clinical pathology

(pathology) A medical specialty encompassing the diagnostic study of disease by means of laboratory tests of material from the living patient.

The branch of general pathology directed to the diagnosis and monitoring of diseases through the examination of blood, body fluids, secretions, and tissue biopsy specimens for chemical, morphological, microbiological, and immunological abnormalities. A variety of test procedures and microscopic examinations are performed in clinical pathology laboratories, located within hospitals, clinics, and stand-alone facilities, either privately owned or operated under the auspices of local, state, and federal public health agencies. Clinical laboratories function to confirm a clinical impression, establish or rule out a diagnosis, monitor therapy, and help establish a prognosis. Under guidelines established by the Clinical Laboratory Improvement Act of 1988, laboratory services are divided into several levels of activities. Many of the less complicated waived tests may be performed in physicians' office laboratories; more complex tests can be performed only within hospital laboratories or in laboratories where sophisticated state-of-the-art equipment is available.

The clinical pathology laboratory is commonly divided into four major subdisciplines or sections: chemical pathology, hematology, immunohematology (blood bank), and microbiology. Each includes several subsections of activity.

Chemical pathology

The chemical constituents of serum, urine, and other body fluids are analyzed in the chemical pathology section of the laboratory. Chemical tests are then clustered into functional groups to help diagnose diseases of the heart, kidney, liver, gastrointestinal tract, and other organ systems. Simple and high-volume tests are usually performed in the routine chemistry laboratory; more complicated, labor-intensive tests are carried out in a special chemistry section.

In the toxicology subsection of the chemical pathology laboratory, the blood, urine, and other body fluids are analyzed for the presence of drugs and substances of abuse. An equally important application of toxicology testing is to measure the blood levels of therapeutic drugs to assure that concentrations are adequate to treat the disease but not so high as to cause toxic side effects.

Radioimmunoassay allows trace quantities of substances to be measured. This procedure involves adding radioactive chemical markers to properly prepared specimens and detecting their binding to the substances being measured.

Because the use of radioactive substances requires licensure of personnel presents problems in disposal of biohazard waste, and requires expensive analytic instruments, most laboratories have substituted one or more enzyme-linked immunoassay (ELISA) techniques, in which specific enzymes instead of radioisotopes are linked to protein markers.

Electrophoresis is used to separate and identify proteins and lipoproteins.

Urinalysis is still carried out by manual procedures in many laboratories, relying on reagent-impregnated filter paper strips for the detection of protein, glucose, ketones, hemoglobin, bilirubin, and other substances, and microscopic observations for casts, crystals, red and white blood cells, and bacteria. However, automated instruments are used in higher-volume laboratories. Specific gravity is measured by a mass gravity meter, and urine chemistries by standard reflectance spectrophotometry; microscopic analysis is facilitated with an automated intelligent microscopy system. See also Immunoassay; Radioimmunoassay; Serology; Toxicology; Urinalysis.

Hematology

The cornerstone of clinical laboratory work in hematology is the assessment of the cellular elements (red blood cells, white blood cells, and platelets) in blood samples. The blood cells may be enumerated, either by manual cell-counting techniques or by automated particle-sensing and -sizing instruments. Microscopic observation of stained peripheral blood smears is limited to assessing the morphology of atypical cells as they may appear in cases of dysplastic syndromes and overt leukemias. In complicated cases where the diagnosis of a hematologic disorder cannot be made by study of the peripheral blood smear, a bone marrow examination may be necessary. Special procedures are available by which specific markers on the surfaces of blood cells can be detected by using specific reactants, thereby providing a means for classifying leukemias and other hematologic disorders.

Flow cytometry is an extension of the particle-sensing principles used in the automated blood counting instruments. In the flow cytometer, cells are dispersed in fluid suspension and channeled through a narrow-bore tube; as each cell passes by a narrow laser light beam, an optical signal is translated into electronic pulses that are measured and analyzed in the computer processor. Signals may be scattered light that relates to the mass of each cell, or to the magnitude and type of fluorescence produced, which reflects the internal structures of cells depending upon the dyes used to stain the cells. Coagulation is an important subdiscipline of hematology involving the study of clotting and bleeding disorders. People especially prone to developing blood clots often receive anticoagulant drugs in order to “thin” the blood. The amount of anticoagulant given must be regulated, based on the results of a clotting test known as the prothrombin time. A test known as the partial thromboplastin time is also frequently performed in clinical laboratories to assist in the assessment of patients who have one of various bleeding disorders. The evaluation of platelets in the peripheral blood is also an important step in evaluating patients with coagulopathies. See also Blood.

