A single positive broth culture in suspected chronic prosthetic joint infection
A 67-year-old man with a right total hip arthroplasty 3 years ago reports 6 months of deep groin pain and night aches. He is afebrile. CRP 14 mg/L, ESR 32 mm/h, WBC 8.4 x 10^9/L. He completed a 10-day course of oral cefuroxime 3 weeks ago for presumed cellulitis, and the joint aspiration was done 12 days after the antibiotic stopped. Synovial fluid: 3200 nucleated cells/microlitre, 82% neutrophils, no crystals, Gram stain shows no organisms. The fluid was inoculated into two pairs of blood culture bottles; one aerobic bottle from the first pair turned positive at 4 days with coagulase-negative staphylococcus. The direct agar plates and the other three bottles remain negative at 5 days.
1. How should this single broth-only coagulase-negative staphylococcus be interpreted, and what additional information would change your interpretation?
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A single coagulase-negative staphylococcus (CoNS) from one of four bottles, with negative direct plates, is most often a skin-flora contaminant, and it does not by itself establish prosthetic joint infection. Several features, however, argue for caution rather than dismissal: CoNS is the commonest cause of chronic, low-grade prosthetic joint infection; the patient has a compatible pain syndrome, an inflammatory synovial fluid (3200 cells/microlitre, 82% neutrophils) and modestly raised CRP/ESR; and recent cefuroxime will have suppressed competing organisms and reduced colony burden, so a low-inoculum pathogen can appear in broth only.
Information that would shift the interpretation: the same organism grown from a prior aspirate or from multiple bottles; a short time to positivity suggesting a higher burden; colony counts; growth on direct plates; prior microbiology from the joint; and, above all, sonication fluid and multiple periprosthetic tissue cultures taken at revision. Alpha-defensin and leukocyte esterase are useful adjuncts but lose specificity and sensitivity in the setting of recent antibiotics and metallosis.
2. What is your next step in management and laboratory workup, and how would you interpret one positive tissue culture out of five at revision?
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Do not commit this patient to a prolonged oral or suppressive antibiotic course on the strength of a single broth-only isolate. Discuss with orthopaedics and infectious diseases; stop antibiotics if clinically safe and allow an antibiotic-free interval (ideally 2 weeks) before repeat aspiration, accepting that a further aspiration carries a false-negative risk and may not settle the question. Send the repeat fluid for cell count and differential, alpha-defensin, and culture inoculated both into blood culture bottles and onto agar, and ask the laboratory to hold cultures 14 days for indolent organisms, including Cutibacterium.
At revision, send at least three to five separate periprosthetic tissue samples plus the explanted components for sonication, and hold cultures 7-14 days. One positive of five tissue cultures with CoNS is usually contamination if histology is negative, but the same organism from two or more samples, or a single positive with a compatible histology and sinus tract, satisfies accepted diagnostic criteria for infection. The clinical decision (one-stage versus two-stage revision, antibiotic choice and duration) should rest on the combined picture, not on a single culture.