What are laboratory tests?

Indications and Procedures

Clinical laboratory testing is a vital element in diagnosis. After physical examination and the taking of the patient’s medical history, the physician will often request that specific tests be performed on blood, urine, or other body fluids. Appropriate specimens are collected and forwarded to the laboratory for specimen processing.

Blood
is the most common specimen submitted for testing in the clinical laboratory. In a hospital or large referral laboratory, there may be special personnel, called phlebotomists, employed to collect blood. In a small office laboratory, blood may be collected by the attending physician or nurse. Blood is collected in a syringe or in special tubes that may contain anticoagulants.

Urine
is the next most common laboratory specimen and is collected as a result of a single void (random urine specimen) or for a time period of twenty-four hours or more. In the latter case, the collection container may also contain substances that act as a preservative. If a long-term urine specimen is necessary, it is very important for the patient to follow the directions regarding collection. Failure to follow these directions can lead to erroneous laboratory results.

Less commonly collected specimens include
cerebrospinal fluid, gastric (stomach) fluid, and amniotic fluid. Cerebrospinal fluid is usually collected by a physician by direct sampling with a needle (lumbar puncture, or spinal tap). Gastric fluid is obtained by the insertion of a gastric collection tube. Amniotic fluid is collected by an obstetrician in the process called
amniocentesis, in which a sample of the fluid surrounding the fetus is removed by the insertion of a needle through the mother’s abdomen. Frequently, laboratory tests are also ordered on infectious material associated with a wound or surgical incision.

A major aspect of specimen collection is ensuring that the sample is correctly labeled and that no mix-up of specimens has occurred. Part of this process may involve checking identification armbands or asking patients or nursing staff to confirm identification. While this procedure may be exasperating to the patient or nursing personnel, it is a necessary part of detecting errors.

Immediately after the specimen is received in the laboratory, documentation of time of receipt and the tests requested is made, which is referred to as logging in the specimen. Each sample receives a special code called an accession number. The test performance and results are tracked with this number, since multiple specimens can be received on a single patient in a given day. This process is usually computerized and may use bar code labeling in a process very similar to that used for automatic cash-register pricing of grocery items.

In large hospital or referral laboratories, the processing center is responsible for distributing the sample to the laboratory sections, where various tests are performed. Since each test requires a specific amount of sample, specimen processing also involves determining that the correct amount of fluid has been collected and reserved for proper performance of the test.

For blood specimens, many laboratory determinations are made regarding plasma, or serum, which is the liquid component of blood that contains no cells. The whole blood specimen is separated into cellular and liquid components by centrifugation. The sample is spun rapidly so that the force of the spin sediments the cells, with the serum or plasma layer on top.

Once the specimen is distributed to the pertinent laboratory sections, testing is done using a variety of analytical techniques. The testing methodology is almost as varied as the types of analyses requested. A few general statements, however, are applicable. Automation is the guiding force behind laboratory test methodology development. Routinely ordered tests are done with instruments specifically designed to perform a group or panel of tests, rather than each test being performed individually by a technologist using manual chemistry methods. Automation coupled with computerization has greatly increased laboratory efficiency, decreased turnaround time (the time required for a test to be performed and results to be reported to the physician), eliminated human errors, and allowed more tests to be performed on smaller sample material. The latter advantage is particularly important for pediatric specimens, in which sample size is usually an important consideration. Automation also eliminates much of the technologist’s contact with the specimen, considerably reducing the risk of spreading infectious diseases.

Each section of the laboratory is responsible for a specific set of tests. The chemistry section performs chemical analyses of body fluids. Panels of tests related to kidney, heart, and liver function are also done. In addition, tests to measure amounts of therapeutic drugs, hormones, blood proteins, and cancer-related proteins are accomplished with immunoassay techniques. The development of antibody-related techniques has revolutionized testing in all areas of the clinical laboratory. The ability to customize antibody production and adapt it to specific analytical requirements has allowed the continual development of new tests and methodologies.

