Pleural, Pericardial, and Peritoneal Fluid Analysis

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Pleural, Pericardial, and Peritoneal Fluid Analysis

Learning Objectives

Key Terms1⁎ *

Physiology and Composition

The lungs, heart, and abdominal organs are surrounded by a thin, continuous serous membrane, as well as by the internal surfaces of the body cavity wall. A space or cavity filled with fluid lies between the membrane that covers the organ (visceral membrane) and the membrane that lines the body wall (parietal membrane) (Fig. 10.1). Each cavity is separate and is named for the organ or organs it encloses. The lungs are individually surrounded by a pleural cavity, the heart by the pericardial cavity, and the abdominal organs by the peritoneal cavity. The serous membranes that line these cavities consist of a thin layer of connective tissue covered by a single layer of flat mesothelial cells. Within the membrane is an intricate network of capillary and lymphatic vessels. Each membrane is attached firmly to the body wall and the organ it surrounds; however, the opposing surfaces of the membrane—despite close contact—are not attached to each other. Instead, the space between the opposing surfaces (i.e., between the visceral and parietal membranes) is filled with a small amount of fluid that serves as a lubricant between the membranes, which permits free movement of the enclosed organ. The cavity fluid is created and maintained through plasma ultrafiltration in the parietal membrane and absorption by the visceral membrane. The name serous fluid is a general term used to describe fluids that are an ultrafiltrate of plasma and therefore have a composition similar to that of serum.

The process of fluid formation and absorption in the pleural, pericardial, and peritoneal cavities is dynamic. Fluid formation is controlled simultaneously by four factors: (1) permeability of the capillaries in the parietal membrane; (2) hydrostatic pressure in these capillaries; (3) oncotic pressure (or colloid osmotic pressure) produced by the presence of plasma proteins within the capillaries; and (4) absorption of fluid by the lymphatic system (Box 10.1). Hydrostatic pressure (i.e., blood pressure) forces a plasma ultrafiltrate to form in the cavity; at the same time, plasma proteins in the capillaries produce a force (oncotic pressure) that opposes this filtration.

The permeability of the capillary endothelium regulates the rate of ultrafiltrate formation and its protein composition. For example, increased permeability of the endothelium will cause increased movement of protein from the blood into the cavity fluid. When this occurs, the now protein-rich fluid in the cavity further enhances the movement of more fluid into the cavity. Such an accumulation of fluid in a body cavity is termed an effusion and indicates an abnormal or pathologic process. The lymphatic system, or the fourth component in cavity fluid formation, plays a primary role in removing fluid from a cavity by absorption. However, if the lymphatic vessels become obstructed or impaired, they cannot adequately remove the additional fluid, resulting in an effusion. Other mechanisms can cause effusions, and they may occur with a variety of primary and secondary diseases, including conditions that cause a decrease in hydrostatic blood pressure (e.g., congestive heart failure, shock) and those characterized by a decrease in oncotic pressure (i.e., disorders characterized by hypoproteinemia).

A pleural, pericardial, or peritoneal effusion is diagnosed by a physical examination of the patient and on the basis of radiographic, ultrasound, or echocardiographic imaging studies. The collection and clinical testing of pleural, pericardial, and peritoneal fluids play an important role in determining the type of effusion present and in identifying its cause.

Specimen Collection

The term paracentesis refers to the percutaneous puncture of a body cavity for the aspiration of fluid. Other anatomically descriptive terms denote fluid collection from specific body cavities. Thoracentesis, for example, refers to the surgical puncture of the chest wall into the pleural cavity to collect pleural fluid, pericardiocentesis into the pericardial cavity, and peritoneocentesis (or abdominal paracentesis) into the peritoneal cavity. The term ascites refers to an effusion specifically in the peritoneal cavity, and ascitic fluid is simply another name for peritoneal fluid.

Collection of effusions from a body cavity is an invasive surgical procedure performed by a physician using sterile technique. Unlike cerebrospinal fluid and synovial fluid collections, serous fluid collections from effusions in the pleural, pericardial, and peritoneal cavities often yield large volumes of fluid. Consequently, the amount of fluid obtained often exceeds that needed for diagnostic testing. Note that at times, additional or repeat puncture procedures are necessary to remove a recurring effusion from a cavity for therapeutic purposes, such as when the effusion is compressing or inhibiting the movement of vital organs.

Before serous fluid is collected from a body cavity, the laboratory should be consulted to ensure that appropriate collection containers are used and suitable volumes are obtained (Table 10.1). In microbiological studies, the percentage of positive cultures obtained increases when a larger volume of specimen (10 to 20 mL) is used or when a concentrated sediment from a centrifuged specimen (50 mL or more) is used to inoculate cultures.

