Blood cultures are a procedure healthcare providers use to diagnose systemic infections, like bacteremia or sepsis, which is a life-threatening response to infection. This test involves collecting a small amount of blood and placing it into specialized bottles containing nutrient broth to encourage any present microorganisms to grow. The reliability of this diagnostic tool depends entirely on a precise collection process, as procedural error can lead to misleading results. Obtaining an accurate culture quickly is paramount for guiding effective treatment with antibiotics.
The Two Types of Culture Bottles
A standard blood culture collection involves two distinct types of bottles, each designed to detect different kinds of bacteria. The environmental needs of the microorganisms determine which bottle they are placed in for growth. These bottles contain a liquid medium, or broth, that provides the necessary nutrients for any bacteria in the blood sample to multiply.
One bottle is designated as “aerobic” and is intended for bacteria that require oxygen to survive and replicate. The second bottle is “anaerobic,” specifically formulated for organisms that cannot grow in the presence of oxygen. Together, the pair of bottles maximizes the chance of identifying the infectious agent, covering the broadest possible spectrum of potential pathogens.
Both bottles are typically inoculated during a single venipuncture, which is defined as one blood culture “set.” For adult patients, collection guidelines generally recommend obtaining two or three of these sets from separate sites. This increases the sensitivity of the test and helps distinguish a true infection from a contaminated sample.
The correct volume of blood is also a significant factor, with 8 to 10 milliliters per bottle being the standard for an adult set.
The Essential Order of Draw
When collecting blood cultures using a winged infusion set, often called a butterfly needle, the aerobic bottle must be filled first. This specific sequence is based on the physical design of the collection equipment and the needs of the microorganisms. The tubing that connects the butterfly needle to the collection adapter contains a small amount of air.
If the anaerobic bottle were filled first, the air trapped inside the tubing would enter the bottle, severely compromising its oxygen-free environment. Introducing oxygen can inhibit or prevent the growth of strict anaerobic bacteria, leading to a false-negative result. By filling the aerobic bottle first, the air is safely diverted into the container that requires oxygen, preserving the specialized environment of the anaerobic bottle.
Filling the aerobic bottle first also serves a secondary purpose by flushing the collection system before the blood enters the anaerobic environment. This action helps to wash out any potential surface contaminants, such as skin flora or residue from the antiseptic solution, that may have adhered to the needle or the tubing. This procedural step reduces the risk of inadvertently introducing common skin bacteria into the anaerobic bottle.
It is worth noting that this order of draw is primarily for collections using a butterfly device. When blood is collected directly into a syringe and then transferred to the bottles, some protocols suggest filling the anaerobic bottle first to minimize the chance of air from the syringe entering the container. Regardless of the collection method, the blood culture bottles are always collected before any other blood test tubes, such as those for coagulation or chemistry, to maintain sterility.
Diagnostic Impact of Improper Technique
Failing to follow the correct collection sequence or using inadequate technique can have profound consequences for patient care by leading to inaccurate results. The two primary procedural errors are contamination, which causes false-positive results, and insufficient volume or air introduction, which can cause false negatives. A false-positive result occurs when skin-dwelling bacteria are mistakenly introduced into the sample during collection, making it appear that the patient has a bloodstream infection when they do not.
Contaminated blood cultures can result in a patient being unnecessarily treated with broad-spectrum antibiotics. This carries risks like secondary infections and contributes to the global problem of antimicrobial resistance. Patients with contaminated blood cultures often have an increased length of hospital stay and greater associated healthcare costs.
Conversely, a false-negative result means a true infection is missed, which can delay life-saving treatment for conditions like sepsis. If the anaerobic bottle is filled improperly, or if the blood volume collected is too low, the few bacteria present may not be able to multiply enough to be detected. Inadequate blood volume is a frequent cause of false negatives, as the probability of capturing the causative organism increases directly with the amount of blood cultured. The inability to identify the pathogen prevents clinicians from targeting the infection with the most appropriate medication, impacting patient recovery and outcomes.