The
hematology section is responsible for monitoring the levels of blood cells and clotting factors. Other specialized tests to diagnose cancer of the blood cells may also be done. Blood typing and donor testing are technically hematology-related tests, but they are usually reserved for a separate section designated as
blood bank or transfusion services. Transfusion service is a specialty in its own right and is almost always reserved for hospital-associated laboratories.

Microbiology is the section where body fluids are checked for infectious microorganisms. Once an organism is identified, the section can also determine which antibiotics may be useful for treatment by performing antibiotic susceptibility tests.

As the laboratory tests are performed, the results are recorded and reported to the physician. Computerization has permitted the transfer of patient results directly from the instrument performing the test to the patient’s file, eliminating many tedious and error-prone clerical functions.

For hospital and reference laboratories, a laboratory director—either a physician (usually a pathologist) who specializes in laboratory medicine or a scientist with doctoral level training in a laboratory specialty—monitors the performance of the laboratory, helps physicians with the interpretation of ambiguous or complex laboratory results, and provides guidance on the introduction of new tests or instrumentation. Most laboratories also have a section supervisor or administrator who is an experienced medical technologist to oversee the daily laboratory routine.

Uses and Complications

Because of the variety of laboratory testing, it is impractical to cover its applications in depth in a brief review. Instead, a few illustrative tests that are performed often or are associated with familiar disorders will be presented. The most frequently ordered laboratory tests are serum glucose tests, serum electrolyte (salt) level measurements, and complete blood count (CBC) tests.

The maintenance of blood glucose (sugar) levels is essential for body activity and brain function. The laboratory measurement of blood glucose is one of the oldest known procedures performed in the clinical laboratory. It is part of the diagnostic procedures used to monitor and test for
diabetes mellitus. Glucose and electrolyte testing are performed in the chemistry section of the laboratory, while a CBC takes place in hematology. Certain levels of electrolytes—sodium, chloride, potassium, and calcium—are needed for proper cardiac function. An abnormal level of these salts could also indicate possible hormonal or kidney malfunction. The CBC is a measure of the cell populations that carry oxygen (red blood cells), fight infection or invasion by foreign substances (white blood cells), and activate the blood-clotting mechanism (platelets). The white cell population is elevated in infections but also in cases of leukemia (malignant growth of a white cell population). More specialized testing is needed when leukemia is suspected. An instrument called a flow cytometer can be used to count and detect subtypes of white cells. These data, along with a pathologist’s microscopic examination of a blood smear and the results of clinical examination, are used to arrive at a diagnosis of the specific type of leukemia present. The identification of the cell population causing the cancer is important for determining treatment and prognosis.

A deficiency of red cells or their oxygen-carrying hemoglobin
molecule is called anemia. It can be caused by iron deficiency and other impairments of red cell production, chronic bleeding, or accelerated red cell destruction (hemolysis). Each of the causes must be either confirmed or ruled out through additional testing or by clinical examination.

Platelet deficiency is a major cause of clotting disorders, although many other causes of bleeding disorders exist. The specific defect can be determined by measuring the clotting time and by using special immunoassays to measure clotting substances in the blood.

Many hormonal (endocrine) disorders can be diagnosed through laboratory testing. For example, the thyroid, the regulator gland for body metabolism, can produce a variety of symptoms when it is not functioning properly. Thyroid testing is the most common endocrine-related laboratory procedure requested by physicians. The blood levels of thyroid hormone and of the pituitary factor that stimulates the thyroid gland are measured in the laboratory using immunoassay methods. These types of assays can also be used to monitor other hormones involved in fertility, growth, and the function of the adrenal gland (the gland that helps maintain sugar metabolism and electrolyte balance).

