Theses authors retrospectively reviewed 208 revision arthroplasty cases. For all patients included in the study, at least two culture specimens (synovial fluid and/or tissue) were obtained (mean, 4.2 ± 1.4 samples; range, two to seven samples). Tissue and fluid were cultured anaerobically for a mean (and standard deviation) of 13.1 ± 3 days. A positive culture was found for ninety-two (44%). P. acnes was identified in culture in sixty-one (29%) of the cases; in seven of those cases, coagulase-negative Staphylococcus (CONS) was also identified. Organisms identified in the other culture-positive cases included isolated CONS (twenty-two cases), Enterobacter cloacae (two cases), diphtheroid bacilli (two cases), and one case each of methicillin-sensitive S. aureus (MSSA), Staphylococcus epidermidis, Staphylococcus lugdunensis, Peptostreptococcus magnus, and Enterococcus gallinarum.
This study concerned 46 cases that had P. acnes only positive cultures. Cases were categorized into one of two groups for analysis: probable true positive or probable contaminant (false-positive) on the basis of culture results and perioperative findings.
Among the cases that were positive the time to P. acnes culture growth was significantly shorter (p = 0.002) in the probable true-positive culture group compared with the probable contaminant group (median of five days compared with nine days). Among the thirty-seven cases in the probable true-positive group, no culture result turned positive after eleven days, whereas in the probable contaminant group, cultures turned positive after this time point in 44% (four of nine) of the cases. There were also significantly fewer days to P. acnes culture growth among cases with a higher number of positive cultures (p = 0.001) and a higher proportion of positive cultures (p < 0.001), regardless of group classification.
Comment:
There is no such thing as a false positive culture - if bugs grow the culture is positive. There is no practical way of determining whether the bacteria cultured existed in the surgical wound or whether the specimen was contaminated by the operating room or laboratory environment.
Both the percent of specimens that are culture positive and the time for cultures to become positive depend on the number of bacteria present in the shoulder, so it is not surprising that the two are related as shown by this plot of the data from this study.
Both the percent of specimens that are culture positive and the time for cultures to become positive depend on the number of bacteria present in the shoulder, so it is not surprising that the two are related as shown by this plot of the data from this study.
The clinical manifestations of a shoulder infection are also expected to be related to the number of bacteria in the shoulder, so those shoulders in the upper left corner are more likely to have clinical findings that those in the lower right corner.
Our interpretation of the data in this study is that the percent of cultures that are positive, the time it takes for the cultures to become positive, and the degree of clinical manifestation of the infection are all eflections of the severity of the infection. There is no additional benefit in trying to assign some culture results to a 'contaminant' or 'false positive' category. This information on the severity of the infection may influence the decision about how the infection should be managed.
We need to remember:
(1) Shoulders containing greater numbers of bacteria are more likely to have clinical manifestations.
(2) Shoulders containing greater numbers of bacteria are more likely to have a a high percentage of cultures that are positive.
(3) Shoulders containing greater numbers of bacteria are more likely to have shorter times for cultures to become positive. The reason is that a culture becomes positive when the laboratory technologist can see growth on the culture plate. This growth is visible when a threshold number of bacteria have grown. The time for this threshold to be reached depends on the amount of bacteria streaked out on the culture plate - the size of the innoculum. The size of the innoculum, in turn, depends on the number of bacteria in the shoulder.
Thus the authors' conclusion"There were also significantly fewer days to P. acnes culture growth among cases with a higher number of positive cultures (p = 0.001) and a higher proportion of positive cultures (p < 0.001), regardless of group classification."
We all continue to struggle with the best way to manage patients with these different types of culture results. Progress comes in small steps, but one of the most important ones is recognition of the need, as explained here, to obtain at least 5 specimens from tissue or explants, to culture these specimens on aerobic and anaerobic media, to hold the cultures for 17 days, and to report the results in a way that indicates the source, the number of specimens and the number of specimens that were culture positive, as in this example: Propi: Tissue: 2+/3, Explants: 3+/3, Fluid 0+/1
Thus the authors' conclusion"There were also significantly fewer days to P. acnes culture growth among cases with a higher number of positive cultures (p = 0.001) and a higher proportion of positive cultures (p < 0.001), regardless of group classification."
We all continue to struggle with the best way to manage patients with these different types of culture results. Progress comes in small steps, but one of the most important ones is recognition of the need, as explained here, to obtain at least 5 specimens from tissue or explants, to culture these specimens on aerobic and anaerobic media, to hold the cultures for 17 days, and to report the results in a way that indicates the source, the number of specimens and the number of specimens that were culture positive, as in this example: Propi: Tissue: 2+/3, Explants: 3+/3, Fluid 0+/1
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