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Diagnosing periprosthetic infection: false-positive intraoperative Gram stains
Margret Oethinger1, Debra K Warner, Susan A Schindler
1Department of Clinical Pathology, The Cleveland Clinic, Cleveland, OH, USA.
Background:
Intraoperative Gram stains have a reported low sensitivity but high specificity when used to help diagnose periprosthetic infections. In early 2008, we recognized an unexpectedly high frequency of apparent false-positive Gram stains from revision arthroplasties.
Questions/Purposes:
The purpose of this report is to describe the cause of these false-positive test results.
Methods:
We calculated the sensitivity and specificity of all intraoperative Gram stains submitted from revision arthroplasty cases during a 3-month interval using microbiologic cultures of the same samples as the gold standard. Methods of specimen harvesting, handling, transport, distribution, specimen processing including tissue grinding/macerating, Gram staining, and interpretation were studied. After a test modification, results of specimens were prospectively collected for a second 3-month interval, and the sensitivity and specificity of intraoperative Gram stains were calculated.
Results:
The retrospective review of 269 Gram stains submitted from revision arthroplasties indicated historic sensitivity and specificity values of 23% and 92%, respectively. Systematic analysis of all steps of the procedure identified Gram-stained but nonviable bacteria in commercial broth reagents used as diluents for maceration of periprosthetic membranes before Gram staining and culture. Polymerase chain reaction and sequencing showed mixed bacterial DNA. Evaluation of 390 specimens after initiating standardized Millipore filtering of diluent fluid revealed a reduced number of positive Gram stains, yielding 9% sensitivity and 99% specificity.
Conclusions:
Clusters of false-positive Gram stains have been reported in other clinical conditions. They are apparently rare related to diagnosing periprosthetic infections but have severe consequences if used to guide treatment. Even occasional false-positive Gram stains should prompt review of laboratory methods. Our observations implicate dead bacteria in microbiologic reagents as potential sources of false-positive Gram stains.
Insights
False-positive Gram stains in periprosthetic infection diagnosis were caused by dead bacteria in reagents. Reviewing laboratory methods is crucial to prevent inaccurate results and guide appropriate patient treatment.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Infectious Diseases
Background:
- Intraoperative Gram stains for periprosthetic infections show low sensitivity and high specificity.
- An increased frequency of false-positive Gram stains was observed in revision arthroplasty cases.
Purpose of the Study:
- To identify the source of false-positive Gram stain results in revision arthroplasty procedures.
- To describe the cause of erroneous diagnostic test outcomes.
Main Methods:
- Calculated sensitivity and specificity of Gram stains using microbiologic cultures as gold standard.
- Investigated specimen handling, processing, and staining methods.
- Implemented a modified method (Millipore filtering) and prospectively evaluated results.
Main Results:
- Retrospective analysis showed 23% sensitivity and 92% specificity.
- Identified nonviable bacteria in commercial broth reagents as the cause.
- Post-modification, sensitivity was 9% and specificity 99%.
Conclusions:
- False-positive Gram stains, though rare in periprosthetic infections, can have serious treatment implications.
- Dead bacteria in microbiologic reagents are implicated as a source of false positives.
- Laboratory method review is essential when false-positive results are suspected.
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