruments allow for about 80% of tests in clinical chemistry to be highly automated. The remaining 20% of technically complex and highly specialized tests that are less automated or performed manually can consume more than half of laboratory staff time.

Automation has not only enhanced speed and accuracy, but has also allowed computerized reporting of test data. Often different analyzers from different laboratories can be linked, enabling laboratories to produce a single report of a patient's tests from all laboratories. For hospital patients these reports can be generated daily, or even more frequently, and can show data from previous days for purposes of comparison or creating comprehensive records. Results from the laboratory can even be displayed directly on terminals in patient care units.

Clinical laboratories use all of the technologies described below in analyzing test data.

Immunoassays. This group of laboratory techniques is used to identify such diverse substances as infectious diseases, hormones, vitamins, drugs, cardiac enzymes, and antigens (proteins) associated with cancer. These techniques are exquisitely sensitive and, in some instances, capable of measuring less than a billionth of a gram of a biological substance.

Immunoassay technology is based on the antigen-antibody response, that is, the ability of the body to develop antibodies (proteins made by the immune system) to protect against antigens (proteins on the surface of invaders such as bacteria, viruses, or allergens). Since there is a specific antibody for each specific antigen, it is possible to test for one using the other. That is, if one is placed in close proximity to the other, such as happens when a small amount of a known antigen is placed in a test tube that is then filled with blood that contains an antibody for it, they will bind together and can be identified by using a special marker.

Immunoassay testing relies on tagging antibodies or antigens with different markers such as radioisotopes, enzymes, or fluorescent chemicals, in order to make the substance tested for visible. For example, in radioimmunoassay (RIA), the antigens are tagged with a radioisotope; in some nonisotopic immunoassay, with a special dye that shows up as glowing particles under a fluorescent microscope. In enzyme-linked immunosorbent assay (ELISA)-an increasingly popular technique used in such diverse circumstances as testing for allergies and antibodies to the HIV virus-the antibodies are tagged with enzymes.

There are many types of immunoassays. In addition to RIA and ELISA, the most common ones include enzyme-multiplied immunoassay technique (EMIT), fluorescence polarization immunoassay, and enzyme immunoassay.

Spectrophotometry. This technique measures the intensity of color formed when the substance being tested for reacts with added chemical reagents. It is the basis for most automated tests performed on multichannel analyzers. Applications vary from highly automated tests to simple procedures for blood sugar testing that can be performed at home.

Electrophoresis. This technique is based on the differences in movement of electrically charged particles under the influence of an electrical current. The size, shape, and electrical charge of the particles will affect the direction, distance, and rate of movement and can be used to distinguish between two substances, such as different proteins. There are several variations on this technique, including the following:

Serum protein electrophoresis. Separates serum proteins, which is useful in diagnosing and monitoring multiple myeloma and related disorders, evaluating and monitoring chronic inflammatory conditions, and evaluating and managing kidney disease, liver disease, and nutritional status.

Immunoelectrophoresis. Proteins are separated in an electrical field and further identified by reaction with specific antibodies. This is most often used for diagnosing and classifying multiple myeloma, and also for diagnosing some immunodeficiency states.

Hemoglobin electrophoresis. Separates the various types of hemoglobin (red pigment in red blood cells). Used in diagnosing sickle-cell anemia, thalassemia, and related congenital blood disorders.

Chromatography. A technique for separating substances on the basis of their molecular size, or physical or chemical properties, which is used to measure drugs, some proteins, and hormones, among other substances. Variations of this technique are based on the medium in which the chromatography is performed. These include high-performance liquid chromatography (HPLC), thin-layer chromatography (TLC), and gas-liquid chromatography (GLC).

Mass Spectrometry. In combination with chromatographic techniques described above, this technology makes possible very specific identification and measurement of substances on the basis of their physical structure. It is widely used in screening for illicit drug use, to provide definitive confirmation when an initial immunoassay screen is positive.

Atomic Absorption and Flame Emission Spectrometry. When a solution is converted to the gaseous state in a hot flame, dissolved metals, depending on the conditions, will either emit or absorb light at a wavelength that is characteristic of that element. A type of spectrophotometer measures the amount of light emitted or absorbed. Many trace metals can be measured by these techniques, including the amount of lead in the blood, a measurement of great importance in pediatrics.

Specific Ion Electrode. The potential, or voltage, of specially designed electrodes is altered when exposed to certain elements (ions) in the blood that are critical for normal metabolic functioning. The current generated by the electrode is a measure of the amount of the substance present. This technology is used in many multichannel automated analyzers, as well as smaller instruments that are devoted to urgent laboratory services. Critically important tests that employ specific ion electrodes include those used to measure levels of the electrolytes sodium, potassium, bicarbonate, and calcium.

