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.
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