Coagulase-negative staphylococci in a dialysis catheter: contaminant or pathogen?
Mr. D, a 58-year-old on maintenance hemodialysis through a tunneled cuffed catheter, develops chills mid-session and a temperature of 38.9 °C. Two blood culture sets are drawn 30 minutes apart, one from the catheter and one from a peripheral vein. At 13 hours the automated instrument signals positive: both catheter bottles and the peripheral anaerobic bottle. Gram stain of all three bottles shows gram-positive cocci in clusters. At 18 hours the subculture shows a single colony type that is catalase positive, coagulase negative, white, and nonhemolytic. The nurse asks whether this is 'just a skin contaminant,' and the infectious disease fellow says to work it up 'only if it matters.' The laboratory has MALDI-TOF, a cefoxitin 25 µg disk, and a penicillin MIC panel available.
1. Using only the information given, argue whether this represents true bacteremia or contamination. What additional facts would most change your conclusion?
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Several features favor true bacteremia rather than contamination:
- Multiple bottles and both sites positive. Contaminants usually appear in one bottle of one set; growth in both a catheter and a peripheral set, including two of two catheter bottles, is unusual for a skin-flora contaminant.
- Short time to positivity (13 h). Rapid growth implies a high organism burden; contaminants typically appear after 24–48 h.
- A single, pure morphotype on subculture is consistent with a genuine bloodstream isolate (though it does not exclude contamination).
- Clinical context: fever with rigors during dialysis through a tunneled catheter is the classic presentation of catheter-related bloodstream infection with coagulase-negative staphylococci.
Facts that would change the conclusion: the number of sets drawn overall and how many were positive; whether the peripheral set was truly peripheral (not drawn through the line); colony counts or growth in only one of several bottles; whether the same organism was recovered previously; presence of a prosthetic device; and whether the patient had already received antibiotics before the draw.
Because the pretest probability of true bacteremia is high and therapy hinges on the answer, the isolate should be identified and worked up rather than dismissed.
2. What identification and susceptibility testing should be performed and reported, and how should oxacillin resistance be detected in this organism?
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Identification: Confirm species by MALDI-TOF (or equivalent). Species matters - Staphylococcus lugdunensis and S. schleiferi behave far more aggressively than S. epidermidis, and S. lugdunensis should essentially never be dismissed as a contaminant.
Susceptibility: Perform and report a full panel for this isolate, because it has been judged clinically significant. Practical points:
- Oxacillin/cefoxitin: test with the cefoxitin 25 µg disk (zone ≤24 mm indicates resistance for coagulase-negative staphylococci) or by an oxacillin MIC with the coagulase-negative staphylococcal breakpoint (susceptible at ≤0.25 µg/mL). Do not apply S. aureus breakpoints.
- Penicillin: test with a penicillin MIC or beta-lactamase test; most coagulase-negative staphylococci produce beta-lactamase.
- Avoid reporting agents that are intrinsically inactive (e.g., ceftazidime, aztreonam) and suppress unreliable bug-drug combinations.
- A cefoxitin-resistant isolate should be reported as resistant to all beta-lactams, and mecA/mecC confirmation sent to the reference laboratory if epidemiologically important.
Report with a comment that the isolate was recovered from multiple sets and is being treated as a true bloodstream isolate.