Specimens From Sterile Body Sites Fluids

In response to infection, fluid may accumulate in any body cavity. Infected solid tissue often presents as cellulitis or with abscess formation. Areas of the body from which fluids are typically sent for microbiologic studies (in addition to blood and cerebrospinal fluid [see Chapters 52 and 55]) include those in Table 61-1.

Pleural Fluid

The parietal pleura, a serous membrane of the thoracic cavity (see Chapter 53), lines the entire thoracic cavity. The outer surface of each lung is also covered by the visceral pleura (Figure 61-1). Pleural fluid is a collection of fluid in the pleural space, normally found between the lung and the chest wall (see Figure 61-1). The fluid usually contains few or no cells and has a consistency similar to that of serum but with a lower protein content When excess amounts of this fluid are present, it is called an effusion, or transudate, and is often the result of cardiac, hepatic, or renal disease. Pleural fluid that contains numerous white blood cells and other evidence of an inflammatory response (an exudate) is usually caused by infection, but malignancy, pulmonary infarction, or autoimmune diseases in which an antigen-antibody reaction initiates an inflammatory response may also be responsible. The material collected from the patient by needle aspiration (thoracentesis) is submitted to the laboratory as pleural fluid, thoracentesis fluid, or empyema fluid. Exudative pleural effusions that contain numerous polymorphonuclear neutrophils, particularly those that are grossly purulent are called empyema fluids. Empyema usually occurs secondary to pneumonia, but other infections near the lung (e.g., subdia phragmatic infection) may seed microorganisms into the pleural cavity.

Peritoneal Fluid

The peritoneum is a large, moist, continuous sheet of serous membrane that lines the walls of the abdominal-pelvic cavity and the outer coat of the organs contained within the cavity (Figure 61-2). In the abdomen, these two membrane linings are separated by a space called the peritoneal cavity, which contains or abuts the liver, pancreas, spleen, stomach and intestinal tract, bladder, and fallopian tubes and ovaries. The kidneys occupy a retroperitoneal (behind the peritoneum) position. Within the healthy human peritoneal cavity is a small amount of fluid that maintains moistness of the surface of the peritoneum. Normal peritoneal fluid may contain as many as 300 white blood cells per milliliter, but the protein content and specific gravity of the fluid are low. During an infectious or inflammatory process, increased amounts of fluid accumulate in the peritoneal cavity, a condition called ascites. The fluid, often called ascitic fluid, contains an increased number of inflammatory cells and an elevated protein level.

Agents of infection gain access to the peritoneum through a perforation of the bowel, through infection within abdominal viscera, by way of the bloodstream, or by external inoculation (as in surgery or trauma). On occasion, as in pelvic inflammatory disease (PID), organisms travel through the natural channels of the fallopian tubes into the peritoneal cavity.

Primary Peritonitis. There are two major types of infections in the peritoneal cavity: primary and secondary peritonitis. In primary peritonitis, no apparent focus of infection is evident. The organisms likely to be recovered from specimens from patients with primary peritonitis vary with the patient's age. The most common etiologic agents in children are Streptococcus pneumoniae and group A streptococci, Enterobacteriaceae, other gram-negative bacilli, and staphylococci. In adults, Escherichia coli is the most common bacterium, followed by S. pneumoniae and group A streptococci. Polymicrobic peritonitis is unusual in the absence of bowel perforation or rupture. Among sexually active young women. Neisseria gonorrhoeae and Chlamydia trachomatis are common etiologic agents of peritoneal infection, often in the form of a perihepatitis (inflammation of the surface of the liver, called Fitz-Hugh-Curtis syndrome). Tuberculous peritonitis occurs infrequendy in the United States and is more likely to be found among persons recendy arrived from South America, Southeast Asia, or Africa. Fungal causes of peritonitis are not common, but Candida spp. may be recovered from immunosuppressed patients and padents receiving prolonged antibacterial therapy.

Secondary Peritonitis. Secondary peritonitis is a sequel to a perforated viscus (organ), surgery, traumatic injury, loss of bowel wall integrity because of destructive disease (e.g., ulcerative colitis, ruptured appendix, carcinoma), obstruction, or a preceding infection (liver abscess, salpingitis, septicemia). The nature, location, and etiology of the underlying process govern the agents

Table 61 -1 Areas of the Body from Which Fluids Are Submitted to the Microbiology Laboratory

Body Area

Fluid Name(s)


Thoracentesis or pleural or empyema fluid

Abdominal cavity

Paracentesis or ascitic or peritoneal fluid


Synovial fluid


Pericardial fluid

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