November 01, 2008
5 min read

Is that joint infected? Diagnosing the difficult post-arthroplastyinfection

Joint aspiration, synovial fluid white blood cell count are usefuldiagnostic tools.

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Infection is among the most common yet difficult to diagnose causes of failure following total hip (THA) and knee arthroplasty (TKA). The diagnosis of infection can be challenging as the clinical presentation can be subtle and there is no gold standard for diagnosis. The differentiation between septic and aseptic failure, however, is critical as treatment is fundamentally different if the surgeon identifies infection as the cause of failure.

The early postoperative period

In the early postoperative period, the diagnosis of infection can be particularly challenging. Normal postoperative pain is compounded by expected inflammation at the surgical site and associated edema in the extremity. While the patient who presents in the early postoperative period with fever, an increasing pain pattern and wound drainage has an obvious infection, in most patients the diagnosis is more difficult to make.

While it is tempting to initiate antibiotic treatment at the first signs or symptoms of infection, a better strategy is to definitely rule in or out an infection with a joint aspiration.

Craig J. Della Valle, MD
Craig J. Della Valle

Javad Parvizi
Javad Parvizi

In the case of a TKA, joint aspiration is easily performed in the office while for a THA, aspiration is typically performed in the fluoroscopy suite either by the operating surgeon or a radiologist.

Obtain cultures

In either case, a large diameter needle is helpful — typically 20 gauge or larger — to facilitate aspiration. If fluid is obtained, it should be sent for a synovial fluid white blood cell count with differential and a full set of aerobic, anaerobic, fungal and AFB (acid fast bacilli) cultures.

While data now from several centers has identified the synovial fluid WBC count and differential as a useful perioperative diagnostic tool for the detection of infection, less is known about the utility of these tests when applied to the early postoperative period.

An 18-gauge needle is used to aspirate the hip joint intraoperatively
An 18-gauge needle is used to aspirate the hip joint intraoperatively, once the deep fascia has been entered but prior to opening the hip capsule.

Image: Della Valle CJ

Hemarthrosis from a recent operative procedure may complicate interpretation of the synovial fluid WBC, however, formulas have been developed to assist the surgeon in correcting for a “bloody tap.” If the synovial fluid WBC is clearly positive (>50,000 WBC/ml3) then operative treatment is indicated. If the picture is less clear, the patient can be admitted to the hospital, placed on intravenous antibiotics — preferably vancomycin to cover the most common pathogens that may cause an acute postoperative infection — and the patient’s clinical picture and synovial fluid cultures can be observed until the results are known or the clinical picture clarifies.

Algorithm for the use of preoperative and intraoperative tests
Algorithm for the use of preoperative and intraoperative tests to diagnose periprosthetic sepsis. Source: Della Valle CJ

Image: Della Valle CJ

Chronic infection

Diagnosis of the chronically infected joint is oftentimes similarly challenging. Evaluation should begin with a good history and physical examination: Determine if the joint has been painful ever since the time of surgery, if it drained for a prolonged period after surgery or if the patient was placed on antibiotics or returned to the OR soon after the index procedure. All of these historical points suggest infection.

The patient’s past medical history should also be scrutinized for risk factors that point toward infection such as a history of diabetes, inflammatory arthritis, skin disorder (such as psoriasis), chronic renal failure or anything else, such as a history of an organ transplant that would cause the patient to be immuno-compromised. The physical exam should focus on the wound; evidence of an active or healed sinus clearly indicates infection.

Multiple reports have identified the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) as excellent screening tools for ruling out infection; if both of these are normal, the presence of infection is very unlikely.

Synovial fluid cell count

Several studies from multiple centers have shown that the synovial fluid WBC count is the best perioperative test available for identifying periprosthetic joint infection. When evaluating a TKA, the optimal cut-off point has been reported to be between 1,100 and 3,000 WBC/ml. Although less commonly used, the synovial fluid WBC count is also useful in evaluating a THA; at our center we have used this intraoperatively for some time and a recent analysis has shown that the optimal cut-off value is 3,000 WBC/ml, if the ESR and CRP are both elevated.

Typically, we use an 18-gauge needle to aspirate the hip joint intraoperatively, once the deep fascia has been entered but prior to opening the hip capsule (Figure). Benefits of the synovial fluid WBC count include the ability to use it pre- or intraoperatively, it is inexpensive, objective and available to all orthopedic surgeons. The percentage of polymorphonuclear cells in the synovial fluid WBC count has also been found to useful with a differential of between 60% and 80% being optimal for differentiating infected from noninfected cases.

Other intraoperative tests

Tips for diagnosing suspected infection following joint arthroplasty

Preoperative joint aspiration for culture is also a useful test, however not as useful as the cell count. Patients should be off of antibiotics for at least 2 weeks prior to aspiration. When used routinely prior to revision THA, the rate of false positive results has been shown to be prohibitive and thus it should be used selectively in patients who have an elevated ESR and/or CRP.

It is clear-based on several publications, that an intraoperative Gram stain is a poor test and should not be relied upon to rule out infection. Reports on the precision of intraoperative frozen sections have shown variable results. The optimal criteria for infection is controversial, however an average of between five and 10 polymorphonuclear cells in the five most cellular fields is most commonly cited. Problems with this test include its subjective nature and the potential for sampling error if the wrong areas are sent for analysis.

Bringing it all together

The best screening tests for ruling out chronic infection are the ESR and CRP; if both are negative, infection is unlikely. In both the acute and chronic setting, joint aspiration is an excellent test; the synovial fluid WBC count has been shown to be a useful diagnostic tool when either performed pre- or intra-operatively and the cell count differential may be diagnostic as well. If joint aspiration is performed pre-operatively, the infecting organism can oftentimes be identified.

A note from the editors:

Look for the next installment of Infection Watch in the January issue of Orthopedics Today.

For more information:

  • Craig J. Della Valle, MD, associate professor, Department of Orthopaedic Surgery, Rush University Medical Center, can be reached at 1725 West Harrison, Suite 1063, Chicago, IL 60612; 312-432-2350; e-mail: He is a consultant for Zimmer and he and his institution have received research support from Zimmer. He has received paid travel from Smith-Nephew and Stryker.
  • Javad Parvizi, MD, FRCS, can be reached at the Rothman Institute, 925 Chestnut St., 5th Floor, Philadelphia, PA 19107. 267-339-3617; e-mail: He receives research support from and is a consultant to Stryker, he receives miscellaneous funding from Johnson & Johnson and is a consultant to Smith & Nephew.


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