Paper of the Week: The Incidence And Incubation Period of False-Positive Culture Results in Shoulder Surgery

Paper of the Week: The Incidence And Incubation Period of False-Positive Culture Results in Shoulder Surgery

ICM Philly August 24, 2021

Tyler R. McCarroll, MD, Ryan R. Jaggers, MD, Robert A. Cagle, MD, Thomas E. Davis, MD, PhD, Brenda L. Easton, LPN, Chris T. Curless, MHA, ATC, LAT, Gary W. Misamore, MD

J Shoulder Elbow Surg. 2021 Mar;30(3):538-543
doi 10.1016/j.jse.2020.06.030

Summary by Tyler Radack, BS

The lack of a concise diagnostic algorithm in the identification of postoperative shoulder infections (PSIs) places significant emphasis on the reliability of intraoperative cultures to identify potential infections. Thus, the incidence of false positivity in these intraoperative cultures must be recognized to determine their diagnostic strength and decrease the probability of unnecessary treatment. A major confounder in this determination is C. acnes, isolated in up to 70% of periprosthetic shoulder infections.1 Additionally, a majority of positive cultures for C. acnes are typically identified within 7-10 days, though some advocate for longer incubation times.2,3 McCarroll et al., sought to determine the rate of false-positivity of intraoperative cultures from noninfected patients. Furthermore, they attempted to demystify the relationship between increasing incubation time and culture growth rate.

This was a single-surgeon prospective cohort study consisting of two groups of 50 consecutively enrolled, noninfected patients receiving either primary open or arthroscopic repair from 2015 to 2017. All included patients had an identifiable mechanical source of shoulder symptoms. Patients were excluded if they had a history of shoulder surgery, a shoulder or systematic inflammatory disorder, had been administered glenohumeral injections within the past six months, or were suspected of having an infection. Three samples were obtained for each patient, one superficial, one deep, and one control: a swab waved over the incision. Furthermore, all included samples were incubated for 28 days.

In the open surgery group, 8 out of the 47 patients (17.0%) resulted in positive results from 9 of the 141 cultures (6.3%). C. acnes and coagulase-negative S. aureus were the most commonly isolated organism in 33% and 22% of the cultures, respectively. In the arthroscopic group, 5 out of the 48 patients (10.4%) had positive cultures from 9 of the 144 cultures (5.5%) taken. MRSA and coagulase-negative S. aureus were the most commonly isolated organism, representing 50% and 25% of positive results, respectively.

McCarrol et al, concluded the false-positivity rate for open and arthroscopic shoulder surgery was 17% and 10.4%, respectively. Furthermore, the incidence of false-positive results was not increased by prolonging incubation time to 28 days. Limitations of this study include an inability to prove false-positive results since any shoulder injection at any time could cause preoperative seeding of bacteria. Additionally, clinical follow-up of six months many not be long enough to determine if positive cultures were inconsequential and generalizability of these results to other surgeons, surgery centers, or laboratories is limited.


  1. Chuang MJ, Jancosko JJ, Mendoza V, Nottage WM. The Incidence of Propionibacterium acnes in Shoulder Arthroscopy. Arthrosc J Arthrosc Relat Surg. 2015;31(9):1702-1707. doi:10.1016/j.arthro.2015.01.029
  2. Padegimas EM, Lawrence C, Narzikul AC, et al. Future surgery after revision shoulder arthroplasty: the impact of unexpected positive cultures. J Shoulder Elbow Surg. 2017;26(6):975-981. doi:10.1016/j.jse.2016.10.023
  3. Bossard DA, Ledergerber B, Zingg PO, et al. Optimal Length of Cultivation Time for Isolation of Propionibacterium acnes in Suspected Bone and Joint Infections Is More than 7 Days. Patel R, ed. J Clin Microbiol. 2016;54(12):3043-3049. doi:10.1128/JCM.01435-16
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