Tuesday, June 18, 2013

Positive cultures in revision shoulder arthroplasty - should they be 'unexpected' and how do we know their 'clinical meaning'?

Clinical meaning of unexpected positive cultures (UPC) in revision shoulder arthroplasty.

This study reports the positive cultures that were unexpected by the authors in 15% (107 of 678) of shoulders that they revised for reasons other that clinically obvious infection. Male sex was a risk factor for positive cultures. Preoperative blood tests and intraoperative pathology were not helpful in making the diagnosis. The most prevalent bacteria were Propionibacterium acnes (n = 68) and Staphylococcus epidermidis (n = 21).

The authors used some complex definitions:
*true infections: another positive culture with the same organism from the shoulder after the original positive culture at the time of the revision. [Note that in order to meet this definition, the patient either needed a second revision surgery or an aspirate after the initial revision. Since the initial clinical presentation of shoulders did not suggest infection and since the initial initial positive cultures were unexpected, it is difficult to know the indications for re-revision or aspiration after the first revision. It seems that persistent presence of organisms may be overlooked in patients not having repeat surgery or aspiration]. 11 patients met these criteria. Curiously, antibiotic use and number of positive cultures did not influence the rate of true infections.
*true contaminant: negative cultures from a second revision surgery if the patient was not treated with antibiotics after the first revision [again a second surgery is required for the shoulder to meet this definition, so 'true contamination could not be diagnosed without a second surgery]
*possible contaminant: revision surgery improved the patient's clinical status without antibiotic treatment [this definition excludes the possibility that shoulders with infections could be improved by a revision surgery without antibiotic treatment -.e.g. that a release of adhesions might improve a patient with truly positive cultures]
*possible infection: antibiotics were administered without subsequent cultures
*undetermined: patients with positive cultures at a second surgery that were different than those cultured at the original revision or patients who were not treated with antibiotics who did not improve after the revision or patients who had incomplete data.

The authors concluded that unexpected positive cultures were prevalent in revision shoulder arthroplasty, but that in at least 25% of cases, these cultures "had no clinical relevance".

Comment: Propionibacterium is frequently cultured at the time of revision arthroplasty when there is no obvious clinical evidence of infection. Whether or not Propionibacterium is recovered at the time of revision arthroplasty depends on a number of critical factors: the number of samples submitted for culture, the source of the samples submitted, the media on which the samples are cultured, how long the cultures are held, and whether the patient received antibiotics prior to harvesting the cultures. In the retrospective study of a database reported by the authors, it is difficult to know the sampling and culturing protocol for these patients, some of who had their revision surgery back in the 1970's. What has been learned recently is that Propionibacterium is often cultured in shoulders revised for stiffness or component loosening. Trying to distinguish 'true infection' from 'contaminant' under these circumstances is not at all clear, as is evidenced by the complex definitions suggested in this article.

Thus, for now, the most unassailable approach is to report the results of the cultures obtained, rather than trying to interpret the clinical meaning when we do not know the role that organisms may play in prompting revision surgery. Positive cultures should not be unexpected in revision shoulder arthroplasty.

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