Prospective analysis of preoperative and intraoperative investigations forthe diagnosis of infection at the sites of two hundred and two revision total hip arthroplasties

Background: Total hip arthroplasty is a commonly performed procedure in the United States and Canada that is associated,vith a definite risk of postop erative infection. Moreover, diagnosing an infection after total hip arthro plasty can present a challenge as there are no preoperative tests that are consistently sensitive and specific for infection in patients who need a re vision arthroplasty. The present prospective study was performed to evaluat e a variety of investigations for the diagnosis of infection at the site of a previous arthroplasty in order to determine if any combination of diagno stic studies could be used to determine which patients are at risk for a po stoperative wound infection. Methods: We prospectively analyzed the preoperative and intraoperative inve stigations used for the diagnosis of infection in 178 patients who had a to tal of 202 revision hip replacements. Clinical data were collected preopera tively. Investigations to determine the presence or absence of infection in cluded a white blood-cell count, measurement of the erythrocyte sedimentati on rate, measurement of the level of C-reactive protein, preoperative aspir ation of the joint, intraoperative gram-staining and culture of periprosthe tic tissue, a white blood-cell count in synovial fluid, and examination of intraoperative frozen sections. Frozen sections were analyzed in a blinded fashion without knowledge of clinical or laboratory data. Patients receivin g antibiotics at the time of aspiration or collection of specimens for intr aoperative culture were excluded from the analysis of those investigations, regardless of the results of the cultures. A positive result (suggestive o f infection) was clearly defined for each of the investigations. Results: Thirty-five hips (17 percent) were determined to be infected on th e basis of clinical findings and positive results, according to the defined criteria, of investigations. With inflammatory conditions excluded, the se nsitivity, specificity, positive predictive value, and negative predictive value were 0.82, 0.85, 0.58, and 0.95, respectively, for the erythrocyte se dimentation rate and 0.96, 0.92, 0.74, and 0.99, respectively, for the leve l of C-reactive protein. All patients who had a periprosthetic infection ha d an elevated erythrocyte sedimentation rate or level of C-reactive protein , but not always both. When patients who were receiving antibiotics were ex cluded, the results of aspiration of the joint were 0.86 for sensitivity, 0 .94 for specificity, 0.67 for the positive predictive value, and 0.98 for t he negative predictive value. Intraoperative studies revealed sensitivities , specificities, positive predictive values, and negative predictive values of 0.19, 0.98, 0.63, and 0.89, respectively, for gram-staining of specimen s of the most inflamed-appearing tissue; 0.36, 0.99, 0.91, and 0.90, respec tively, for the white blood-cell count in synovial fluid; and 0.89, 0.85, 0 .52, and 0.98, respectively, for a neutrophil count in synovial fluid of mo re than 80 percent. The sensitivity, specificity, positive predictive value , and negative predictive value were 0.80, 0.94, 0.74, and 0.96, respective ly, for the frozen sections and 0.94, 0.97, 0.77, and 0.99, respectively, f or the intraoperative cultures. Conclusions: The combination of a normal erythrocyte sedimentation rate and C-reactive protein level is reliable for predicting the absence of infecti on. Aspiration should be used when the erythrocyte sedimentation rate or th e C-reactive protein level is elevated or when a clinical suspicion of infe ction remains. We found the gram stain to be unreliable. Examination of int raoperative frozen sections is useful in equivocal cases or when hematologi cal markers may be falsely elevated because of an inflammatory or other con dition.