Friday, June 19, 2015

Propionibacterium in open shoulder surgery - cautionary tale: "false positives" or contamination

The Incidence of Propionibacterium acnes in Open Shoulder SurgeryA Controlled Diagnostic Study

These authors reported on 177 shoulders without evidence of infection undergoing a first time open deltopectoral approach to the shoulder. Three pericapsular soft-tissue samples were taken from the shoulder for bacterial culture and were incubated for fourteen days. A sterile sponge was also analyzed in parallel with the tissue cultures. In addition, similar cultures were obtained from patients who had undergone previous shoulder surgery.

Overall, 20.5% of surgeries (twenty-four of 117) yielded at least one specimen removed for culture that was positive for bacterial growth, and 13.0% of sterile control specimens (seven of fifty-four) had positive culture growth. P. acnes represented 83.0% of all positive cultures (thirty-nine of forty-seven) at a median incubation time of fourteen days. Among the subjects who had not undergone previous surgery, 17.1% (fourteen of eighty-two) had at least one positive P. acnes culture. Male sex was associated with a greater likelihood of bacterial growth (p < 0.01), and patients who had undergone previous surgery and had received two or more preoperative corticosteroid injections had a higher likelihood of bacterial growth (p = 0.047).

Comment: We always wince when the phrase 'false positive culture' is used. There really is no such thing. Either a culture is positive or it is not. The question is, when a culture is positive, 'Where did the bacteria come from?' In the case of this study, we must ask where the bacteria recovered from 7 of 54 'sterile' gauze sponges sent for culture came from: the sponges themselves, the instruments, the air, the OR personnel, or the laboratory. This 13% 'noise' in the culture results from supposedly sterile sponges creates a real problem in sorting out the ratio of the signal to the noise. As a result of the high noise ratio, no significant difference was found between the proportion of sterile control specimens (13.0%) that eventually yielded positive culture growth and the proportion (20.5%) of all shoulders included in the study that yielded tissue cultures positive for bacterial growth (p 0.234), those shoulders that had had no previous surgery that yielded bacterial culture growth (p 0.409), and those shoulders that had had previous surgery that yielded bacterial culture growth (p 0.128).

One of the ways assess the contamination rate in an institution is to compare the results from male and female patients as shown in the results reproduced below from one of our prior studies.

In a similar note, if we look at the data from the recent study, we see that the rate of positive cultures for the sterile sponges is twice that for cultures from female patients. Makes you wonder what's up with the sponges.

The bottom line is that institutions need to implement process for identifying and controlling the rates of specimen contamination if we are to understand their data on bacterial cultures.


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