Sterile body fluids

Fluids collected from normally sterile body sites are priority specimens and should be treated as STATs.

Pleural (Thoracentesis / Empyema) Fluids

Infection of the pleural space may result in severe morbidity and mortality. Therefore rapid and accurate microbiological assessment is required. Any organism found in pleural fluid must be considered significant (although specimen contamination may occur during collection).

Peritoneal (Ascites) Fluids

Peritonitis may be classified as primary (spontaneous) or secondary. Primary peritonitis usually occurs in someone with pre-existing ascites (e.g. patients with chronic liver disease) in which there has been no entry into the abdominal cavity. Secondary peritonitis occurs after surgery or trauma to the abdomen. Although enteric Gram negative organisms are the most common isolates associated with these types of infections, polymicrobial infection is also common.

Synovial (Joint) & Pericardial Fluids

These are normally sterile fluids. Infection of these fluids may be due to a variety of different organisms as a result of contamination at the time of surgery/trauma or hematogenous spread.

Amniotic Fluids

Amniotic fluid surrounds the developing fetus in utero and infection may result in severe morbidity and mortality to the mother and fetus. Any organism isolated must be considered significant (although contamination may occur during collection).

“Sterile” urines

When urine is collected as a percutaneous surgical fashion is is treated as a sterile body fluid. The two types of samples most commonly seen are:

  • Nephrostomy urine when the nephrostomy tube is first inserted (almost always done in radiology).
  • Suprapubic bladder aspirate (Tap)

Other Fluids

Several other specimens are occasionally submitted for culture including tympanocentesis fluid, intraocular fluids (vitreous and aqueos), hydrocele fluid, cyst fluid.

Setup NOTE – Chest tube drainage will be treated as a superficial wound specimen – see Wounds (Superficial) – Wound swabs / Abscess swabs / Drainage

If a clotted specimen is received, emulsify the coagulated material using a sterile swab or loop.

Centrifuge the specimen (3000 rpm x 10 minutes).

Transfer the supernatant into a sterile container and refrigerate until the final report is issued. Label the container as supernatant.

Use the sediment to prepare the smears and inoculate the culture media.

Direct Examination

Gram stain


Media Incubation
Blood Agar (BA) CO2, 35°C x 48 hours
Chocolate Agar (CHOC) CO2, 35°C x 48 hours
MacConkey’s Agar (MAC) O2, 35°C x 48 hours
Wilkins Chalgren Agar (WC) AnO2, 35°C x 48 hours
WC Nalidixic acid – Tween 80 (NAT) AnO2, 35°C x 48 hours
WC Nalidixic acid – Vancomycin (NAV) AnO2, 35°C x 48 hours
Thioglycolate broth (THIO) O2, 35°C x 48 hours

If fungal culture is requested, add:

Sabouraud’s Heart Infusion Agar (SABHI) O2, 30°C x 4 weeks
SABHI with Chloramphenicol (SABHI-C) O2, 30°C x 4 weeks

Interpretation Examine the BA, CHOC, MAC and THIO at 24 and 48 hours. Examine the WC, NAT and NAV after 48 hours incubation.

If no growth on the culture plates but evidence of growth in THIO, perform a Gram stain and sub-culture the THIO onto BA, CHOC, WC and other media as appropriate and incubate and process as above. All isolates are to be identified.

If the Gram stain is positive and no growth in 48 hours, re-incubate all plates an additional 48 hours.


Gram stain

Report with quantification the presence or absence of organisms and WBCs.


Negative Report
“No growth”

Positive Report
Report all isolates with quantitation with appropriate susceptibilities.

If isolated from THIO only – add comment “From broth culture only, indicative of small numbers and/or contamination.”

Telephone results of a positive Gram stain and all positive cultures to the ward / ordering physician.

References P.R. Murray, E.J. Baron, M.A. Pfaller, R.H. Yolken. 2003. Manual of Clinical Microbiology, 8th ed. ASM Press, Washington, D.C.

H.D. Izenberg. 2003. Blood Cultures-General Detection and Interpretation, p. In Clinical Microbiology Procedures Handbook, 2nd ed. Vol.1 ASM Press, Washington, D.C.