Tag Archives: otitis externa

“Choosing Wisely Bacteriology: Ear swabs for otitis externa”

There is plenty of scope for choosing wisely in the microbiology laboratory. The most obvious targets are actually within infectious serology, a department now essentially in the process of being superseded by molecular methods. However there are lots of opportunities within culture based bacteriology also, with an impressive proportion of superficial swabs being of low clinical value… I will try and review some of these sample types over the next few weeks.

Otitis externa is a common condition, especially in the summer when people go and bathe in rivers and lakes and get their ear canals repeatedly wet with non-sterile water…

The microbiology laboratory receives lots of ear swabs from patients with otitis externa. But in the vast majority of cases, the swab result is absolutely meaningless in terms of managing the infection.

But it is very tempting to take an ear swab nevertheless. Who wouldn’t want to take a swab to a discharging ear!

Bacteria and fungi are usually bit part players only in otitis externa. The actual condition is a vicious circle of infection of ear debris- inflammation- swelling, blockage, leading to more infection and so the cycle goes on.

Releasing the blockage by clearing the debris, along with drying the ear canal are just as effective as antimicrobial drops, if not more so.

Most otitis externa swabs grow Pseudomonas aeruginosa or Staphylococcus aureus. A few grow Candida or Aspergillus species. Others simply grow a bacterial soup! (Our lab doesn’t report more than two organisms from an ear swab)

It actually doesn’t matter that much…

And antimicrobial susceptibilities are essentially useless as well. The treatment of otitis externa is with topical agents and it is well documented that the clinical response to topical antimicrobials is poorly correlated with their in-vitro susceptibility patterns.

Mild cases of otitis externa can often be managed with acetic acid drops alone (a drying agent with some anti-bacterial activity).

More severe cases usually get drops which often contain a bit of everything; a broad spectrum anti-bacterial, an anti-fungal, and a bit of steroid to reduce the inflammation.

So ear swabs should be reserved for recalcitrant cases of otitis externa, where the clinician is at the stage of discussing the case with an ENT specialist.

For the remainder, who cares that much what the swab grows…

From a choosing wisely perspective, how do we approach this? One option is to reject all ear swabs from otitis externa patients unless the clinical details suggested recalcitrant infection. Alternatively a comment could be added to every ear swab result saying that ear swabs are not indicated for otitis externa, except in special circumstances.

Time to act…


“Hearing aids”


I have never found ear swabs a very satisfactory microbiological sample, the outer ear canal usually being filled with all manner of debris and micro-organisms regardless of whether an infection is present or not. (Try processing a swab from your own ear!)

In addition it is often difficult to tell from the request form whether the swab has been taken from the external part of the ear or right inside the canal.

I doubt whether the results of many ear swabs ever affect the clinical outcome of the patient.

Nevertheless I have a few rules for ear swabs, to help both the scientists and myself make the results generated as sensible as possible.

  • If respiratory organisms (H. influenzae, S.pneumoniae, M. catarrhalis) isolated from a toddler ear swab, it is likely to be an otitis media with a perforation. (there is also the possibility that the organism has got into the ear from the mouth via the finger of the toddler). I tend to do susceptibilities on such isolates on the rationale that a systemic antibiotic may be indicated for recalcitrant cases.
  • If Staphylococcus aureus and Streptococcus pyogenes are isolated together from an “ear swab” then the chances are it is an impetiginous lesion. Therefore I do susceptibilities on the staphylococcus (but not the S. pyogenes, see “Excuses”)
  • Rule of three. If three or more organisms are isolated from an ear swab then you are essentially wasting your time identifying every member of the zoo. What help is this going to be to the clinicians.¬†Would simply report as “Mixed flora” or “Mixed Coliforms.”
  • With the exception of the first two points above I would hardly ever perform susceptibilities on isolates from ear swabs. I am not aware of any evidence that doing this improves patient outcomes. The one exception to this would be where there is clinical details of “mastoiditis”. In this clinical scenario, if there is a pure or dominant organism, or a¬†Pseudomonas aeruginosa, it should be worked up.

Ear swabs are over-worked, over processed and over-interpreted in clinical microbiology laboratories all over the world. 

The results of ear swabs can sometimes help with clarifying the aetiology of the ear symptoms, as described above. However they do little to affect the management of the patient, and we should bear this in mind when processing them…..