Synovial fluid white blood cell count of 10,000 may indicate an early hip infection

Issue: May 2012

SAN FRANCISCO — While the standard diagnosis of a chronic infection after total hip arthroplasty takes into account erythrocyte sedimentation rate, C-reactive protein and synovial fluid white blood cell count, the efficacy of using these measures to determine infection in the early postoperative period remains unclear.

At The Hip Society 2012 Specialty Day Meeting, Craig J. Della Valle, MD, discussed the utility of these tests for detecting infection in the first 6 postoperative weeks and provided an algorithm to determine the preferred treatment for early postoperative infections.

“The three numbers that I would ask you to remember would be 100 mg/L for the [C-reactive protein] CRP, 10,000 WBC/uL for the [white blood] cell count, and 90% for the differential,” Della Valle, who is an Orthopedics Today Editorial Board member, said.

Measures of infection

In a retrospective review, Della Valle and colleagues found that among 2,409 consecutive primary total hip arthroplasties (THA), 32 patients underwent re-operation within 6 postoperative weeks. In that group, they identified 9 patients with infection and 23 aseptic cases. The researchers found no significant difference between mean erythrocyte sedimentation rate (ESR) for the infected vs. aseptic cases (86 mm/hr vs. 75 mm/hr).

A fluoroscopic image showing hip aspiration.

Image: Della Valle CJ

“We did find, however, that the CRP was helpful,” Della Valle said. The investigators found mean CRP values of 190 mg/L in the infected patients and 29 mg/L in the aseptic patients. “In all aseptic cases, the CRP was less than 57,” he said, noting normal was CRP of 8 mg/L with the optimal cut-off of about 100 mg/L.

In addition, the mean synovial fluid white blood cell (WBC) count was approximately 65,000 WBC/uL for infected patients and 2,000 WBC/uL for the aseptic cases. “We found the synovial fluid WBC count was the best test in the early postoperative period, with an optimal cut-off point of approximately 10,000 white blood cells, which is quite a bit higher than the 3,000 WBC/uL which we would normally advocate for diagnosis of a chronic infection. Our concern has been that if you use that standard 3,000 WBC/uL number, that you could be over-diagnosing infection and reoperating on patients unnecessarily,” Della Valle said.

Further, the differential was also helpful with an optimal cut-off point of approximately 90% polymorphonuclear cells. Finally, all infected cases had a synovial fluid WBC count of greater than 10,000 WBC/uL or a differential of greater than 90% polymorphonuclear cells. Della Valle noted that while the numbers of study patients were small, the findings of this study correlate well with a larger study the authors had previously published in conjunction with Javad Parvizi, MD, at the Rothman Institute that looked at diagnosis of early postoperative infection following total knee arthroplasty.

Choice of treatment

Della Valle noted the literature lacks evidence for common treatment strategies for infection, such as irrigation and debridement and one- and two-stage exchanges, during the early postoperative period. Although an irrigation and debridement combined with an exchange of the modular femoral head and liner is used most commonly, he noted that the results are discouraging with at best a 50% success rate which is probably worse in cases where the infecting organism is Staphylococcus aureus and/or it is a resistant organism. In light of that, Della Valle and colleagues performed a decision analysis to determine the best treatment for a relatively healthy patient who developed an infection within 6 weeks of primary cementless THA.

“It suggests that if your success rate with irrigation and debridement is greater than 60%, then an irrigation and debridement is probably the optimal treatment strategy,” Della Valle said. “However, if your success rate is less than 60% with an irrigation and debridement or your success rate with a one-stage exchange is greater than 69%, then that may be the optimal treatment option.”

Della Valle highlighted the advantages of a single-stage exchange, which he said include replacing a colonized implant.

Further he noted that,“In the first 6 weeks postop it is never going to be easier to get out a cementless cup and stem. Once you remove those implants, your exposure also increases dramatically and you get a much better shot of doing a good debridement and debriding the bony surfaces.”

Della Valle cautioned, however, that these findings are theoretical and that studies to validate the utility of a one-stage exchange clinically are ongoing.

“However, given the generally poor results of an irrigation and debridement, a one-stage exchange is something that the surgeon can consider in the acute postoperative period,” he said. – by Gina Brockenbrough, MA

References:

Bedair H, Ting N, Moric M, et al. Mark Coventry Award: Diagnosis of infection in the early post-operative period following primary total knee arthroplasty: The utility of synovial fluid white blood cell count. Clin Orthop Relat Res. 2011. 469: 34-40.
Della Valle CJ. Diagnosis and management of early infection. Presented at The Hip Society 2012 Specialty Day. Feb. 11. San Francisco.

For more information:

Craig J. Della Valle, MD, can be reached at Rush University Medical Center, 1611 West Harrison, Suite 300, Chicago, IL 60611; 312-432-2350; email: craigdv@yahoo.com.
Disclosure: Della Valle is involved with CD Diagnostics.