Automatic Blood Cell Counters. Used in the hematology laboratory for automated counting and typing of blood cells (CBC). These are commonly used for evaluating and monitoring anemia, infection, bleeding, leukemias, and cancer chemotherapy, for screening before surgery, and for general health screening.

Flow Cytometry and Molecular Diagnostics. Two relatively new techniques that are used in both anatomical pathology and clinical pathology laboratories. (For more information, see the discussion of new diagnostic technologies below.)

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Table 4.2

Common Blood Profile Tests and What They Most Often Mean

Test Results and Interpretation

Test results from clinical pathology laboratories are usually represented as numbers, or occasionally, as positive or negative, meaning that the substance or disorder being tested for is or is not present. The numerical values are usually expressed as the amount of a substance present in a given quantity of body fluid. For example, the amount of phosphorus is given in milligrams per deciliter (100 mL) of blood (mg/dL).

For most numerical values, there is a range of what is considered normal. These so-called reference ranges can vary with such factors as sex and age. For example, the normal concentration of serum alkaline phosphatase, a bone enzyme, may be up to three times greater in growing children than in adults. Reference ranges can also vary according to the population being considered. For example, hospitalized patients may have a range of results that differs from that of healthy outpatients.

Although reference ranges are often relatively wide, any given individual may have a narrower range of what is normal. Thus, you may have a test result that falls within the normal range for the general population and still have a disorder. There is no way to perfectly estimate ranges because, in general, only 95% of a population is statistically described by a reference range. Thus, it is also possible for the remaining 5% to fall outside a reference range and still be healthy.

Finally, reference ranges can vary from laboratory to laboratory, depending on the technology used to analyze test results. For these reasons, reference ranges are not given in this book.

Specificity and Sensitivity. The most reliable tests are those that are both sensitive and specific, and thereby minimize the incidence of false-positive and false-negative results. "Sensitivity" refers to the ability of a test to correctly identify individuals who have a given disease or disorder. "Specificity" refers to the ability of a test to correctly exclude individuals who do not have a given disease or disorder.

A false-positive result indicates a disease in a patient who in fact does not have the disease, and these must be minimized to achieve high specificity. A false-negative result indicates that there is no disease in a patient who does have the disease, and these must be minimized to achieve high sensitivity.

An ideal test that is 100% sensitive and specific would detect everyone with a given disease but no one without it. Few, if any, tests achieve this, although many come close. Yet even with the most specific and sensitive tests, misapplication can cause difficulties. For example, even a highly specific test used in a segment of the population that is known to have very few cases of a disease (defined as a low prevalence) can have a high likelihood of yielding a false-positive result in healthy individuals. An illustration of this would be the PSA test for prostate cancer performed on males less than 40 years old: many of the positive results will be false.

Tests are only a piece of the diagnostic process; your doctor's judgment is critical. Presented with clinical laboratory results, your doctor must interpret the information in the context of your history and physical examination, as well as other diagnostic tests. If your test results are negative but your doctor suspects (based on your symptoms and complaints and the physical exam) that you have a specific disease, he or she may elect not to rule it out but to repeat the test or order a different test for the same disorder. The results of several tests taken together can provide a better evaluation of a disorder than a single test alone.

Alternatively, if you have an abnormal test result but no signs or symptoms of a particular condition, your doctor may suspect a false-positive result, which might be caused by elements in your diet or a medication you're taking, or simply because you're one of a small percentage of the population that falls outside the reference range. Finally, if your tests were ordered for monitoring purposes, your doctor may consider changes in results to be more important than any absolute value.

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.

PATIENT TIP

Medications can interfere with some tests. Always advise your doctor of any drugs you are taking. Sometimes a medication must be stopped before a test is performed, but usually notification is all that is necessary.

DID YOU KNOW?

The development of immunoassay technology was considered so significant that its inventors were awarded a Nobel Prize. This technology has become so ubiquitous that it is done manually in specialized laboratories, on automated instruments, in doctors' offices, and in some instances, even at home, as is the case with over-the-counter pregnancy tests.

PATIENT TIPS

• Abnormal test results do not always indicate disease.

• Findings can be affected by factors ranging from medications to diet to athletic conditioning.

• Some tests are more likely than others to produce false-positive results. There is also the possibility of laboratory error.

• Diagnosis or treatment should not be finalized on the basis of a single test, especially if you have no symptoms.

Based on The Yale University School of Medicine Patient's Guide to Medical Tests by Barry L. Zaret M.D., Senior Editor, Copyright © 1997 by Yale University School of Medicine and G. S. Sharpe Communications, Inc. Published under license from Houghton Mifflin Company.
©2000 Microsoft Corporation. All rights reserved.

The above article was copied from http://content.health.msn.com/yale_books/Diagnostics/yale_lab_tests_clinical_pathology.htm, which unfortunately no longer exists. I do however feel it appropriate to still credit "MSN Health with WebMD" for the article.

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