Immunoassay methodology has also permitted the routine laboratory testing of therapeutic drugs as well as of drugs of abuse. In the past, the technology for analyzing drugs in biological fluids involved expensive, labor-intensive techniques that were impractical for routine laboratory use. With the introduction of immunologically based testing for drugs, however, it became possible to monitor patients on antibiotics, immunosuppressive agents, cardiac drugs, and antiseizure medication. Testing has been automated so that these drug levels can be performed as routine laboratory procedures. Assay results can be used to establish an individual dosage schedule so that dosage is maintained in the therapeutic range and does not exceed the concentration threshold, leading to toxic effects, or decline to values too low to achieve adequate treatment (subtherapeutic levels).

A continuing research effort is directed toward developing specific diagnostic cancer tests. These tests could be used to screen patients for tumors in order to detect them early, when therapy would be most effective. Substances that appear in body fluids coincident with the growth of tumors are referred to as tumor markers. The ideal tumor marker would appear only in patients afflicted with a specific type of cancer. Its concentration would reflect the size of the tumor as well as the presence of metastasis, in which tumor cells migrate from the initial cancer site to other sites in the body.

The ideal tumor marker has not yet been discovered. Most have not been specific or sensitive enough to use as a screening tool for detecting tumors, although they have been useful for monitoring the effects of therapy. One example of a useful marker is prostate-specific antigen (PSA). The level of this protein in serum is very low when the prostate gland is normal. When prostate cancer is present, however, the serum level, as measured by immunoassay, is elevated. The test can also be used for screening, provided that any positive result is confirmed by clinical examination. It is also used following prostate surgery or radiation therapy in order to determine the completeness of tumor removal. Continually high or rising levels of PSA in the serum following treatment indicate that residual tumor is still present.

In the
microbiology department, the culturing of body fluids and antibiotic susceptibility studies allow the selection of the most appropriate antibiotic for treatment. The course and duration of treatment can then be followed in the chemistry laboratory using the therapeutic drug monitoring techniques discussed above. When an infection is suspected, body fluids are cultured or incubated with media selected to grow only specific microorganisms. Antibiotic susceptibility studies are performed by culturing the organism with various antibiotics until growth is arrested. Many strains of bacteria and other microorganisms will become resistant to an antibiotic that had proven effective previously, and patients who are allergic to some antibiotics may need to be treated with an alternative regimen.

The detection and identification of viruses has become a subspecialty in microbiology with distinctly different culturing techniques. Newer immunoassay methods and other biotechnologically based methods have made virus diagnosis easier.
Acquired immunodeficiency syndrome (AIDS) testing is a prime example of the application of immunoassay techniques to virology testing. A detection technique that required growth of the
human immunodeficiency virus (HIV) in the laboratory would be extraordinarily difficult and tedious. It would also be prohibitively expensive and time-consuming to screen large populations such as blood donors and high-risk groups. Instead, laboratory screening for HIV uses an automated immunoassay technique based on the detection of patient antibodies to virus-specific antigens. Although this test is very specific, the possibility of false positives is greatly minimized by confirming all positive screening results with another antibody test called a Western blot. In this test, a serum sample from a suspected HIV-positive patient is applied to a membrane impregnated with virus proteins. The virus proteins are localized at a characteristic position determined by their migration rate when the membrane coated with virus proteins is subjected to an electric field in a process called protein electrophoresis. After the membrane has been treated with patient serum and color development reagents, the presence in the patient sample of an antibody to one or more of these proteins is revealed as a colored stripe on the membrane. A combination of the two tests is a cost-efficient and extremely accurate procedure to confirm a suspected diagnosis of HIV infection.

Perspective and Prospects

According to a 2002 study of the history of the clinical laboratory by J. Büttner, the concept of the modern hospital laboratory was first documented in 1791, when French physician and chemist Antoine-François de Fourcroy wrote that in hospitals, “a chemical laboratory should be set up not far away from a ward having twenty or thirty beds.” Büttner asserts that the two suppositions necessary for the creation of these laboratories were the idea that the results of laboratory examinations can be used as “chemical signs” in medical diagnosis and a new concept of disease that was the result of the “birth of the clinic” at the end of the eighteenth century.

