Tag Archives: wound swabs

“Bringing the patient into the laboratory…”

Photo courtesy of Cabalari

Sometimes we can process a wound swab from start to finish without ever knowing why it has been taken. It could literally be anything from a “burst pimple”, to a burns patient with a severely infected skin graft.

Insisting on clinical details goes some way to solving this problem, but even then, the details provided may not accurately convey the type or severity of the infection.

Wouldn’t it be great if we had each patient right in front of us whilst processing the wound swab? Obviously this is not practical. However  if we could view a photographic image of the infection, it would immediately contextualise the wound swab that we are processing.

With the current advances in electronic requesting, such a facility is not out of the question. Dermatologists use a lot of digital imaging these days. So why not microbiologists?

Pictures are often better than words…

And when validating the report, I could then have even more information at hand:- The result, the Kiestra digital plate images, the request form and a digital image of the infection site on the patient.

Quality assurance by using all the evidence.

… And requestors might also think twice about sending in swabs of burst pimples  if they also needed to provide a picture of the infection as well!


p.s. There are many dubious reasons for taking microbiological samples, as there are often multiple agendas at play…

“Painting Pictures”

A wound swab arrives into the microbiology laboratory..

…Because there are no clinical details we don’t know whether a Gram stain might be worthwhile.

Because there are no clinical details, we don’t know whether to add anaerobic culture or set up a yeast agar for this  swab.

Because there are no clinical details, we don’t know whether this Pseudomonas aeruginosa isolate on the plate is potentially significant…

Microbiology laboratories all over the world (including our own) receive hundreds of wound swabs every day. Often there are no clinical details or only a very cursory acknowledgement to this requirement, such as “?infection”, or “discharging wound”.

You know the story well…

So often we are left to process and report the wound swab with no idea of what is actually wrong with the patient, and what exactly we should be doing with the swab.

In my opinion clinical details involve two main elements, which overlap to a certain extent:

  • Contextualisation- What type of infection are we dealing with here? Taking the example of skin infections; Is it an impetiginous lesion, is it a boil, is it an area of cellulitis, is it a post-surgical wound, is it a burn, etc, etc? By providing clinical context we can start to work out what we are looking for and ascertain the importance of potential pathogens on the agar plates. Is there anything we should know about the patient? Are they immunocompromised? Do they have antibiotic allergies? Are they failing treatment? Is there anything unusual about the clinical presentation? Was it caused by a bite? Was there overseas travel?
  • Justification- Taking the example of skin infections again, most patients with impetigo & boils, along with many other types of uncomplicated skin infections, do not need a wound swab sent to the lab. For most skin infections, swabbing should be the exception, not the rule. So what was it about this particular patient that prompted the clinician to send a wound swab? Was it because the patient failed first line treatment? Was it because they had an associated fever? Was it because the infection was getting rapidly worse?

Here are a few examples of hypothetical clinical details that both contextualise and justify the swab.

  • “Impetiginous lesions on face getting worse despite topical anti-septic treatment. Past history of MRSA colonisation.”
  • “Area of spreading cellulitis L lower leg. Patient diabetic.”
  • “Carbuncle L buttock, patient has an anaphylactic allergy to penicillin.”
  • “Cat scratch to R hand. patient now has fever and increasing erythema. Commenced on amoxycillin-clavulanate.”

It’s not rocket science, nor does it have to be a story.

If the clinician is required to contextualise and justify the laboratory request, then the advantages are two-fold. Not only can the scientist paint a picture in their head of what is going on with the patient, and process accordingly, but also the clinician may start to think twice about why they are sending a particular sample to the laboratory in the first place…



“Molecular diagnosis of wound infections: The Holy Grail.”

The slice of pie taken by the molecular department in the microbiology laboratory is increasing, slowly but surely.

On the contrary, the proportion of culture based microbiology is inexorably declining.

Many clinical microbiology laboratories are now switching, or looking at switching over to molecular diagnosis of enteric pathogens.

Molecular diagnosis of pathogens causing vaginitis and pharyngitis will not be far behind, and a few labs have already moved in this direction.

However diagnosis of wound infections remains firmly culture based. There is not even much in the literature with regards to molecular diagnosis of wound infections…

Is it even possible?

So what are the difficulties?:

  • Potential number of pathogens: There are several pathogens or putative pathogens that are able to cause wound infections. This makes PCR based molecular diagnosis more difficult. However, on the flip side, 95% of wound infections in a general clinical setting are caused by two pathogens, Staphylococcus aureus and Streptococcus pyogenes (Group A streptococcus).
  • Lack of susceptibility information: Molecular diagnosis of resistance determinants is still a little behind phenotypic culture based testing. However with regards to the two main pathogens as above, PCR analysis can easily differentiate between MSSA and MRSA, and susceptibility data for Streptococcus pyogenes is only required for a small proportion of patients who have anaphylactic reactions to penicillin.
  • “Over-sensitivity”: It is always nice to know which are the dominant organisms within a wound. Culture is relatively good at this. However molecular methods are starting to be able to quantify to some extent (e.g by playing with the cycle threshold cut-off).
  • Cost: The culture of a standard wound swab might cost $10 or so, even when overheads are included. In my experience molecular tests have to be performed in very large volumes to even get close to this kind of price. However wound swabs do arrive into the laboratory in very large volumes!

If it was easy it would have been done by now… I suspect it is the 4 things above acting together as a “bundle”, as opposed to any one insurmountable barrier which has contributed to the lack of progress up until now.

However it will come in some form or other, you can be sure of that…

Here is a potential solution I have thought of:

  • Any wound swab accompanied with clinical details suggesting an unusual pathogen e.g. animal bites (Pasteurella), water exposure (vibrios, aeromonas), immunocompromise,  would still undergo routine “catch-all” bacterial culture.
  • The rest (the vast majority) would be subjected to a multiplex PCR for the detection of MSSA, MRSA and Streptococcus pyogenes (and possibly Group G streptococcus also). A positive result would be reported routinely. A  negative result would have an accompanying comment to suggest contacting the laboratory if the patient’s symptoms were persisting, or if further susceptibilities are required, so that culture could be set up if necessary.

In this way molecular diagnostics could be performed on (a good proportion of) wound swabs at a relatively low cost.

In addition multiplex PCRs could be developed specifically for infections in specific clinical situations, e.g. post animal bite,  etc., etc.

Automated bacteriology culture systems like Kiestra TLA would not have happened if the industry didn’t think that there was at least another 20 years or so of culture based bacteriology left, and they are probably right. But I don’t think it will be too long before commercial laboratory diagnostics companies start looking closely at “wholesale” molecular options as above.

And who knows, whole genome sequencing might come in and completely disrupt PCR based molecular diagnostics, and the picture might change again…

The future’s uncertain and the end is alway near