A lab strategy to detect prosthetic joint infections
By Robert Muder, MD, Hospital Epidemiologist, Pittsburgh VA Medical Center.
Synopsis: Increasing the incubation time of tissue specimen culture from seven to 14 days increases the rate of culture positivity by one-third.
Source: Schäfer P et al. Prolonged bacterial culture to identify late periprosthetic joint infection: A promising strategy. Clin Infect Dis 2008; 47:1,403-1,409.
Accurate microbiologic diagnosis of prosthetic joint infection (PJI) is problematic. Infecting organisms reside in a biofilm, and standard culture techniques appear to have suboptimal sensitivity. Schäfer et al studied 284 patients with suspected late (> 2 months after implantation) PJI. They took 10 tissue samples at exploration and submitted five for culture and five for histologic examination. Specimen were inoculated onto trypticase soy agar with 5% sheep blood, chocolate agar, MacConkey II agar, and brain heart infusion broth for aerobic culture. Anaerobic culture was performed using Schaedler agar with sheep blood and Schaedler broth. All cultures were incubated for 14 days. They defined a positive result as two different specimens positive for phenotypically identical organisms, or one specimen with growth plus a positive histologic result. This was defined as greater than five polymorphonuclear leukocytes in at least 10 high-power fields.
A total of 110 specimens (39%) were classified as representing infection. Forty-seven cases were classified as contaminants; that is, a single positive culture without histologic evidence of inflammation. At seven days, only 74% of samples from infected patients had positive cultures. Enterobacteriaceae, staphylococci, streptococci, enterococci were most often detected before day seven. Propionibacterium, other gram-positive bacilli, and Peptostreptococcus typically grew within days seven and 13 (86%); 79/92 of the cases with two or more positive cultures also were positive by histologic criteria.
Commentary
Differentiating PJI from aseptic joint failure is often difficult for a number of reasons. The symptoms of PJI and aseptic failure overlap to a considerable degree. Microbiologic diagnosis is hampered by the fact that in late PJI, the organisms typically reside in a biofilm in which they are often metabolically inactive and present in relatively low number. A number of studies suggest that routine culture techniques are relatively insensitive in yielding a microbiologic diagnosis. These have suggested that the diagnostic yield can be increased by various means, including sonication of the removed implant1 or amplification of 16s bacterial ribosomal rRNA2; these techniques may be impractical in many clinical laboratories. Further, there is no universally agreed-upon "gold standard" for the diagnosis of PJI, so the optimal method remains uncertain.
The study by Schäfer et al does not completely resolve this controversy. However, their study has several things to recommend it. The technique is straightforward and readily adoptable by clinical laboratories with minimal increase in cost, material, and effort. The correlation between culture results and histologic evidence, while not 100%, is encouraging. As we don't yet have a practical, precise, and universally agreed-upon definition of PJI, the strategy reported by Schäfer et al definitely shows promise.
References
- Trampuz, et al. Sonication of removed hip and knee prostheses for diagnosis of infection. N Engl J Med 2007; 357:654-663.
- Tunney MM, et al. Detection of prosthetic hip infection at revision arthroplasty by immunofluorescence microscopy and PCR amplification of the bacterial 16S rRNA. J Clin Microbiol 1999;37:3,281-3,290.
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