During revision of shoulder arthroplasties presenting with pain, stiffness or prosthetic loosening, surgeons frequently obtain Cutibacterium-specific cultures of deep tissues and prosthetic explants because:

(1) Cutibacterium is the bacterium most commonly isolated from shoulder periprosthetic infections and (2) Cutibacterium periprosthetic infections (CPJI) typically have a stealth type presentation without the fever, chills, swelling, erythema, tenderness, and elevated serum or joint fluid inflammatory markers characteristic of the obvious periprosthetic infections from other organisms that can complicate hip and knee arthroplasties.

While some authors refer to positive cultures in the absence of obvious evidence of periprosthetic infection as unexpected, these cultures are obtained because surgeons expect some of their shoulder revisions to be sufficiently culture positive to merit treatment for CPJI.

As an example, a young healthy man who had an initially normal post-arthroplasty course for 9 months and then develops otherwise unexplained pain and stiffness of the shoulder and who has high loads of Cutibacterium on a swab of the unprepared skin over the shoulder would be expected to be at high risk for positive Cutibacterium cultures obtained at the time of revision. In such a case multiple deep tissue and explant cultures would be submitted for aerobic and anaerobic cultures to be observed for at least 14 days. Because the cultures were expected to be positive, the revision would probably consist of a single stage prosthesis exchange after thorough debridement and lavage followed by a course of antibiotics starting immediately after surgery and continued until the results of the cultures were final. Subsequent treatment would be guided by the results of the cultures.

The authors of Evidence-Based Approach to Managing Unexpected Positive Cultures in Shoulder Arthroplasty point out that positive cultures are commonly found when surgeons send intraoperative cultures to rule out periprosthetic joint infection in failed arthroplasties that do not have obvious clinical or radiographic signs of infection.

They conducted a review of 22 studies reporting on positive cultures from patients who did not have clinical or laboratory findings consistent with infection preoperatively. Studies that included patients with obvious periprosthetic infections were excluded.

These authors report that the primary finding from their study was that the rate of positive deep tissue cultures was 27.5% in revision shoulder arthroplasty; Cutibacterium, accounted for 76.4% of these cases. Only 3 of the 22 studies investigated the culture rate of control cultures of inanimate specimens (sterile gauze or suture), The pooled rate of positive cultures in these three studies was 20.1%

The utilization of antibiotics and treatment regimens varied across these studies. Patient reported outcomes and re-operation rates did not differ between patients with positive and those with negative cultures.

Based on the results of this review, the authors recommend that at least five cultures be obtained at revision arthroplasty and that two or more positive cultures are suggestive of true bacterial colonization. Cultures that have 2+ or more growth on agar plates indicate a much greater bacterial burden compared to only one colony or growth only in the broth. The senior author’s preferred protocol is typically to put all revisions on a 14 day course of oral doxycycline while awaiting final culture data.

The authors state that the positive culture rate in revision arthroplasty (27.5) was "only slightly greater than the control rate of culture positivity for sterile, inanimate objects (20.1%)". However, as pointed out above, the false positive rate of 20.1% was based on only three studies, two of which cultured sterile sponges and one of which cultured sterile suture. The 20.1% figure may not accurately reflect the false positive in most medical centers. For example, in the study What do Positive and Negative Cutibacterium Culture Results in Periprosthetic Shoulder Infection Mean?, two sterile gauze samples were cultured for Cutibacterium at 11 different institutions. These samples grew Cutibacterium in only 3 of 22 samples at two of the 11 institutions, the other 9 institutions had no growth for the sterile samples. Furthermore, the strength of culture positivity was significantly lower in these negative controls compared to positive specimens (p<0.001). No negative control had >1 quadrant of Cutibacterium growth on agar plates. In sum, only 2 of the 11 institutions cultured Cutibacterium from the sterile controls and the amount of growth from these sterile controls was significantly less than that for the samples containing various concentrations of Cutibacterium.

Similarly in Preoperative Skin-Surface Cultures Can Help to Predict the Presence of Propionibacterium in Shoulder Arthroplasty Wounds, only 2 of 50 sterile control specimens were culture positive for Cutibacterium and both of these control samples had no more than minimal growth.

Finally, in The efficacy of topical preparations in reducing the incidence of Cutibacterium acnes at the start and conclusion of total shoulder arthroplasty: a randomized controlled trial, none of the 101 control swabs that had been placed within the sterile field on the surgical trolley for the case duration were culture positive for Cutibacterium.

Thus it seems essential that each institution assess its own rate of positive cultures for sterile control specimens, rather than assuming that all institutions have the same false positive rate.

The importance of culturing multiple deep tissue and explant specimens in revision shoulder arthroplasty - even if there is no obvious evidence of infection - is demonstrated in Single-Stage Revision Is Effective for Failed Shoulder Arthroplasty with Positive Cultures for Propionibacterium: revised shoulder arthroplasties having 2 or more positive cultures for Cutibacterium and treated with single stage revision and appropriate antibiotic therapy had at least as good 4 year outcomes as for revised shoulder arthroplasties with no or minimal Cutibacterium growth.

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