One Positive Blood Culture Bottle in a Patient with Pneumonia
A 70-year-old man is admitted with fever, productive cough, and a right lower lobe infiltrate. Two sets of blood cultures are drawn peripherally before antibiotics. At 38 hours, the anaerobic bottle from set 2 turns positive with gram-positive cocci in clusters; the other three bottles remain negative at 5 days. The organism is later identified as coagulase-negative Staphylococcus; it is susceptible to all tested antibiotics. The patient has no central line, no prosthetic material, no IV drug use, and no skin breakdown. He improves on ceftriaxone and azithromycin.
1. Is this likely a true bacteremia or contamination? Explain the reasoning.
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This is most consistent with contamination, based on the following:
- Only 1 of 4 bottles is positive; true bacteremia usually grows in multiple bottles or multiple sets.
- The organism is coagulase-negative Staphylococcus, the most common skin flora contaminant.
- Time to positivity of 38 hours is relatively slow for a true bacteremia from a virulent organism; contaminants often grow later.
- The patient has no central line, prosthetic material, or IV drug use, and no skin source.
- The clinical course fits pneumonia, and he is improving on therapy that does not reliably cover coagulase-negative staphylococci.
None of these features alone is decisive, but together they make contamination far more likely than true bacteremia.
2. What should the laboratory and clinician do with this result?
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- Communicate the result with interpretation: one of four bottles, coagulase-negative Staphylococcus, likely contaminant, but correlate clinically.
- Do not automatically treat with vancomycin for a single contaminant bottle in a stable patient without a line or prosthesis.
- Do not repeat cultures routinely if the patient is improving; if concern persists, draw two new sets from separate sites before changing antibiotics.
- If repeat cultures grow the same organism in 2 or more bottles or from both sets, the probability of true bacteremia rises and the patient needs evaluation for a source and appropriate therapy.
- Document the contamination so it is not carried forward as a diagnosis of bacteremia.
3. Five days later, two of four bottles from a repeat draw grow coagulase-negative Staphylococcus, and the patient develops new fever and hypotension. How does your interpretation change?
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The interpretation shifts toward true bacteremia:
- Growth in multiple bottles/sets greatly increases the likelihood of a true bloodstream infection.
- New hemodynamic instability makes a contaminant much less likely and requires urgent evaluation.
- The patient now needs a source search: examine for an occult central line, prosthetic device, endocarditis, or intra-abdominal source; obtain an echocardiogram if indicated.
- Antibiotics should cover the organism based on susceptibility, and therapy should be tailored once the source is addressed.
- The laboratory should ensure the isolate is saved and that repeat cultures are drawn before antibiotic changes when feasible.
The lesson: the number of positive bottles, time to positivity, organism, and clinical context must be interpreted together, not by organism name alone.