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Using White Blood Cell Counts to Measure Infection in a Total Knee Replacement

How can the doctor tell if a total knee replacement (TKR) is infected? The patient history, an exam, and lab results are helpful. Since lab studies are so important, researchers want to know if the lab values for infection are the same in a knee with an implant compared to a knee without an implant.

Fluid from the knee can be removed and checked for infection. The number of white blood cells (WBCs) and neutrophils are counted. Neutrophils are the WBCs that destroy bacteria in the body. The American College of Rheumatology sets the levels for bacterial infection. They say more than 50,000 WBCs and more than 75 percent neutrophils suggest infection. Normal joint fluid has less than 200 WBCs and fewer than 25 percent neutrophils.

Should these same values be used if a knee with an implant is infected? That's what these doctors tried to find out. They looked at the records of 440 patients with TKRs. Eighty-six had WBCs measured before their TKR was revised. There were 50 knees free of infection and 36 knees with a bacterial infection.

The authors found a much higher WBC count in the infected knees. The infected group had 25,591 WBCs compared to 645 in the uninfected group. After studying the results of the lab values, the researchers found the best lab values for infection in a knee with an implant.

More than 2,500 WBC and greater than 60 percent of neutrophils can be used as measures of bacterial infection in TKRs. Both must be present at the same time. Using synovial fluid is one more test doctors can use to find infection in a TKR before repairing or removing it.

References: J. Bohannon Mason, MD, et al. The Value of White Blood Cell Counts before Revision Total Knee Arthroplasty. In The Journal of Arthroplasty. December 2003. Vol. 18. No. 8. Pp. 1038-1043.

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