One Positive Anaerobic Bottle After Knee Arthroplasty
A 64-year-old man is on day 6 after a total knee arthroplasty. He has a temperature of 38.7°C, a knee that is warm but not draining, WBC 13.8 ×10^9/L and CRP 88 mg/L. Two sets of blood cultures (four bottles, 8–10 mL each) are drawn 30 minutes apart from separate venipunctures before antibiotics; the central line is not used. At 34 hours one anaerobic bottle from the first set signals positive, and the Gram stain shows Gram-positive cocci in clusters. The other three bottles remain negative at 5 days. The isolate grows only in the anaerobic bottle and is identified at 48 hours as Staphylococcus epidermidis, coagulase-negative, susceptible to vancomycin. The orthopaedic team asks: "Is this a contaminant? Should we start vancomycin and take the knee out?"
1. Interpret the likelihood that this is contamination versus true bacteraemia, using the number of bottles, time to positivity, organism and clinical context.
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Several features favour contamination: only one of four bottles is positive, the organism is a coagulase-negative staphylococcus (the commonest skin commensal), growth occurred only in the anaerobic bottle, and the time to positivity of 34 hours is relatively long, suggesting a low organism burden. A single positive bottle with a skin commensal has a low positive predictive value for true bacteraemia.
However, contamination cannot simply be declared. Coagulase-negative staphylococci are a leading cause of prosthetic joint infection, and this patient has a prosthesis, fever, a warm knee and raised inflammatory markers. The blood culture is not the primary diagnostic test for a prosthetic joint infection, but a positive culture in this context must be taken seriously and correlated with the joint itself.
Useful criteria for true coagulase-negative staphylococcal bacteraemia include two or more positive bottles, the same organism in repeated sets, an intravascular device, and clinical signs of sepsis. None of these is clearly met here. The correct answer to the clinician is that the result is more likely a contaminant than true bacteraemia but that it is not dismissible, and that the decision depends on the joint and on repeat cultures rather than on this single bottle alone.
2. What additional information do you need, and what do you recommend?
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Information to gather: how the skin was prepared (chlorhexidine, allowed to dry), the exact volume inoculated into each bottle, whether the bottles were filled before or after other tests, whether the patient had received antibiotics in the preceding two weeks, whether there is any indwelling device (the central line was not used but is still a risk), and whether the knee has been aspirated.
Recommendations: notify the clinical team of the Gram stain immediately - that is the laboratory's most time-critical action - and complete identification and susceptibility testing on the isolate, saving it for comparison. If the patient is clinically stable, obtain two further sets of blood cultures from separate sites before starting antibiotics. Aspirate the knee for cell count, crystals and culture; a joint aspirate growing the same organism is far more informative than a single blood bottle. Consider echocardiography if bacteraemia is confirmed. If the patient becomes septic or unstable, start empirical vancomycin after cultures rather than waiting.
Removing the prosthesis is not a decision for a single positive blood culture; it requires evidence of joint infection, usually from aspirate and clinical assessment.
3. A second set also grows Staphylococcus epidermidis with the same susceptibility pattern 12 hours later. How does your interpretation change, and how should the laboratory report it?
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Two positive bottles from separate venipunctures with the same organism and susceptibility pattern markedly increase the probability of true bacteraemia, although it still does not prove that the knee is the source. The laboratory should report both isolates with a comment noting the concordance, and should ensure that the isolates are saved so that a joint aspirate isolate, if any, can be compared by susceptibility pattern or molecular typing.
The clinical implications change too: persistent coagulase-negative staphylococcal bacteraemia in a patient with a prosthesis warrants a search for a deep source - joint aspiration, assessment of the central line, and consideration of endocarditis. Vancomycin is reasonable empirical therapy while the source is defined, but the definitive decision about the prosthesis depends on the joint aspirate and the clinical picture.
The teaching point is that the laboratory's role is to supply the discriminating facts (number of positive bottles, time to positivity, organism, susceptibility concordance, collection quality) quickly and clearly, and to avoid both over-calling a single bottle and dismissing it in a patient with a prosthesis.