Aspiration, cultures, preop antibiotics recommended to combat TKA infections

Get in-office joint fluid aspirations cultured to diagnose infection, identify the bug that caused it.

Issue: September 2008

SAN FRANCISCO — Robert L. Barrack, MD, recommends staying up-to-date on methods of diagnosing total knee arthroplasty infections, identifying germs that cause them and treating this sometimes devastating complication.

“There is no single test that is 100% accurate,” he said at the Knee Society/American Association of Hip and Knee Surgeons Specialty Day Meeting held during the American Academy of Orthopedics Surgeons 75th Annual Meeting.

Better diagnostics may be on the horizon, like serum tests that identify interleukin-6 markers and positron emission tomograph scans, although some are still experimental.

Robert L. Barrack, MD
Robert L. Barrack

Barrack, of St. Louis, reviewed the importance of aspirations, pre- and intraoperative cultures, frozen sections, checking nucleated cell counts and identifying types of cells. He said, “Early, accurate diagnosis is … a critical step in effectively managing an infected knee.”

Nucleated cells

Among available tests, probably the most underutilized is aspiration of fluid from the knee joint, which can be performed quickly in the office without fluoroscopy or other imaging, he told Orthopedics Today.

After obtaining a few ccs of fluid, you should culture the aspirate to determine which bacteria it grows, how many nucleated cells are present and what percent are polymorphonuclear (PMN) cells, Barrack said. Aspirations may be repeated.

He found in a study performed in conjunction with investigators at Tulane University, New Orleans, and Thomas Jefferson University Hospital, Philadelphia, “If the total number of nucleated cells is over 2,000 or 3,000 and the percentage of PMNs is 60% or 70% or higher, then the chances the knee is infected are high,” Barrack said.

The accuracy is about the same as a culture for predicting total knee arthroplasty (TKA) infection, he said.

Culture reliability

Be aware of false negatives in cultures from patients on antibiotics within a month of testing, such as those whose revision surgery was performed soon after the primary. “We won’t even bother to aspirate that knee for 6 weeks unless the patient is in extreme pain or has shown signs of sepsis,” Barrack said.

Pre- and intraoperative cultures are about 95% reliable and are invaluable for identifying the infecting organism. Results approach 90% to 95% accuracy with such tests as erythrocyte sedimentation rate (ESR), C-reactive protein and cell counts with PMN differential, he said.

Find out for sure if a knee is infected before re-implanting the prosthesis during revision surgery. When in doubt, surgeons recommend using an antibiotic spacer.

Fluorodeoxyglucose-positron emission tomograph scans are not currently indicated for diagnosing TKA infection but hold promise for aiding in the process.

Images: Barrack RL

Frozen section

Intraoperative frozen tissue samples can aid diagnosis when infection is uncertain, if results of other tests are contradictory or the knee seems infected but cultures are negative and the ESR is mildly elevated, Barrack said.

These can be performed intraoperatively in 30 minutes. Take tissue from the most suspicious area that is inflamed, red or purulent, near where the infection started or settled.

Tell the lab specifically what to look for and examine the five most cellular fields and count the number of PMNs per high-powered field.

“If it averages greater than 5, there’s a greater than 80% chance of infection. If it averages greater than 10 there’s more than a 90% chance of infection,” Barrack said.

Using preop antibiotics

“It is a common practice to withhold antibiotics in a revision so you can get cultures. Based on all the work we did, that is not a good practice to follow,” Barrack told Orthopedics Today.

He rarely withholds the drugs if the suspicion of infection is high. Even if all tests are normal and risk of infection is low, Barrack won’t withhold them in 80% to 90% of cases. “If somebody doesn’t have an infected knee, you are increasing their risk of that surgery resulting in infection,” he explained.

From their multicenter study, investigators concluded gram stains are inaccurate. “You can really make a case for not doing them because only 10% or 20% of infected knees will have a positive gram stain.” Barrack said.

For more information:

Robert L. Barrack, MD, can be reached at Washington University School of Medicine, 660 S. Euclid Ave., Campus Box 8233, Department of Orthopedics, St. Louis, MO 63110; 314-747-2592; e-mail: barrackr@wustl.edu. He has no direct financial interest in any products or companies mentioned in this article.

Reference:

Barrack RL, Parvizi J, Ghanem E, et al. Diagnosis of TKA infection: Current state of the art. Symposium VII: Complications do happen: How to avoid them and what to do. Presented at the Knee Society/AAHKS Specialty Day Meeting. March 8, 2008. San Francisco.