Immunohematology (blood bank)

The primary function of the blood bank is to ensure that blood taken from a donor will be safe to give to a recipient (that is, will not produce a transfusion reaction or transmit an infectious disease). Blood from a recipient must be subjected to compatibility testing against the blood of each donor to ensure that no atypical reaction occurs. Donor-recipient blood compatibility must be established for both the ABO blood group system and the Rh system. The recipient's plasma must also be subjected to an antibody screen to detect any atypical antibodies. If an atypical antibody is found, donor units must be carefully screened to select for transfusion only those with red cells lacking the antigens corresponding to the recipient's atypical antibody. Transfusion therapy has become a highly advanced discipline. It requires the separation of whole blood into a variety of components—packed red blood cells, platelet concentrates, fresh-frozen plasma, and cryoprecipitate.

Donor recruitment programs to maintain an adequate supply of blood are provided by many blood banks. Other services include outpatient therapeutic transfusions; phlebotomy of individuals who have too many red blood cells; and plasmapheresis, a procedure where plasma or platelets can be separated from the withdrawn blood and the formed elements (red cells and platelets) can be immediately retransfused, eliminating the chance for inducing anemia or deficiencies of other blood components. See also Blood groups.

Microbiology

The primary task of the clinical microbiologist is to identify as quickly as possible any microorganism in a specimen that represents the possible causative agent of an infectious disease. Presumptive identification of microbes can be made by microscopically examining direct mounts of an appropriate portion of the specimen or thin smears that have been stained with one of a variety of dyes. Rapid presumptive diagnoses can also be made by directly testing specimens with a variety of immunological reagents.

The practice of general bacteriology still follows traditional culture methods that were introduced by Robert Koch before the turn of the century. Specimens are applied to the surface of a variety of agar culture media for the purpose of recovering in pure culture any bacterial species that may be clinically significant. Gram stains may determine the cellular morphology and staining characteristics of the bacteria, and a variety of rapid, direct tests can be performed to provide an early identification.

Bacterial identifications and antibiotic susceptibility tests may be performed in a variety of packaged systems. These systems typically use lyophilized, dried chemical substrates or antibiotics at various concentrations that are contained within microwells that are stamped into plastic polystyrene cards or trays. These cards or trays are placed in a 95°F (35°C) incubator after the microwells have been inoculated with a suspension of a pure culture of the bacterium to be identified.

The availability of monoclonal antibodies and nucleic acid probes, in which a highly specific portion of antigen or a small segment of nucleic acid can be tagged with either a fluorescent or an enzyme-linked detector, revolutionized the ability to detect specific microbes in biologic specimens and rapidly confirm the results of a culture.

The laboratory identification of fungi and the diagnosis of fungal infections is similar to that described for the bacteria. Specimens are inoculated on special fungal media, the plates are incubated for periods as long as 4 weeks, and the growth of any mold or yeast is identified morphologically and biochemically. Nucleic acid probes are available to quickly confirm any fungus colony suspected of being one of the dangerous pathogens. The laboratory identification of parasites involves detecting microscopically the typical forms in body fluids and secretions. Viruses can live only in viable cells and, for the most part, can survive briefly outside human or animal hosts. Therefore, culture techniques must use embryonated eggs, cell culture suspensions, thin cell sheets called monolayers, or laboratory animals. Species of viruses are identified by observing their ability to produce certain cytopathic effects in the cells where they are growing or to cause recognizable diseases in laboratory animals.

Clinical immunology is that discipline in which infectious diseases are diagnosed by detecting antibodies in serum and other body fluids. In practice, immunologic and serologic techniques are used to diagnose an infectious disease when the agent may be too difficult to recover in culture. See also Clinical microbiology; Culture; Monoclonal antibodies.

1. The practice of pathology as it pertains to the care of patients.
2. The subspecialty in pathology concerned with the theoretical and technical aspects of laboratory technology that pertain to the diagnosis and prevention of disease.