Tuesday, February 6, 2018

Thoughts about "two positive cultures"

Many surgeons diagnose a periprosthetic infection when two specimens from a revision arthroplasty grow out the same organism. 

In considering the application of this criterion, four factors should be kept in mind. 

(1) the importance of the denominator: the chances of obtaining two positive cultures rises with the number of specimens sent for culture. If three specimens are sent and one is culture positive, the joint does not meet the criterion. However, the odds of each subsequent culture being positive in this example is one out of three. Thus if two more specimens are submitted from the same shoulder, it is very likely that one of the two would be positive, bringing the total of positive cultures to two. In that case the shoulder diagnosis would change from being uninfected to being infected just by changing the number of specimens submitted.

(2) the source of the specimen affects its likelihood of being culture positive: joint fluid specimens are less likely to be culture positive than tissue or explant cultures from the same shoulder (see this link).

(3) the media used in the culture of a specimen affects its likelihood of being culture positive: broth, aerobic and anaerobic cultures used together are most sensitive (see this link).

(4) cultures are not simply positive or negative: as shown in this article, "Characterizing the Propionibacterium Load in Revision Shoulder Arthroplasty: A Study of 137 Culture-Positive Cases" (see this link). While some positive cultures grow out only one colony or are only positive in the broth, others have 2+ growth on agar plates, indicating a much greater bacterial load.

So, while commonly used, the criterion of 'two or more positive cultures' needs to be applied being mindful of these four factors.

=====
The reader may also be interested in these posts:



Consultation for those who live a distance away from Seattle.

Click here to see the new Shoulder Arthritis Book.

Click here to see the new Rotator Cuff Book

Information about shoulder exercises can be found at this link.

Use the "Search" box to the right to find other topics of interest to you.

You may be interested in some of our most visited web pages including:shoulder arthritis, total shoulder, ream and runreverse total shoulderCTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'

See from which cities our patients come.

See the countries from which our readers come on this post.