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Laboratory Shldies

Most of the evaluation process for diagnosing an infectious disease is

based on la bora tory studies. These studies are performed to ( 1) isolate

the microorganisms from various body fluids or sites; (2) directly

examine specimens by microscopic, immunologic, or genetic techniques; or (3) assess specific antibody responses to the pathogenS

This diagnostic process is essential to prescribing the most specific

medical regimen possible for the patient.

Hematology

During hematologic studies, a sample of blood is taken and analyzed

to assist in determining the presence of an infectious process or organism. Hematologic procedures used to diagnose infection include leukocyte count, differential white blood ceH (WBC) count, and antibody measuremenr.6

Leukocyte COllnt

Leukocyte, or WBe, count is measured to determine whether an

infectious process is present and should range between 4,500 and

1 1 ,000 ceHs/�1.7 An increase in the number of WBCs, termed leukocytosis, is required for phagocytosis (ceHular destruction of microorganisms) and indicates the presence of an acute infectious process? A decreased WBC count from baseline, termed leukopenia, can indicate

altered immunity or the presence of an infection that exhausts supplies of certain WBCS.7 A decreased WBC count relative to a previously high count (i.e., becoming more within normal limits) may indicate the resolution of an infectious process.7

Differemial White Blood Cell Count

Five types of WBCs exist: lymphocytes, monocytes, neutrophils, basophils, and eosinophils. Specific types of infectious processes can trigger alterations in the values of one or more of these ceHs. Detection of these changes can assist in identification of the type of infection

present. For example, an infection caused by bacteria can result in a

higher percentage of neutrophils, which have a normal range of 2.0-

7.5 x 109/1iter. In contrast, a parasitic infection wiH result in increased

eosinophils, which have a normal COunt of 0.0-0.45 x 109/liter.7

Antibody Measurement

Antibodies develop in response to the invasion of antigens from new

infectious agents. Identifying the presence and concentration of spe-

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cific antibodies helps in determining past and present exposure to

infectious organisms.s

Microbiology

In microbiology studies, specimens from suspected sources of infection (e.g., sputum, urine, feces, wounds, and cerebrospinal fluid) are collected by sterile technique and analyzed by staining, culture, or

sensitivity or resistance testing, or a combination of all of these.

Staining

Staining allows for morphologic examination of organisms under a

microscope. Two types of staining techniques are available: simple

staining and the more advanced differential staining. Many types of

each technique exist, but the differential Gram's stain is the most

common.1I

Gram's stain is used to differentiate similar organisms by categorizing them as gram-positive or gram-negative. This separation assists in determining subsequent measures to be taken for eventual identification of the organism. A specimen is placed on a microscope slide, and the following six-step process is performed":

I.

Crystal violet stain applied to stain all organisms

2.

Iodine stain applied ro enhance reaction between cell wall

and primary stain

3.

Ethyl alcohol or acerone applied

4.

Safranin 0 or carbolfuchsin applied ro stain gram-negative

organisms pink to red

5.

Alcohol or acerone applied to decolor gram-negative

organisms

6.

Safranin reapplied for visualization

A red specimen at completion indicates a gram-negative organism,

whereas a violet specimen indicates a gram-positive organism.'"

Culture

The purpose of a culture is ro identify and produce isolated colonies

of organisms found within a collected specimen. Cells of the organism are isolated and mixed with specific mediums that provide the proper nourishment and environment (e.g., p H level, oxygen con-

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AClffE CARE HANDBOOK FOR PHYSICAL TI-I(RAI'ISTS

tent) needed for the organism to reproduce into colonies. Once this

has taken place, the resultant infectious agent is observed for size,

shape, elevation, texture, marginal appearance, and color to assist

with identification.9

Sensitivity and Resistance

When an organism has been isolated from a specimen, its sensitivity

(susceptibility) to antimicrobial agents or antibiotics is tested. An

infectious agent is sensitive to an antibiotic when the organism'S

growth is inhibited under safe dose concentrations. Conversely, an

agent is resistallt to an antibiotic when its growth is not inhibited by

safe dose concentrations. Because of a number of factors, such as

mutations, an organism's sensitivity, resistance, or both to antibiotics

is constantly changing.lo

Cytology

Cytology is a complex method of studying cellular structures, functions, origins, and formations. Cytology assists in differentiating between an infectious process and a malignancy and in determining

the type and severiry of a present infectious process by examining cellular characteristics.s,1 1 It is beyond the scope of this chapter, however, to describe all of the processes involved in studying cellular structure dysfunction.

Body Fluid Examination

Plellral Tap

A pleural tap, or thoracentesis, is the process by which a needle is

inserted through the chest wall into the pleural cavity to collect pleural fluid for examination of possible malignancy, infection, inflammation, or any combination of these. A thoracentesis may also be performed to drain excessive pleural fluid in large pleural effusions.11

Pericardiocentesis

Pericardiocentesis is a procedure that involves accessing the pericardial space around the heart with a needle or cannula to aspirate fluid for drainage, analysis, or both. It is primarily used to assist in diagnosing infections, inflammation, and malignancies and to relieve effusions built up by these disorders. 13

Synovial Fluid Analysis

Synovial fluid analysis, or arthrocentesis, involves aspirating synovial

fluid from a joint capsule. The fluid is then analyzed and used to assist

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in diagnosing infections, rheumatic diseases, and osteoarthritis, all of

which can produce increased fluid production within the joint.14

Gastric Lavage

A gastric lavage is the suctioning of gastric contents through a nasogastric rube to examine the contents for the presence of sputum in patients suspected of having tuberculosis (TB). The assumption is that

patients swallow sputum while they sleep. If sputum is found in the

gastric contents, the appropriate sputum analysis should be performed to help confirm the diagnosis of TB. 12.IS

Peritolleal Fluid Analysis

Peritoneal fluid analysis, or paracentesis, is the aspiration of perironeal fluid with a needle. It is performed to ( 1) drain excess fluid, or ascites, from the peritoneal cavity, which can be caused by infectious

diseases, such as TB; (2) assist in the diagnosis of hepatic or systemic

malfunctions, diseases, or malignancies; and (3) help detect the presence of abdominal trauma.12.IS

Other Studies

Imaging with plain x-rays, computed tomography scans, and magnetic resonance imaging scans can also help identify areas with infectious lesions. In addition, the following diagnostic studies can also be performed to help with the differential diagnosis of the infectious process. For a description of these studies, refer to the sections and chapters indicated below:

• Sputum analysis (see Diagnostic Testing in Chapter 2)

• Cerebrospinal fluid (see Lumbar Puncture in Chapter 4)

• Urinalysis (see Diagnostic Tests in Chapter 9)

• Wound cultures (see Wound Assessment and Acute Care Management of Wounds in Chapter 7)

Infectious Diseases

Various infectious disease processes, which are commonly encountered in the acute care setting, are described in the following sections.

Certain disease processes that are not included in this section are

described in other chapters. Please consult the index for assistance.