Table 10.1

Suggested Serous Fluid Specimen Requirements
Physical Examination Volume Acceptable Containers
Color and clarity   Recorded at bedside by physician and noted on test request form
Microscopic Examination
Cell counts, differential 5–8  mL EDTA
Cytology study (PAP stain, cell block) 50  mL recommended; 15–100 mLa Plain tube/container, sodium heparin, EDTA
Chemical Examination
Glucose 3–5  mL Plain tube, sodium fluoride
Protein, lactate dehydrogenase, amylase, triglyceride, cholesterol, others 5–10  mL Plain tube, sodium heparin
pH (pleural fluid) 1–3  mL Heparinized syringe; anaerobically maintained
Microbiological Studies
Gram stain, bacterial culture 10–20 mLb Sterile container; SPS, none, sodium heparin
Acid-fast stain and culture 15–50 mLb Sterile container; SPS, none, sodium heparin


EDTA, Ethylenediaminetetraacetic acid; PAP, Papanicolaou; SPS, sodium polyanetholsulfonate.

aNo upper limit to the amount of fluid that can be submitted; large volumes of fluid enhance the recovery of cellular elements.

bLarge fluid volumes may facilitate the recovery of viable microbial organisms.

Normally, serous fluids do not contain blood or fibrinogen, but a traumatic puncture procedure, a hemorrhagic effusion, or an active bleed (e.g., from a ruptured blood vessel) can result in serous fluid that appears bloody and clots spontaneously. Therefore to prevent clot formation, which entraps cells and microorganisms, sterile tubes coated with an anticoagulant such as sodium heparin or liquid ethylenediaminetetraacetic acid (EDTA) are used to collect fluid specimens for the microscopic examination and microbiological studies. In contrast, serous fluid for chemical testing is placed into a nonanticoagulant tube (red top), which will allow clot formation when fibrinogen or blood is present. Serous fluids should be maintained at room temperature and transported to the laboratory as soon as possible after collection to eliminate potential chemical changes, cellular degradation, and bacterial proliferation. Note that refrigeration (4–8°C) adversely affects the viability of microorganisms and should not be used for serous fluid specimens. However, serous fluid samples intended for cytology examination are an exception and can be refrigerated at 4°C when storage is necessary.

A blood sample must be collected shortly before or after the paracentesis procedure to enable comparison studies of the chemical composition of the effusion with that of the patient’s plasma. These studies enable classification of the effusion (transudate or exudate, chylous or pseudochylous), which assists in diagnosis and treatment. Note that for chemical analysis, the same type of specimen collection tube (nonanticoagulant, sodium heparin) should be used for both the fluid specimen and the blood collection (serum or plasma). In addition, specimen transport and handling conditions should be the same to eliminate result variations due to these potential differences.

Transudates and Exudates

An effusion, particularly in the pleural or peritoneal cavity, is classified as a transudate or an exudate. This classification is based on several criteria, including appearance, leukocyte count, and total protein, lactate dehydrogenase, glucose, and bilirubin concentrations; however, because of the overlap among categories, no single parameter differentiates a transudate from an exudate in all patients.1 Table 10.2 lists parameters and the values associated with transudates and exudates.

Table 10.2

Differentiation of  Transudates and Exudates
Parameter Transudates Exudates
Causes Increased hydrostatic pressure Increased capillary permeability
Decreased oncotic pressure Decreased lymphatic absorption
Physical Examination
Clarity Clear Cloudy
Color Pale yellow Variable (yellow, greenish, pink, red)
Clots spontaneously No Variable; often yes
Microscopic Examination
WBC count <1000 cells/μL (pleural) Variable, usually
<300 cells/μL (peritoneal) >1000 cells/μL (pleural)
>500 cells/μL (peritoneal)
Differential count Mononuclear cells predominate Early, neutrophils predominate; late, mononuclear cells
Chemical Examination
Bilirubin ratio (fluid-to-serum) ≤0.6 >0.6
Glucose Equal to serum level Less than or equal to serum level
TP concentration <50% of serum >50% of serum
TP ratio (fluid-to-serum) ≤0.5 >0.5
LD activity <60% of serum >60% of serum
LD ratio (fluid-to-serum) ≤0.6 >0.6
Cholesterol ratio (fluid-to-serum) ≤0.3 >0.3


LD, Lactate dehydrogenase; TP, total protein; WBC, white blood cell.

