A blood culture is a diagnostic test used to detect microorganisms, such as bacteria or fungi, in a patient’s bloodstream, a condition known as bacteremia. This test is a fundamental step in diagnosing serious systemic infections like sepsis. The timely identification of a bloodstream infection is a significant factor in influencing patient outcomes and guiding appropriate treatment.
Purpose and Types of Blood Culture Bottles
Blood culture bottles are sterile containers filled with a nutrient-rich liquid, called a broth, which encourages microorganisms in a blood sample to multiply. This broth also contains anticoagulants, such as sodium polyanethol sulfonate (SPS), to prevent blood from clotting. The ratio of blood to broth is managed to dilute the natural bactericidal properties of blood and support microbial growth.
For a comprehensive diagnosis, the primary types of bottles used are aerobic and anaerobic. Aerobic bottles are designed to support the growth of organisms that require oxygen to survive, such as Staphylococcus aureus or E. coli. Anaerobic bottles create an oxygen-free environment for microbes that cannot grow in the presence of air, like Bacteroides species. Using both types increases the chances of identifying the specific pathogen causing the infection.
To distinguish between them, the bottles have color-coded caps. While colors can vary by manufacturer, a common convention is a blue or green cap for aerobic bottles and a purple or orange cap for anaerobic ones. Specialized bottles also exist, such as pediatric bottles for the smaller blood volumes drawn from infants. Some bottles also contain resins that neutralize any antibiotics a patient may be taking, preventing the medication from inhibiting microbial growth in the sample.
The Blood Collection Procedure
Collecting a blood sample for culture requires a strict aseptic technique to prevent contamination. Before the needle is inserted, the patient’s skin at the venipuncture site is thoroughly disinfected, often with a chlorhexidine solution. This step eliminates bacteria on the skin that could enter the sample and cause a false positive result. Healthcare professionals must also practice hand hygiene and use sterile gloves and equipment.
Standard practice involves drawing multiple blood samples from different veins. Two to three sets of cultures are collected from separate sites to increase the likelihood of detecting microorganisms. A “blood culture set” consists of one aerobic and one anaerobic bottle, both filled with blood from a single venipuncture. Filling the aerobic bottle first is a common procedure to avoid introducing air into the anaerobic bottle.
The volume of blood collected is a factor in the test’s accuracy. For adults, each bottle is inoculated with 8 to 10 mL of blood, as overfilling or underfilling can impact the reliability of the results. Pediatric bottles require a much smaller volume, between 0.5 and 4 mL, depending on the infant’s size. The blood culture tubes are always filled before any other blood samples are drawn for other tests.
Laboratory Analysis and Detection
Once collected, the blood culture bottles are sent to a laboratory and placed into an automated incubation system. These instruments maintain a constant temperature to encourage microbial growth. The system continuously monitors the bottles, and the standard incubation period for a blood culture is five days. A sample with no detected growth is reported as negative.
The detection mechanism relies on the metabolic processes of microorganisms. As microbes consume the nutrients in the broth, they produce carbon dioxide (CO2) gas. A sensor at the bottom of each bottle detects any increase in CO2 concentration. When the CO2 level crosses a threshold, the instrument flags the bottle as positive, alerting staff that a pathogen may be present.
This automated process allows for rapid notification. When an alert indicates a positive culture, laboratory personnel remove the bottle for immediate analysis. This ensures that positive results are identified quickly, allowing for timely communication with the clinical team.
Interpreting Blood Culture Results
The outcome of a blood culture test can be positive, negative, or a contaminant. When a bottle is flagged as positive, the first step is to perform a Gram stain on the sample. This provides a rapid, preliminary identification of the bacteria, classifying them as either Gram-positive or Gram-negative. This information helps guide initial treatment decisions.
Following the Gram stain, the sample is subcultured by spreading it onto an agar plate. This allows the microorganism to grow into visible colonies, enabling a definitive identification of the specific species. Concurrently, antibiotic susceptibility testing (AST) is performed. This test determines which antibiotics will be most effective at killing the identified pathogen, helping doctors select the most targeted treatment.
A negative result means no microbial growth was detected after the full five-day incubation period. A result may also be interpreted as a contamination. If a common skin bacterium is found in only one of several culture sets, it is often presumed to be a contaminant from the blood draw rather than a true bloodstream infection.