False-positive Cultures After Native Knee Aspiration: True or False

Synovial fluid aspiration is a routine practice used by most orthopaedic surgeons to aid in the diagnosis of joint infection. In patients for whom there is a low pretest probability of infection, a positive culture-particularly if it is a broth-only culture-may be considered a contaminant, especially if the bacterial species are skin pathogens. To our knowledge no study has evaluated the incidence of contamination of aspirations from the native knee.

What is the frequency of false-positive cultures among knee aspirations of the native knee?

Two hundred patients, with a total of 200 knees, with the diagnosis of degenerative osteoarthritis undergoing a total knee arthroplasty (TKA) were identified for this study. None of these patients had symptoms, signs, or laboratory studies to suggest the presence of joint infection; a positive culture in this population therefore would be considered contaminated. Thirty-two (16%) patients were excluded secondary to a dry aspiration. One patient was enrolled in the study but did not have the knee aspirated and another patient's specimen was accidentally discarded. Each knee was aspirated under sterile conditions before performing the TKA. The fluid was sent for cell count and culture. If insufficient fluid was obtained for both cell count and culture, culture was performed rather than cell count.

There were no false-positive cultures (zero of 166 [0%]) in aspirations of native knees.

Our study would indicate that when done properly under sterile technique, cultures taken from knee arthrocentesis in patients without prosthetic joints should not be affected by perceived contaminant species. A positive specimen finding on culture should raise a strong suspicion of bacterial septic arthritis. Future studies should include more specimens as well as knees with prior TKA to help further identify the rate of false-positive cultures in knee arthrocentesis in both populations.

Level I, diagnostic study.