Classifying an effusion as a transudate or exudate is important because this information assists the physician in identifying its cause. Transudates primarily result from a systemic disease that causes an increase in hydrostatic pressure or a decrease in plasma oncotic pressure in the parietal membrane capillaries. These changes are noninflammatory and are frequently associated with congestive heart failure, hepatic cirrhosis, and nephrotic syndrome (i.e., hypoproteinemia). Once an effusion has been identified as a transudate, further laboratory testing usually is not necessary.

In contrast, exudates result from inflammatory processes that increase the permeability of the capillary endothelium in the parietal membrane or decrease the absorption of fluid by the lymphatic system. Numerous disease processes such as infections, neoplasms, systemic disorders, trauma, and inflammatory conditions may cause exudates. Additional laboratory testing is required with exudates, such as microbiological studies to identify pathologic organisms or cytologic studies to evaluate suspected malignant neoplasms.

Table 10.3 summarizes various causes of pleural, pericardial, and peritoneal effusions. Unlike pleural and peritoneal effusions, pericardial effusions usually are not classified as a transudate or an exudate. Most often, pericardial effusions result from pathologic changes of the parietal membrane (e.g., because of infection or damage) that cause an increase in capillary permeability; hence the majority of pericardial effusions could be considered exudates.

Table 10.3

Serous Effusions: Types, Mechanism of Formation, and Associated Conditions
Effusion Type Mechanism of Formation Conditions
Pleural and peritoneal Transudates Decreased hydrostatic pressure Congestive heart failure
Hepatic cirrhosis
Decreased oncotic pressure Nephrotic syndrome
Pleural and peritoneal Exudates Increased capillary permeability
      Pleural: lung and metastatic cancers
Peritoneal: hepatic and metastatic cancers
      Systemic disease (e.g., rheumatoid arthritis, systemic lupus erythematosus)
      Gastrointestinal disease (e.g., pancreatitis)
    Decreased lymphatic absorption Tumors/neoplasms (e.g., lymphoma, metastasis)
      Trauma or surgery
Pericardial Not categorized as transudate or exudate Increased capillary permeability due to changes in parietal membrane Infections (e.g., bacterial, tuberculous, viral, fungal)Cardiovascular disease (e.g., myocardial infarction, aneurysms)
      Tumors/neoplasms (e.g., metastatic cancers)
      Systemic disease (e.g., rheumatoid arthritis, systemic lupus erythematosus)


Physical Examination

Reference values for the characteristics of normal serous fluid in the pleural, pericardial, and peritoneal cavities are not available because in healthy individuals, the fluid volume in these cavities is small and the fluid is not normally collected. Only effusions are routinely collected and categorized as a transudate or an exudate (see Table 10.2). Transudates are usually clear fluids, pale yellow to yellow, that have a viscosity similar to that of serum. Because transudates do not contain fibrinogen, they do not spontaneously clot. In contrast, exudates are usually cloudy; vary from yellow, green, or pink to red; and may have a shimmer or sheen to them. Because exudates often contain fibrinogen, they can form clots, thus requiring an anticoagulant (e.g., EDTA, sodium heparin) in the collection tube. The physical appearance of an effusion usually is recorded on the patient’s chart by the physician after paracentesis and should be transcribed onto all test request forms. If this information is not provided, the laboratory performing the microscopic examination should document the physical characteristics of the fluid.

A cloudy paracentesis fluid most often indicates the presence of large numbers of leukocytes, other cells, chyle, lipids, or a combination of these substances. In pleural or peritoneal fluid, a characteristic milky appearance that persists after centrifugation usually indicates the presence of chyle (i.e., an emulsion of lymph and chylomicrons) in the effusion. A chylous effusion is caused by obstruction of or damage to the lymphatic system. In the pleural cavity, this can be caused by tumors, often lymphoma, or by damage to the thoracic duct due to trauma or accidental damage during surgery. Chylous effusions in the peritoneal cavity result from obstruction to lymphatic fluid drainage, which can occur with hepatic cirrhosis and portal vein thrombosis. Note that chronic effusions (as seen with rheumatoid arthritis, tuberculosis, and myxedema) can have a similar appearance, owing to the breakdown of cellular components; they also have a characteristically high cholesterol content. Consequently because of their visual similarity, chronic effusions are often called pseudochylous effusions and are differentiated from true chylous effusions by their lipid composition (i.e., triglycerides, chylomicron content). In a chylous effusion, lipoprotein analysis will show an elevated triglyceride level (i.e., greater than 110 mg/dL) and chylomicrons present, whereas a pseudochylous effusion has a low triglyceride level (less than 50 mg/dL) and no chylomicrons present. Table 10.4 summarizes the characteristics that assist in differentiating chylous and pseudochylous effusions.

