Wounds (Superficial) – Wound swabs / Abscess swabs / Drainage

This section includes specimens from wound swabs, abscess swabs, decubitus ulcers, episiotomies, non-intravenous or non-central line exit sites, chest tube drainage, abdominal drainage, and tracheal swabs. Many different bacterial species can cause infection of these sites but S. aureus, beta-hemolytic streptococci, P. aeruginosa and enteric Gram negative bacilli are frequently implicated.

However samples are often taken from chronic wounds that are not overtly infected and commonly colonized with several species of microorganisms. These samples can generate a large amount of work that has little clinical value.

The presence of polymorphonuclear leukocytes (PMNs) is an indication of inflammation and increases the likelihood that microorganisms found in culture are significant. The presence of squamous epithelial cells without PMNs suggests a sample from a site that is not inflamed and that microorganisms isolated in culture are only colonizing and not causing infection.

Specific rejection criteria

Tubing and drains that have been in the patient and connected to the outside are not processed. The exceptions are intravascular catheter tips.

If the drain container (Jackson-Pratt, Sump etc.) or portion of tubing (Penrose drain, chest tube tip, etc.) are submitted, do not process and add “Tips of drainage devices that have been indwelling in patients are not suitable for culture. Please submit fluid samples preferentially acquired at the time of device placement.”

Setup

Direct Examination

Gram stain

Culture

Note: Irrespective of whether sample was identified as deep or superficial only set up anaerobes if specifically requested.

Media Incubation
Blood Agar (BA) CO2, 35°C x 48 hours
MacConkey Agar (MAC) O2, 35°C x 48 hours
Phenylethyl Alcohol Blood Agar (PEA) CO2, 35°C x 48 hours

For chest tube and trachea (tracheostomy) swabs, add:

Chocolate Agar (CHOC) CO2, 35°C x 48 hours

If anaerobic culture requested (irrespective of whether sample was identified as deep or superficial only set up anaerobes if specifically requested), add:

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

Interpretation

Culture

Examine the aerobic plates after 24 and 48 hours incubation and anaerobic plates after 48 hours incubation.

Count the number of types of organisms.

If there are <3 types in total of organisms isolated, work up significant isolates of any amount of Probable Pathogens. Workup Possible Pathogens if pure growth OR moderate to heavy and obviously predominant growth.

Do not workup Skin Flora.

If there are 3 or more types in total of organisms isolated, work up any amount of Probable Pathogens but do not work up other organisms.

Organisms for workup are categorized as follows:

Probable pathogens

  • Staphylococcus aureus
  • Beta-haemolytic streptococcus groups A, C and G
  • Beta-haemolytic streptococcus group B (for neonates and postpartum patients only)
  • Pseudomonas aeruginosa

For chest tube drainage and tracheal swabs, include:

  • Haemophilus influenzae
  • Streptococcus pneumoniae

For sternal wounds, include:

  • Coagulase-negative-Staphylococcus

Possible pathogens

  • Beta-haemolytic streptococcus group B (except neonates and postpartum patients)
  • Enterococcus species
  • viridans Streptococcus group and Streptococcus anginosus group (except tracheal swabs where they are considered normal flora)
  • Aerobic gram-negative-bacilli other than P. aeruginosa
  • Yeasts
  • Anaerobes

Normal flora

  • Coagulase-negative-Staphylococcus (except sternal wound)
  • Micrococcus spp.
  • Corynebacterium spp.
  • Bacillus spp.
  • Propionibacterium spp.
  • Nonpathogenic Neisseria spp.
Reporting

Gram stain

Report with quantitation the presence of pus cells, squamous epithelial cells and organisms.

Culture

Negative report
“No growth” or “Normal flora”

If possible pathogens are present in insignificant amounts (have not been worked up) do not report or list. Report Normal flora only.

Positive report
Quantitate all significant isolates and report with appropriate susceptibilities. If normal flora is also present, report with quantitation.

References H.D. Isenberg. 2004. Specimen Collection, Transport and Acceptability p. 2.1.1 – 2.1.28. In Clinical Microbiology Procedures handbook, 2nd Edition, Vol 1 ASM Press, Washington, D.C

H.D. Isenberg, 2004. Wound Cultures – Wound and Soft Tissue Cultures, p. 3.13.1.1 – 3.13.1.16. In Clinical Microbiology Procedures Handbook, 2nd Edition, Vol 1 ASM Press, Washington, D.C.

H.D. Isenberg. 2004. Culture for anaerobes p. 4.3.1 – 4.3.9 In Clinical Microbiology Procedures Handbook, 2nd Edition, Vol 1 ASM Press, Washington, D.C.