Address Pre-Analytical Factors Affecting Blood Culture Results

December 2019 - Vol. 8 No. 11 - Page #2
Q&A with Eugenio H. Zabaleta, PhD
Clinical Chemist
OhioHealth Mansfield Hospital


MedicalLab Management: How would you describe the utility of blood cultures to the clinical diagnosis of sepsis?

Eugenio Zabaleta: Sepsis is the body’s overwhelming systemic response to an infection that arises in the bloodstream; a condition that can lead to tissue damage, organ failure, amputations, and death. Thus, sepsis should be treated as a medical emergency, one that requires urgent attention and rapid treatment for survival. However, sepsis can be treated, and lives are often saved by the use of established and proven protocols.

This was clear when CMS introduced its new Sepsis Core Measures in October 2015. These Core Measures are evidence-based process measures aimed at reducing morbidity and mortality.1 In this scenario the rapid detection and identification of blood-borne pathogens is essential to the correct diagnosis and prognosis of sepsis. Despite technological advances, blood cultures (BC) are still the most sensitive laboratory test to detect the presence of bacteria or fungi in the blood of sepsis-suspected patients. They are essential for identifying pathogens causing sepsis and for guiding appropriate antibiotic therapy.

MLM: What variables within the blood draw process have the potential to impact blood culture results?

Zabaleta: According to the current guidelines,2 the following specimen collection and transportation aspects should be recognized, recorded, and standardized, to the extent possible:

  1. Timing of the BC
  2. Number of BC sets
  3. Volume of BC
  4. Distribution of blood between aerobic and anaerobic BC bottles
  5. Disinfection of skin and prevention of BC contamination
  6. The BC collection process itself (to avoid contamination)
  7. The transport of the BC sets to the lab

MLM: Are there critical factors in the recovery of pathogens from blood specimens according to the guidelines?

Zabaleta: Yes, there are. These include volume, blood-to-broth ratio, the media formulation, additives (eg, anticoagulants, resins, charcoal), incubation conditions (eg, temperature, atmosphere, length), bottle agitation, and monitoring of frequency/subculture. Attention to detail regarding all of these factors is critical for a successful program.

MLM: Which of those variables have the greatest influence on blood culture positivity and time to positivity?

Zabaleta: According to the traditional dictum, the volume of blood drawn per culture bottle is the single most important variable in recovering microorganisms from the blood of bacteremic or fungemic patients.2,3 Further, each milliliter of blood, up to 10mL, can increase the growth of bacteria in the blood culture by 3% to 5%.4 So a blood culture containing only 2mL can have a decreased sensitivity of up to 40%.

This is so important that is part of our accreditation process. The College of American Pathology has a requirement (CAP MIC.22640) regarding the need to monitor BC volumes, but it goes even further to emphasize that the laboratory should monitor collected blood volumes and provide feedback to clinical staff (eg, phlebotomists). This is a program that we have installed in our health system.

MLM: Does drawing for blood cultures differ from other blood draws?

Zabaleta: Sample contamination can interfere with the ability to have the right diagnosis, treatment, and prognosis in patients suspected of having sepsis. Therefore, the disinfection of skin is essential, and the collection process has to be aimed at preventing contamination. We have developed education tools for our phlebotomists and nurses that enable best practices in phlebotomy technique. These include PowerPoint presentations and short videos on proper technique, a checklist indicating clear steps to enable proper BC collection, a competency assessment form with visual aids, and there is a retraining form for any blood draw staff that show difficulty in this area.

MLM: What are the primary pathogens that cause sepsis, and what are the symptoms patients present with in the ED?

Zabaleta: Traditionally sepsis was associated with Gram-negative bacteria due to their ability to produce endotoxin. Nevertheless, it is now well established that Gram-positive bacteria and fungi can produce a systemic septic response. Regarding specific organisms that cause sepsis, this depends on the suspected source of the sepsis (eg, pneumonia, UTI, skin/soft tissue infection, etc), and where the infection was acquired (eg, community acquired versus nosocomial).

MLM: Which lab disciplines benefit most when proper blood cultures are taken?

Zabaleta: That is an interesting and even provocative question. The likely expected answer is the microbiology department, but in reality, maintaining a proper blood culturing program benefits the entire laboratory. If the BC result is incorrect, regardless of whether it is a false negative (not able to recover the microorganism) or false positive (suspected contamination), the treating physician will be sent down the wrong path of diagnosis. Among other drawbacks, this will likely result in overutilization of unnecessary testing and underutilization of necessary testing.

MLM: What are the most important tools for blood culture volume compliance?

Zabaleta: The most important element of proper BC volume compliance is the commitment of laboratorians to work and clearly communicate with our blood collectors, whether they work for the lab (ie, phlebotomists) or not (ie, nurses). Data collection is important as well, but how this is done is less crucial. The important part is focusing on how to use data to improve patient care and safety. In all cases, this includes collecting the correct blood volume that will allow the laboratory to obtain the most accurate results.

MLM: Do you employ different strategies for training phlebotomists versus nurses on proper blood draws?

Zabaleta: It is important to realize that even though phlebotomists and nurses are performing the same task in drawing blood for BC, they have different operational workflows. Whereas phlebotomists’ roles are task intensive and tied to laboratory procedures, nurses have a wider set of responsibilities and are much more front-facing in terms of managing and treating patients. So, when educating nurses, it is best to close the patient-care loop by directly relating blood draw best practices to their impact on the patient.

MLM: What is a reasonable compliance rate for QA, and how is this best determined?

Zabaleta: When we did our study of BC volume compliance, we realized that periodic education is necessary not only to achieve compliance, but to sustain optimal results, and thereby improve the care and safety of our patients. A compliance rate of 80% and a collection volume higher than 8mL was set for our collectors as a quality assurance measure.

MLM: What other technologies can assist hospital laboratories with improving blood culturing?

Zabaleta: Without question, rapid detection and identification of blood-borne pathogens is essential to the correct diagnosis and prognosis of sepsis. With this in mind, there are opportunities to employ rapid molecular testing platforms for positive blood cultures. These platforms, when used in conjunction with the traditional BC test, along with a solid antimicrobial stewardship program, will help ensure your patients receive the best possible care, as quickly as possible.


  1. Dellinger RP, Levy MM, Rhodes A, et al. Surviving Sepsis Campaign: International Guidelines for Management of Severe Sepsis and Septic Shock: 2012. Crit Care Med. 2013;41(2):580–637.
  2. Principles and Procedures for Blood Cultures, Clinical Laboratory Standards Institute, M47-A Vol. 27, No. 17; May 2007.
  3. Dunne WM Jr., Nolte FS, Wilson ML. Cumitech IB, Blood Cultures III. 1997. Coordinating ed, Hindler JA. ASM Press, Washington D.C.
  4. Mermel LA, Maki DG. Detection of bacteremia in adults: Consequences of culturing in inadequate volume of blood. Ann Intern Med. 1993;119(4):270-272.

Eugenio H. Zabaleta, PhD, is a clinical chemist at OhioHealth Mansfield Hospital in Mansfield, Ohio and a part-time lecturer at Cleveland State University’s graduate clinical chemistry program. Eugenio graduated from the Catholic University of Cordoba (Argentina) with a degree in biochemistry and received his PhD in chemistry from the University of Akron. His training in clinical pathology was at the Hospital Provincial San Roque in Cordoba. In Argentina, he was the laboratory medical director at the Clinica del Sol, a clinic devoted to mother and child’s care, with neonatology intensive care service.

Dr. Zabaleta also presented on this subject as part of the professional posters at the 2019 AACC Annual Meeting.


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