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White blood cell count in presence of elevated ESR, CRP level predicts infection status in patients undergoing total hip arthroplasty, researchers report

Friday, October 03 2008 | Comments
Evidence Grade 1 What's This?
In a recent study, a synovial fluid cell count of >3,000 white blood cells/mL was the most predictive perioperative test for determining the presence of periprosthetic infection when combined with an elevated preoperative erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level in patients undergoing revision total hip arthroplasty.

"... no completely reliable diagnostic test is currently available for establishing the presence of infection at the site of a total hip arthroplasty," the authors of the study wrote.

To evaluate the utility of commonly available tests in assessing periprosthetic infection status, researchers prospectively examined data from 235 consecutive hip arthroplasties in 220 patients who were treated with reoperation. Patients were considered to have an infection if they met 2 of the 3 criteria: a positive intraoperative culture, gross purulence at the time of reoperation, and positive histopathological findings.

There were 201 evaluable total hip arthroplasties, among which 55 hips (27.4%) were deemed to be infected. Thirty-four total hip arthroplasties were excluded because of a draining sinus, incomplete data, or a preoperative diagnosis of inflammatory arthritis.

Overall, preoperative ESR and CRP levels, synovial fluid white blood cell count, and synovial polymorphonuclear cell concentration were all significantly greater among patients with infection than in patients without infection (P<.001). Meanwhile, the number of years from primary surgery to revision was significantly greater among patients without infection (P<.001).

The optimal white blood cell count cut-off for the entire patient cohort was 4,200 cells/mL, at which point the hip aspirate white blood cell count had a sensitivity of 84%, a specificity of 93%, a positive predictive value of 81%, a negative predictive value of 93%, and an accuracy of 90%.

Furthermore, combining multiple perioperative diagnostic tests improved the strength of the test battery. When elevated preoperative ESR and CRP level were combined with the hip aspirate white blood cell count, the optimal white blood cell count cut-off fell to 3,000 cells/mL. When either ESR or CRP level was elevated, but not both, the optimal white blood cell count cut-off was 9,000 cells/mL.

The highest accuracy was attained when elevations of ESR and CRP were defined as values above the reference ranges given by the laboratory performing the test. Using this definition, white blood cell count with a cut-off of 3,000 cells/mL had sensitivity of 91%, specificity of 91%, positive predictive value of 95%, negative predictive value of 83%, and accuracy of 91% in the presence of both elevated ESR and CRP.

Additionally, the specificity for non-infection using white blood cell count with a cut-off of 3,000 cells/mL reached 100% when both the preoperative ESR and CRP level were <30 mm/hour and <10 mg/dL, respectively.

Compared with intraoperative cultures--the current gold standard for diagnosing infection at the site of a total hip arthroplasty--the use of hip aspiration for determination of the white blood cell count and differential as a perioperative diagnostic test has the advantages of high accuracy, low cost, and rapid turnaround, and it can be performed either preoperatively or intraoperatively without specialized equipment, the authors concluded. (Schinsky MF, et al. J Bone Joint Surg Am 2008;90:1869-1875.)

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