Culturing PJI: How Many Samples; How Long to Grow?

MSIS study offers evidence-based recommendations
"Periprosthetic joint infection [PJI] can be very difficult to diagnose. Isolating the organism is crucial in directed treatment; however, an organism’s virulence may impact the culture growth,” explained Michael M. Kheir, MD, who presented findings from the paper, “Culturing PJI: A 15-year Study of Samples, Growth Duration, Outcomes, and Organisms,” at the 2017 annual meeting of the Musculoskeletal Infection Society (MSIS).

With their study, Dr. Kheir and his colleagues sought to provide evidence-based recommendations for the optimal number of samples and growth duration, to look at treatment outcomes and their association with percentage of positive cultures, and to explore the microbiology profile over the last 15 years at a single institution.

“The International Consensus Meeting on PJI that was held in 2013 recommended that three to five distinct intraoperative samples be sent for cultures and maintained between 3 to 14 days. While this was a great guideline, the lack of robust evidence for the optimal number of cultures and incubation duration for diagnosis of PJI led us to design the current study.” he said. “The optimal number of cultures and growth duration is dependent on the type of organism. Our study provides evidence that five samples should be obtained at the time of surgery and held for 2 weeks, given that the type of organism is likely to be unknown at the time of surgery in many cases.”

Positive cultures
The researchers conducted a retrospective review of culture samples taken from 711 total joint arthroplasty (TJA) cases (329 hips and 382 knees) treated at a single institution and that met MSIS criteria for PJI. Overall, 2,290 aerobic and anaerobic culture samples had been submitted for analysis between 2000 and 2014. The researchers performed a manual chart review of demographic information, surgical and cultural characteristics, and incidence of subsequent PJI. They defined treatment success based on the criteria established in a previously published Delphi-based consensus.

“Overall, 62.6 percent of all cultures were positive. We further stratified the cultures by organism type,” Dr. Kheir explained.

He noted that under current protocols at the evaluated institution, the percentage of positive cultures for each organism type (not mutually exclusive) is the following:

  • antibiotic resistant organisms: 80.0 percent
  • Staphylococcus aureus: 76.0 percent
  • gram-positive organisms: 68.3 percent
  • coagulase-negative staphylococcus: 63.3 percent
  • non-resistant organisms: 58.9 percent
  • gram-negative organisms: 58.9 percent
  • Pseudomonas spp.: 52.0 percent
  • atypical organisms: 28.8 percent
  • Propionibacterium acnes: 20.0 percent
  • Escherichia coli: 8.0 percent

“Looking at cultures stratified by organism type, we saw wide variability, with resistant organisms requiring the fewest days to culture and the fewest number of cultures needed to obtain positive cultures,” he said. “At the other end of the spectrum, Proprionibacterium acnes [P acnes] is a slower growing organism, needing a higher number of cultures and more days to grow. Overall, when examining the optimal number of cultures, we found that obtaining 5 culture samples provided the greatest yield of positive cultures for diagnosing PJI.

“In determining the optimal number of days,” Dr. Kheir continued, “we found that at 3 days of incubation, only 42.1 percent of cultures turned positive compared to 98.4 percent at 14 days. A majority of organisms were cultured in 5 days or less on average, while 10.8 days were needed for P acnes; 6.6 days were needed for atypical organisms; and 5.2 days were needed for coagulase-negative staphylococci (Fig. 1). Overall, the mean culture duration for positive cultures was higher than negative cultures.


Fig. 1 Percentage of postive cultures vs. days cultured.
Courtesy of Michael M. Kheir, MD

“Although a higher percentage of positive cultures appeared to have lower survivorship in our Kaplan-Meier analysis, it was found to not be associated with treatment failure after adjustment for confounders in our multivariate analysis,” Dr. Kheir concluded.

Stable trends
The microbiology profile remained relatively stable at their institution over the last 15 years, with approximately 80 percent being gram-positive organisms, 13 percent gram-negative, and 7 percent culture-negative. However, over time the researchers noted a significant decrease in gram-positive PJIs and an increase in culture-negative PJIs.

Dr. Kheir said that current literature suggests that extending cultures for 2 weeks may increase culture sensitivity without risk of contaminants. However, he and his colleagues found no studies addressing the cost-effectiveness of growing cultures for 2 weeks compared to 1 week, and suggest that further research may be needed to examine the economic implications.

Regarding limitations, Dr. Kheir noted the retrospective and single institution nature of the study data. “In addition, we excluded polymicrobial samples to observe pure effects of growth on single organisms,” he stated.

“Host factors may have played a confounding role in culture growth, especially in immunocompromised patients, which we did not investigate. We didn’t examine acid-fast bacilli or fungi because of the rarity of such organisms. We did not examine the number of preoperative aspirations, but solely examined intraoperative samples. And there may be regional differences in microbiology profiles,” he concluded.

Dr. Kheir’s coauthors are: Timothy L. Tan, MD; Colin Ackerman, MD; Ronuk Modi BS; Dean D. Tan, BS; and Javad Parvizi, MD, FRCS.

Peter Pollack is the electronic content specialist for AAOS Now. He can be reached at ppollack@aaos.org.

Bottom Line

  • Researchers conducted a retrospective review of 2,290 aerobic and anaerobic culture samples taken from 711 TJA patients who met MSIS criteria for PJI.
  • Of those cultures that were ultimately positive, 42.1 percent were obtained at 3 days and 98.4 percent were obtained at 14 days.
  • A majority of organisms showed growth or colony formation in 5 days or less, while 10.8 days were needed for P acnes, 6.6 days were needed for atypical organisms, and 5.2 days were needed for coagulase-negative staphylococci.
  • For optimal efficacy, five culture samples should be obtained at the time of surgery and held for 2 weeks.