Low Cost Method of Screening For Infection of Total Hip Replacement

For anyone with a total hip replacement (THR), hip pain is a red flag. 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. To avoid the high cost of multiple tests, it would be useful for the surgeon to know which tests to order. 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.

So which tests are sensitive, reliable, and accurate with a low false-positive and high true-positive results? That was the focus of this study on over 200 patients with painful total hip implants. The tests they evaluated included:

  • erythrocyte sedimentation (SED rate)
  • C-reactive protein (CRP)
  • Synovial fluid white blood cell count (WBC)
  • per cent of polymorphonuclear (PMN) in synovial fluid

    Values for test results are always listed with suggested normal reference ranges. The surgeon must use the test results along with patient history and clinical presentation when making the final diagnosis of infection. Until now, the physician has relied upon three things: positive intraoperative culture, visible pus in or around the joint, and the pathology report confirming that the culture was infected.

    But now, the authors present data to support the combined use of several lab tests to make the diagnosis. This provides surgeons with a low-cost method of screening for infection of the hip in patients with a total hip replacement. 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.