During this phase of laboratory development, investigations were performed at patients’ bedsides by physicians themselves. In the period from 1840 to 1855, clinical laboratories were established as operations distinct from hospitals and clinics. Most of these laboratories were developed in German-speaking countries and staffed by scientists who performed tests for the hospitals and taught medical students physiological chemistry. From 1855 onward, the concept of the clinical laboratory spread rapidly, with clinicians assuming directorship roles. The laboratory ultimately serving as a model for clinical laboratories in the United States was established by the renowned pathologist Rudolf Virchow at Berlin University. As the chair for pathological anatomy, he set up a “chemical department” within the institute for pathology in 1856. This laboratory represented a center of clinical chemistry research and established the clinical laboratory as integral to pathology.

Laboratories have evolved as essential but distinctly separate specialties of medical services. Although there is little or no participation in the analytical process by the physicians ordering the tests, a major part of a physician’s diagnostic skill is knowing which tests to order as a supplement to examination and medical history. Laboratory tests cost money and time, and they may be useless in the diagnostic process if not ordered in a judicious fashion. The old medical admonishment to “treat the patient, not the laboratory result” is still an appropriate consideration. Moreover, responsibility for the correct interpretation of the results lies with the attending physician, who has access to all the pertinent patient data.

Laboratory results are usually interpreted with the help of a reference range. Reference ranges ideally represent laboratory values characteristic of a sample population that is free of known disease. If the results lie within this range, however, the laboratory result cannot always be assumed to rule out a specific diagnosis. Since considerable biological variation exists for most laboratory values, diseased individuals can sometimes yield test values in the normal range, and, conversely, healthy individuals can occasionally have low or elevated values.

To verify a diagnosis, all laboratory results and clinical impressions should complement one another. The detection of blood-clotting deficiencies by the hematology department could be related to a poorly functioning liver, which will also be reflected in changes in enzymes and blood proteins measured in the chemistry laboratory. Cardiac disorders are diagnosed not only by examining an electrocardiograph (EKG or ECG) but also by measuring the levels of specific cardiac-related enzymes that rise to abnormally high levels when cardiac blood supply is diminished (such as with myocardial infarction, or heart attack). In summary, the clinical laboratory provides a valuable tool for physicians, but it should never displace clinical examination and medical history as methods of determining the final diagnosis.

Bibliography

Bennington, James L., ed. Saunders Dictionary and Encyclopedia of Laboratory Medicine and Technology. Philadelphia: W. B. Saunders, 1984.

Cavanaugh, Bonita Morrow. Nurse’s Manual of Laboratory and Diagnostic Tests. 4th ed. Philadelphia: F. A. Davis, 2003.

Griffith, H. Winter. Complete Guide to Symptoms, Illness, and Surgery. Revised and updated by Stephen Moore and Kenneth Yoder. 5th ed. New York: Perigee, 2006.

Lab Tests Online. "Understanding Your Tests." American Association for Clinical Chemistry, November 5, 2012.

McPherson, Richard A., and Matthew R. Pincus, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia: Saunders/Elsevier, 2007.

MedlinePlus. "Laboratory Tests." MedlinePlus, June 20, 2013.

National Heart, Lung, and Blood Institute. "What Do Blood Tests Show?" National Institutes of Health, January 6, 2012.

Pagana, Kathleen Deska, and Timothy J. Pagana. Mosby’s Diagnostic and Laboratory Test Reference. 9th ed. St. Louis, Mo.: Mosby/Elsevier, 2009.

Price, Christopher P., and David J. Newman, eds. Principles and Practice of Immunoassay. 2d ed. New York: Stockton Press, 1997.

Wu, Alan H. B., ed. Tietz Clinical Guide to Laboratory Tests. 4th ed. St. Louis, Mo.: Saunders/Elsevier, 2006.