Body Fluid Culture

Definition

Sterile fluid-filled spaces are present at a number of anatomic sites and are subject to infection. Examples of sterile fluids include peritoneal, pleural, pericardial, and synovial/joint fluid. Infections associated with CSF are life threatening and associated with a different etiology of bacterial pathogens, so these cultures are typically processed differently than other sterile fluids. Urine is also processed with different culture techniques because of its connection with the external environment, via the urethra, and pathogenesis of infection. A broad etiology of bacterial pathogens may cause infections of sterile sites, and culture methods are optimized for recovery of organisms present in low concentrations.

Collect sterile fluid cultures from sites associated with signs and symptoms of inflammation, including redness, swelling, pain, heat, fluid accumulation, and pus formation.
Method: Supportive and enriched solid agar (SBA and chocolate agar) and broth media (blood culture media) are typically inoculated; selective/differential agar media, such as MacConkey agar (gram-negative bacilli), CNA, or phenylethyl alcohol agar (gram-positive organisms), should be inoculated for specimens likely to show polymicrobial infection (e.g., peritonitis) or contamination by endogenous flora (e.g., cul-de-sac aspirates). Anaerobic media should be inoculated if there is a significant possibility of anaerobic pathogens. If infection with an uncommon, fastidious pathogen is suspected, the laboratory should be informed so that special cultures may be inoculated.
Turnaround time: Cultures are incubated for up to 7 days. Additional time is required for isolation, identification, susceptibility testing, and further characterization, as needed.

Fluid aspiration is performed after preparation of the puncture site in a manner consistent with a preparation of a surgical site. Submission of specimens from drainage devices is discouraged because of the high incidence of contamination with endogenous flora; direct collection of the sterile fluid is recommended.
Collection of the maximum amount of fluid from the infected site is recommended. Blood culture bottles may be inoculated and is recommended for patients with spontaneous bacterial peritonitis, but a small amount of fluid should be retained for Gram stain and for special culture inoculation, if needed.
Swabs should not be used for fluid collection.
Place fluid into sterile transport tubes; small-volume specimens, or several milliliters from large-volume specimens, should be placed into an anaerobic transport tube. Note: Specimens transported under anaerobic conditions are acceptable for inoculation of cultures for aerobic bacterial, mycobacterial, and fungal cultures.
Submission of several specimens prior to antibiotic therapy may significantly improve sensitivity of culture detection.
The use of anticoagulants is discouraged because of possible inhibition of some pathogens. If anticoagulation is required, heparin or SPS is recommended.
Transport specimens at room temperature; do not refrigerate or freeze specimens.

Expected results: No growth. Infection is not excluded by a negative culture, especially after initiation antibiotic therapy. Uncommon, fastidious pathogens may not be isolated in culture without inoculation of special media.
Positive cultures indicate infection of the sterile site, but cultures that may be contaminated with endogenous flora must be interpreted with caution in the context of quantity or bacterial growth, purity of culture, Gram stain findings, and clinical signs and symptoms. Infected peritoneal fluid may yield numerous aerobic and anaerobic pathogens. Extensive identification and susceptibility testing are usually not clinically useful: final results are often not available until well into therapy, and empirical treatment is usually effective.