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

• Published in: Clinical orthopaedics and related research Vol. 475; no. 7; pp. 1840 - 1843
• Main Authors: Jennings, Jason M., Dennis, Douglas A., Kim, Raymond H., Miner, Todd M., Yang, Charlie C., McNabb, David C.
• Format: Journal Article
• Language: English
• Published: New York Springer US 01-07-2017; Lippincott Williams & Wilkins Ovid Technologies
• Subjects: Arthritis; Arthroplasty (knee); Arthroplasty, Replacement, Knee; Cell culture; Colony Count, Microbial; Conservative Orthopedics; Contaminants; Contamination; False Positive Reactions; Humans; Infection; Infections; Joint diseases; Joint surgery; Knee; Knee Joint - microbiology; Medicine; Medicine & Public Health; Orthopedics; Osteoarthritis; Osteoarthritis, Knee - surgery; Patients; Skin; Sports Medicine; Suction; Surgery; Surgical Orthopedics; Symposium: 2016 Musculoskeletal Infection Society Proceedings; Synovial fluid; Synovial Fluid - microbiology; Periprosthetic Joint Infection; Primary Total Knee Arthroplasty; Septic Arthritis; Synovial Fluid; Total Knee Arthroplasty
• ISSN: 0009-921X; 1528-1132
• DOI: 10.1007/s11999-016-5194-4

Background
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.
Questions/purposes
What is the frequency of false-positive cultures among knee aspirations of the native knee?
Methods
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.
Results
There were no false-positive cultures (zero of 166 [0%]) in aspirations of native knees.
Conclusions
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 of Evidence
Level I, diagnostic study.