Diagnosing periprosthetic infection: false-positive intraoperative Gram stains.
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.


OBJECTIVE
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.
Oethinger
Margret
M
Department of Clinical Pathology, The Cleveland Clinic, Cleveland, OH, USA.
Warner
Debra K
DK
Schindler
Susan A
SA
Kobayashi
Hideo
H
Bauer
Thomas W
TW
D019643:Arthroplasty, Replacement / Q000009:adverse effects* / Q000295:instrumentation; D005189:False Positive Reactions; D005840:Gentian Violet; D016908:Gram-Positive Bacterial Infections / Q000175:diagnosis* / Q000382:microbiology / Q000601:surgery; D006801:Humans; D007430:Intraoperative Care; D007595:Joint Prosthesis / Q000009:adverse effects*; D008828:Microbiological Techniques / Q000592:standards; D009820:Ohio; D010619:Phenazines; D011237:Predictive Value of Tests; D011446:Prospective Studies; D016459:Prosthesis-Related Infections / Q000175:diagnosis* / Q000382:microbiology / Q000601:surgery; D012086:Reoperation; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D013194:Staining and Labeling / Q000592:standards
C035863:Gram's stain; D010619:Phenazines; D005840:Gentian Violet