One Positive Blood Culture Bottle in a Patient with Pneumonia
A 68-year-old man is admitted with community-acquired pneumonia (temperature 38.9 °C, focal right lower lobe infiltrate, WBC 16.5 x 10^9/L). Two sets of blood cultures are drawn from separate venipunctures before antibiotics. At 22 hours, one anaerobic bottle from set 1 turns positive; Gram stain shows Gram-positive cocci in clusters. The isolate is catalase-positive, coagulase-negative, and pyrrolidonyl arylamidase-negative. The other three bottles remain negative at 5 days. The patient has no central venous catheter, no prosthetic joint, valve, or vascular graft, no recent surgery, and no history of injection drug use. He has not had a prior positive blood culture in this admission. The nurse calls asking whether the result means he has a bloodstream infection and needs vancomycin added.
1. Interpret the culture result. What features argue for contamination versus true bacteremia?
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Most likely a skin-flora contaminant. Coagulase-negative staphylococci are the most common blood culture contaminants, and the pattern here is classic:
Favoring contamination:
- Only 1 of 4 bottles positive - a single positive bottle of a typical skin organism has a low positive predictive value for true bacteremia.
- Organism is a constituent of normal skin flora (Staphylococcus epidermidis-like: catalase-positive, coagulase-negative, PYR-negative).
- No indwelling device (no central line, prosthetic valve, joint, or graft) - the major risk factor for true coagulase-negative staphylococcal bacteremia.
- No injection drug use and no repeated positive cultures.
- Clinical picture is explained by pneumonia; there is no hemodynamic instability or sepsis without source.
Cautions that keep you honest:
- Time to positivity and number of bottles are supportive but not definitive; a single positive bottle can represent true bacteremia in a patient with an occult endovascular focus.
- A single anaerobic-bottle-positive result is sometimes seen with a slow-growing or oxygen-sensitive organism, so the isolate must still be identified to species before dismissal.
2. What do you report, what do you tell the provider, and do you perform susceptibility testing?
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Reporting. Report the Gram stain and the identification with an interpretive comment such as 'coagulase-negative Staphylococcus, probable skin flora contaminant; 1 of 4 bottles positive at 22 hours.' Many laboratories do not perform susceptibility testing on a single-bottle coagulase-negative Staphylococcus isolate in a patient without risk factors, because susceptibility results invite treatment of a contaminant and drive vancomycin overuse. That decision should be written in policy, applied consistently, and documented, not made ad hoc.
What to tell the provider:
- This is most consistent with a contaminated culture; it does not establish a bloodstream infection and does not by itself justify adding vancomycin to the pneumonia regimen.
- The pneumonia should be treated on its own merits per community-acquired pneumonia guidance.
- If clinical suspicion of bacteremia persists (persistent fever, hypotension, new murmur, or a device is found on examination), repeat two sets of cultures from fresh sites before considering the isolate significant.
- Recommend review of collection technique (skin antisepsis with chlorhexidine-alcohol, adequate volume per bottle).
Documentation. Notify the provider, note the person notified and the time, and record the interpretive comment in the report.
3. Describe two scenarios in which the same isolate would have to be treated as clinically significant, and what you would do differently.
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Scenario 1 - the patient has a prosthetic device or an intravascular catheter. Coagulase-negative staphylococci are the leading cause of catheter-related and prosthetic-device infections. If a central line or prosthetic valve were present, a single positive bottle would warrant full identification to species, susceptibility testing, and likely device removal or line exchange. Actions change: complete identification with susceptibility, report without the 'probable contaminant' comment, and recommend infectious disease consultation.
Scenario 2 - two or more separate sets positive, especially rapidly (within 12–24 hours). Multiple positive sets for the same organism indicate a sustained bacteremia rather than skin flora introduced at the time of collection. Actions change: full identification and susceptibility, reporting of every positive set, and an urgent call - the concern becomes an endovascular source.
A third scenario worth naming: any patient who is neutropenic, hemodynamically unstable, or on immunosuppression, where even a low-probability isolate may represent true bacteremia and the cost of being wrong is high.
Contrast with a true pathogen: had the single-bottle isolate been Staphylococcus aureus, a Gram-negative bacillus, or Candida, the same culture pattern would always be reported as significant and called urgently regardless of bottle number, because these organisms are rarely skin-flora contaminants.