cluster of staphylococcus aureus

Wounds: when and how to culture them

Wound cultures are important to assist in the diagnosis of infection in a wound. An improperly collected they can create more problems than solve by:

  • Making an inaccurate diagnosis – calling a case a surgical site infection when it is not
  • Leading to unecessary admission, treatment and potential side effects and complications of treatment

Microbiology

So when should wounds be cultured?

  • when there is purulent drainage from the wound
  • Do not culture non-inflammed wounds

How should a wound be cultured?

  • By either aspirating purulent drainage in a sterile syringe using a needless cannula (shown below) or collecting pus in a sterile cup
  • By sending inflammed tissue in a sterile cup containing non-bacteriostatic saline
  • Do not send eschar (dead tissue)
Aspirating pus from an abscess using a syringe with a needless cannula

Aspirating pus from an abscess using a syringe with a needless cannula

Packaging for needless cannula - available in pyxis

Packaging for needless cannula – available in pyxis

What about sending culture swabs?

They should be avoided for the following reason:

  • Higher comtamination rate
  • Lower quantity of actual specimen (low signal to noise ratio), 150 microliters on a swab
  • Of every 100 organisms absorbed on a swab only 3 make it to the culture plate
  • An effective culture requires 6 plates and 1 thio and a gram stain. One swab with 150 microliters really just does not cut it
  • Harder to process in lab
  • Non actionable vague reports such as “mixed flora”
  • Role limited to screening such as MRSA, VRE, pertussis etc..
  • All the above lead to low quality data

Specimen collection
Notice that even 1.5mL of fluid in a small syringe has 100x the specimen than a typical swab

Low quality data leads to

  • Higher lab costs to isolate pathogens from contaminants
  • Harder to interpret
  • Unecessary consults
  • Unecessary treatment

So what is the definition of a superficial incision surgical site infection:

Any one of the following

  • Purulent drainage from site
  • Organism aseptically isolated from site
  • Signs of inflammation at the site
  • Diagnosis of infection by physician

Summary

  • If you don’t think it is infected- don’t culture it!
  • There is no such thing as a routine wound culture
  • Get tissue or fluid (pus)
  • No surface cultures
  • Do not culture non-inflammed wounds
  • Do not put specimens in formalin
  • A few mL in a syringe is better than 10 swabs
  • Do not send multiple swabs of the same specimen