Coagulase-Negative Staphylococcus in Line and Peripheral Blood Cultures
A 64-year-old man is on postoperative day 6 after a pancreaticoduodenectomy. He has a tunneled central line placed on day 1, a temperature of 38.9 °C, and a white cell count of 13.2 × 10⁹/L. The team draws two sets of blood cultures from separate peripheral venipunctures plus one set from the central line lumen. Results at five days: peripheral set 1 grew coagulase-negative Staphylococcus in the anaerobic bottle only, at 43 hours; peripheral set 2 showed no growth; the line set grew coagulase-negative Staphylococcus in both bottles at 19 hours. The laboratory's stated policy is to report Gram stain and organism identification, with susceptibility testing performed only on clinician request for a single positive bottle. The surgical team asks whether to remove the line and start vancomycin.
1. Interpret these results. Which features argue for contamination, and which argue for a true catheter-related bloodstream infection?
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Features favoring a true catheter-related infection: both bottles of the line set are positive, and the line set turned positive at 19 hours versus 43 hours for the peripheral set. A differential time to positivity of two hours or more, with the line culture flagged first, is a recognized criterion supporting catheter-related bloodstream infection. Two separate sites are involved rather than one.
Features favoring contamination: coagulase-negative Staphylococcus is the most common skin commensal contaminant; one peripheral set was completely negative; and the positive peripheral set grew in the anaerobic bottle only, a pattern typical of low inoculum. Differential time to positivity is only moderately specific.
The decisive variable is the species. Staphylococcus lugdunensis is a genuine pathogen that behaves like S. aureus and should never be dismissed as a contaminant; S. epidermidis in this pattern usually is one. Until the organism is speciated, the results are equivocal and do not by themselves justify line removal or vancomycin.
2. What should the laboratory do next, and what should be reported?
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Speciate every isolate - this is the highest-yield step and is inexpensive. If S. lugdunensis is identified, treat it as a pathogen, perform full susceptibility testing on both isolates, and telephone the result. For any isolate from the line set, susceptibility testing is justified because both bottles were positive and the time to positivity was short.
Report each set with its draw site, the number of bottles positive, and the time to positivity, so the clinical team sees the pattern rather than an unqualified 'coagulase-negative Staphylococcus.' Add an interpretive comment noting that a single positive bottle from one site usually represents contamination, whereas multiple positive bottles from a line with an earlier time to positivity warrant repeat cultures and clinical correlation.
Recommend that the team obtain two further peripheral sets before antibiotics are started, since starting vancomycin first will sterilize the cultures and remove the chance to confirm the diagnosis.
3. What reporting policy would you recommend for coagulase-negative Staphylococcus in this institution, and why?
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Neither extreme is right. Performing full identification and susceptibility on every isolate wastes resources and encourages vancomycin use for contaminants; refusing all workup risks missing S. lugdunensis and a genuine line infection.
A workable middle policy: speciate all isolates (cheap, fast, and the single most informative step); perform susceptibility testing when two or more bottles or sets are positive, when the isolate is S. lugdunensis, when the same organism grows from line and peripheral cultures, or when the clinician requests it; and report with an interpretive comment giving bottle count, draw site and time to positivity.
Pair the policy with prospective audit: review every coagulase-negative Staphylococcus result with the antimicrobial stewardship team so that repeated single-bottle positives in the same patient trigger a conversation. Document the policy in the laboratory handbook and review it annually, because the balance shifts with local epidemiology and line practice.