When I was in the hospital with an infection of my hip replacement, they did bunches and bunches of tests before they finally figured out what was wrong. Was this really necessary?

For anyone with a total hip replacement (THR), joint infection is a real possibility. And infection can lead to implant failure. Medical evaluation is important because of the risk of joint infection. Treatment depends on knowing if there is (or isn’t) an infection. But there isn’t one single or individual test that works best to diagnose or rule out infection.

The current standard for testing for joint infection is to remove fluid from the joint and test it for bacteria. Cultures of the joint taken directly from the area during surgery are the most accurate.

But this type of testing is not practical for the patient who doesn’t have an infection or who doesn’t need surgery. And there is a fair amount of false-positive test results with intraoperative cultures due to errors in sampling technique. Not only that, but sometimes patients are given antibiotics before the operation (affecting test results) and the surgeon doesn’t know it.

To avoid the high cost of multiple tests, it would be useful for the surgeon to know which tests to order. A recent study from Rush University Medical Center in Chicago showed that the results of three tests reviewed together make the diagnosis or infection reliable and accurate. The low cost and high specificity make these screening tests attractive and practical.

Here’s what they found:

  • By itself, white blood cell count in synovial fluid is a high-quality diagnostic test.
    Values above 4200 white blood cells/mL are a red flag for infection.

  • A cut-off point of 3000 white blood cells/mL can be used when combined with elevated
    SED rate and CRP level.

  • If only one value is elevated (either SED rate or CRP), then the cut-off point for
    white blood cells is 9000. Anything above this value is a red flag.

  • No infection is present when the preoperative SED rate was less than 30 mm/hour and the CRP was less than 10 mg/dL.

    Using these guidelines, the surgeon can consistently and accurately diagnose hip infection. White blood cell count is very useful and highly accurate in diagnosing hip infection when combined with values for SED rate and CRP.