Table 10.4

Differentiation of Chylous and Pseudochylous Effusions
Parameter Chylous Effusion Pseudochylous Effusion
Physical Examination Milky Milky
Chemical Examination    
Chylomicrons Present Absent
Triglycerides >110 mg/dL (1.2 mmol/L) <110 mg/dL (1.2 mmol/L)
Cholesterol Usually <200 mg/dL (5.2 mmol/L) Usually >200 mg/dL (5.2 mmol/L)
Microscopic Findings Lymphocytes Variety of cell types
Lipid-laden macrophages
Cholesterol crystalsa
Conditions Pleural effusions due to

Chronic diseases such as


aPresence confirms or establishes fluid as pseudochylous effusion.

Blood can be present in transudates and exudates because of a traumatic paracentesis procedure. As with other body fluids (e.g., cerebrospinal fluid, synovial fluid), the origin of the blood is determined by the distribution of blood during paracentesis. If the amount of blood decreases during the collection and small clots form, a traumatic tap is suspected. If the blood is homogeneously distributed in the fluid and the fluid does not clot (indicating that the fluid has undergone fibrinolytic changes in the body cavity—a process that takes several hours), the patient has a hemorrhagic effusion.

Microscopic Examination

The microscopic examination of pleural, pericardial, and peritoneal fluids may include a total cell count of erythrocytes (red blood cells, RBCs) and leukocytes (white blood cells, WBCs), a nucleated cell differential count, cytology studies, and, at times, identification of crystals. Note that in contrast to cerebrospinal fluid, which is always present in the central nervous system regardless of health status, serous fluids (i.e., pleural, pericardial, peritoneal fluids) are normally not present in a sufficient amount in body cavities to be analyzed. In other words, their presence indicates a pathologic process; hence, there are no “normal” reference values for serous fluids.

As with other body fluids, cloudy effusions must be diluted for cell counting using normal saline or another suitable diluent. (See Appendix D for acceptable diluents and their preparation.) Acetic acid diluents are avoided because they cause cells to clump, which prevents accurate cell counting. Cell counts can be performed manually using a hemacytometer or an automated analyzer. For details, see Chapters 16 Chapter 16 Chapter 17 and 17.

In the microbiology laboratory, a Gram stain is performed on serous fluids when requested to aid in the microscopic identification of microbes (see subsection Microbiological Examination).

Total Cell Counts

Total RBC and WBC counts have little differential diagnostic value in the analysis of pleural, pericardial, and peritoneal fluids. No single value for a WBC count can be used reliably to differentiate transudates from exudates; hence these counts have limited clinical use. However, WBC counts in transudates are usually less than 1000 cells/μL, whereas those in exudates generally exceed 1000 cells/μL.

With pericardial fluid, a WBC count of greater than 1000 cells/μL is suggestive of pericarditis, whereas an RBC count or hematocrit of the fluid can assist in identifying a hemorrhagic effusion. With pleural fluid, RBC counts can also be used to identify hemorrhagic effusions. However, high RBC counts (greater than 10,000 cells/μL) are frequently associated with neoplasms or trauma of the pleura. With peritoneal fluid, a WBC count exceeding 500 cells/μL with a predominance of neutrophils (greater than 50%) suggests bacterial peritonitis. However, the volume of peritoneal fluid (or ascites) can change significantly because of extracellular fluid shifts, and these fluid shifts can significantly change the cell count obtained. Hence a wide range of WBC counts can be encountered in peritoneal effusions throughout the course of a disease.

Differential Cell Count

Microscope Slide Preparation

A cytocentrifuged-prepared smear of a body fluid is most often used to perform a differential cell count. Cytocentrifugation is easy and fast, and enables good cell recovery in a concentrated area of the microscope slide. Despite minimal cell distortion, some recognizable artifacts associated with cytocentrifugation are well known and they are listed in Box 17.4. For additional details on slide preparation, dilutions, and diluents, see Chapter 17, subsection “Body Fluid Analysis: Manual Hemacytometer Counts and Differential Slide Preparation.”

Low-Power Examination

Each cytocentrifuged smear should be scanned in its entirety using a low-power objective (10 ×) to detect abnormalities that may be few in number yet apparent at this magnification, such as malignant cell clumps or other significant findings (see Fig. 9.4). In addition, this overview gives the microscopist a glimpse of the fluid’s composition, that is, its cellularity, density, and cell distribution on the slide (Fig. 10.2). Note that it does not matter whether this evaluation is done before or after the nucleated cell differential.

Oct 18, 2022 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Pleural, Pericardial, and Peritoneal Fluid Analysis
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