Saturday, July 22, 2017

False-positive culture - is there such a thing?

Before reading the below, check this post: Propionibacterium in open shoulder surgery - cautionary tale: "false positives" or contamination

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

These authors sought to determine the frequency of false-positive cultures among aspirations of 200 knees with the diagnosis of degenerative osteoarthritis undergoing a total knee arthroplasty (TKA). None of these patients had symptoms, signs, or laboratory studies to suggest the presence of joint infection.Thirty–two (16%) patients were excluded secondary to a dry aspiration. Each knee was aspirated under sterile conditions before performing the TKA. In the operating room, the knee was cleaned with chlorhexidine wipes and then sterilely prepped with Duraprep1 and allowed to dry. The surgeon performing the aspiration was sterilely gloved and gowned with body exhaust suits at the time of aspiration. An 18-gauge spinal needle with a stylet was placed into the knee through the superior lateral parapatellar position, the stylet was removed, and then the knee was aspirated. Immediately after aspiration, synovial fluid was applied on a leukocyte esterase strip as part of a previous study. The remaining fluid was sent for synovial fluid WBC count and culture. If only minimal fluid was available, culture was preferentially obtained over cell count and fluid analysis. The synovial fluid was plated in aerobic conditions on sheep agar, chocolate agar, and MacConkey agar plates, which were held at 35" to 37" C. The anaerobic conditions included a Brucella blood agar with kanamycin and Vancomycin, both held in an anaerobic pouch at 35" to 37" C.

There were no false-positive cultures in the aspirations of the knees.

Comment: This study demonstrates that sterile technique in the operating room and in the microbiology laboratory may prevent the specimens being contaminated by the environment or the laboratory.

Two points should be made: (1) The term ‘false-positive’ seems problematic. If a culture is positive it is positive - unless the laboratory technician recorded the result incorrectly. Frequently, however, the term ‘false-positive’ is used to refer to a positive culture when the observer doesn’t believe the cultured bacteria originated from the specimen submitted. Unfortunately, unless perfect harvesting, handling and culturing methods are used, one cannot be absolutely sure whether cultured bacteria originated from the joint, the air, the surgical gloves, the specimen container, or the laboratory. For this reason, it is important to know the results of control culture specimens to verify the sterility of the environment and specimen management. As an example, Mook et al (The Journal of Bone and Joint Surgery. 97(12):957–963, JUN 2015) found a 13% positive Propionibacterium culture rate for a 1/2 X 1/2 sq cm square of sterile gauge that was placed in a sterile specimen container and set aside during shoulder surgery. These cultures were positive (not falsely positive), although the source of the cultured organisms was not clear.

(2) Propionibacterium are anaerobic organisms now recognized as being pathogenetic in arthroplasty and spine surgery. In that these organisms are sensitive to Kanamycin and Vancomycin, one wonders if the use of Kanamycin and Vancomycin in the anaerobic media used in this study may have interfered with the ability of these cultures to detect Propionibacterium.

The reader may also be interested in these posts:

Consultation for those who live a distance away from Seattle.

Click here to see the new Shoulder Arthritis Book.

Click here to see the new Rotator Cuff Book

Information about shoulder exercises can be found at this link.

Use the "Search" box to the right to find other topics of interest to you.

You may be interested in some of our most visited web pages including:shoulder arthritis, total shoulder, ream and runreverse total shoulderCTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'

See from which cities our patients come.

See the countries from which our readers come on this post.