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how many cultures need to be taken at revision shoulder arthroplasty?

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Propionibacterium grew in twenty-three of thirty cultures of specimens obtained preoperatively from the unprepared epidermis over the area where a skin incision was going to be made for a shoulder arthroplasty; males had a greater average degree of positivity than females. 

Twelve of twenty-one male subjects and zero of twenty female subjects who had cultures of dermal specimens obtained during revision shoulder arthroplasty had positive findings for Propionibacterium. 

Twelve of twenty male subjects and only one of twenty female subjects had positive deep cultures.. 

The positivity of dermal cultures for Propionibacterium was significantly associated with the positivity of deep cultures for this organism.

If Propionibacterium was present in deep tissues, it was likely that it would be recovered by culture if four different deep specimens were obtained and cultured for a minimum of seventeen days on three different media: aerobic, anaerobic, and broth.

They studied 160 primary RSAs (128 females and 32 males, mean age 74 years), excluding patients with obvious infection or an invasive shoulder procedure in the prior 6 months. 

In 90 cases, 11 cultures were obtained.  10 cultures were obtained in the other 70 cases (culture #10 was a sterile sponge to detect false positives). To determine the minimum number of cultures needed to detect Cutibacterium

Two out of the 70 sterile sponges cultured turned out to be positive for Cutibacterium, giving a false positive rate of 2.8%.

There were 42 patients with positive cultures: 20/32 of the males (69%) and 22/128 of the females (17%).

When considering the the 23% of patients with positive deep tissue cultures, the sensitivity to detect Cutibacterium in relation to the number of specimens is shown in the chart below

Comment: The second study above demonstrates that Cutibacterium can be recovered from a subtantial percentage of patients having primary reverse total shoulder arthroplasty. This study of older predominantly female patients needs to be considered in light of the fact that younger male patients are substantially more likely to have positive deep cultures. The post from earlier today (see this link) is of interest.
While the number of positive cultures is of relevance, recent evidence indicates that the degree of positivity is of greater importance in interpreting the results of deep cultures for Cutibacterium. The authors of Characterizing the Propionibacterium Load in Revision Shoulder Arthroplasty A Study of 137 Culture-Positive Cases  reported on 137 revision shoulder arthroplasties from which a minimum of 4 specimens had been submitted for culture and at least 1 was positive for Propionibacterium. Standard microbiology procedures were used to assign a semiquantitative value (0.1, 1, 2, 3, or 4), called the Specimen Propi Value, to the amount of growth in each specimen. The sum of the Specimen Propi Values for each shoulder was defined as the Shoulder Propi Score, which was then divided by the total number of specimens to calculate the Average Shoulder Propi Score.

The number and percentage of positive specimen specific cultures of material obtained from the stem explant, head explant, glenoid explant, humeral membrane, collar membrane, other soft tissue, fluid per shoulder ranged from 1 to 6 and 14% to 100%. 

A high percentage of specimens (mean, 43%; median, 50%) from the culture-positive shoulders showed no growth. 

Only 32.6% of the fluid cultures were positive in comparison with 66.5% of the soft-tissue cultures and 55.6% of the cultures of explant specimens. 

The average Specimen Propi Value (and standard deviation) for fluid specimens (0.35 ± 0.89) was significantly lower than those for the soft-tissue (0.92 ± 1.50) and explant (0.66 ± 0.90) specimens (p < 0.001). 

The Shoulder Propi Score was significantly higher in men (3.56 ± 3.74) than in women (1.22 ± 3.11) (p < 0.001). Similarly, men had a significantly higher Average Shoulder Propi Score (0.53 ± 0.51) than women (0.19 ± 0.43) (p < 0.001).

This investigation suggests that Propionibacterium is unevenly distributed within culture-positive revised shoulders. As a result, the number of specimens and their source (explant, soft tissue, or fluid) have major influences on the culture results for a revised shoulder arthroplasty.

We have subsequently learned to identify patients at high risk for positive deep cultures at revision for failed arthroplasty: young male patients with highly positive preoperative cultures of the skin overlying the intended skin incision and having high serum levels of testosterone who develop shoulder pain and stiffness after an initial “honeymoon” period of good comfort and function. In  these patients multiple deep tissue and explant specimens are sent for culture while wound prophylaxis (Betadine lavage, topic antibiotics), prosthesis exchange, and antibiotic treatment are considered for managing a likely infection pending the results of the cultures.

Our current practice is to take 5 deep tissue or explant specimens cultured on aerobic, anaerobic and broth media for at least 14 days.

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