Why Do Some Positive Blood Cultures Fail to Grow Bacteria?

News 24 4 月, 2025

A positive blood culture result with no bacterial growth has long puzzled clinical microbiologists. Distinguishing a false positive from other causes of culture failure can be challenging. Here, we summarize and analyze the common causes of false positives and culture failures in blood culture to aid in judgment.

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Environmental Factors

1. Drastic Temperature Fluctuations
   Large swings in lab temperature can cause fluctuations in blood culture growth curves, leading to false positives. It’s recommended to maintain a consistent room temperature between 15–25°C and avoid placing instruments in direct sunlight or near air vents.
2. Power Supply Issues
   Instability in voltage or sharing circuits with high-power equipment like autoclaves or centrifuges may lead to growth curve disturbances. Use a voltage stabilizer and avoid connecting blood culture instruments to the same line as large devices.

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Instrument and Bottle-Related Factors

1. Instrument Maintenance
   Regular calibration and maintenance are critical. Ensure no culture bottles are inside the instrument during calibration, as this can cause false positives.
2. Blood Culture Bottles
   Ensure proper sample handling and timely loading onto instruments. If not loaded immediately, store at room temperature. Storage above 35°C for more than 12 hours or refrigeration (2–8°C) may cause false negatives or positives.
   Before use, check labels, expiration dates, and media appearance. Store bottles at room temperature, away from light.

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Sample Factors or Other Causes

1. Blood Volume
   Too much or too little blood may lead to CO₂ production by blood cells themselves, mimicking microbial growth. For tumor patients with high blood cell metabolism, false positives may occur, and currently there’s no perfect solution for such cases.
2. False Positive Judged from Culture Failure
   Positive bottles require smear staining and subculture. Columbia blood agar is commonly used, but nutrient-demanding bacteria may fail to grow. If smear shows bacteria but culture fails, switch to richer media (e.g., blood-supplemented BHI agar).
   Note: Some bacteria are self-lysing—delays in subculture post-positive signal may cause failure.

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How to Judge False Positives

If a bottle is flagged as positive but no bacteria are recovered:

1. Direct Smear from Blood Culture Bottle
   All flagged-positive bottles must be stained and examined microscopically. If positive on smear but no growth on plates, it’s likely a subculture failure. For SN or FN positive bottles, use anaerobic plates and incubate in anaerobic conditions. For PF, SA, or FA positives, use blood-supplemented BHI agar.
2. Repeat Culture
   If smear is negative and no growth occurs, draw 10 mL from the flagged-positive bottle into a new blood culture bottle. If it turns positive again, subculture failure is likely. If it remains negative after 5 days, it’s likely a false positive.
3. Growth Curve Evaluation
   Check for curve continuity and fluctuations. Disruptions may indicate instrument issues. Contact the manufacturer for inspection and calibration if needed.

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Conclusion

Blood culture remains the gold standard for diagnosing sepsis and identifying bloodstream pathogens for susceptibility testing. Ensuring optimal functioning of the blood culture system and routine calibration reduces false positives and missed detections, enhancing result accuracy and saving time in the lab.