Blood cultures are diagnostic tests performed to identify microorganisms in a patient’s bloodstream. They are crucial for diagnosing serious infections, including bacteremia and fungemia, which can lead to life-threatening conditions like sepsis. Detecting specific pathogens helps healthcare providers determine the most effective course of treatment.
The Purpose of Blood Cultures and Contamination Defined
Blood cultures pinpoint microorganisms causing bloodstream infections. Identifying these pathogens allows selection of appropriate antibiotic or antifungal therapies, guiding targeted treatment. Without this information, broad-spectrum antibiotics might be used, which can be less effective or contribute to antibiotic resistance.
Blood culture contamination occurs when microorganisms not present in the patient’s bloodstream are introduced into the sample during collection. This leads to a false-positive result, indicating a non-existent infection. Accurate results are paramount; misdiagnosis can lead to unnecessary medical interventions or delay correct treatment.
The Leading Cause of Contamination
The most common source of blood culture contamination is the patient’s skin flora. Despite meticulous cleaning, the skin surface harbors various commensal bacteria. These microbes can be dislodged and enter the sample as the needle passes through the skin.
Coagulase-negative Staphylococci, like Staphylococcus epidermidis, are frequently identified as contaminants due to their abundance on skin. Other common skin organisms include Corynebacterium, Bacillus (excluding Bacillus anthracis), Micrococcus, and Cutibacterium acnes. These organisms, harmless on the skin, can multiply in the culture bottle, creating a misleading positive result.
Completely sterilizing the skin is challenging, as some bacteria reside in deeper layers or within hair follicles and sebaceous glands. This means an unavoidable risk of skin bacteria being introduced into the sample. Their presence does not reflect an actual infection within the patient’s bloodstream.
Minimizing Contamination Risk
Measures reduce contamination risk during collection. Key steps include thorough skin antisepsis at the venipuncture site, scrubbing with an antiseptic solution (e.g., chlorhexidine or iodine), and allowing sufficient drying time.
Strict aseptic technique is vital. This includes sterile gloves, avoiding palpation after cleaning, and proper venipuncture. Sterile equipment (needles, collection bottles) prevents environmental contaminants.
Diverting initial blood into a waste tube flushes out potential skin contaminants before sample collection. Ongoing staff training ensures consistent adherence, minimizing false-positive results.
Interpreting Contaminated Results
When a blood culture yields a positive result, differentiation between a true infection and contamination is necessary. This involves considering clinical symptoms (e.g., fever) and evaluating if multiple blood culture sets from different sites show growth of the same organism. If only one set is positive for a common skin organism, it is likely contamination.
The type of bacteria identified provides clues; certain organisms are almost always contaminants, while others indicate true infection. Contaminated results have negative consequences, including repeat blood draws, which can be uncomfortable and delay diagnosis.
Contamination leads to unnecessary antibiotic administration, contributing to resistance and exposing patients to side effects. It prolongs hospital stays and increases healthcare costs due to additional testing and treatments. If contamination is suspected, clinicians repeat the blood culture, relying on clinical judgment and other diagnostic information to determine next steps.