Blood cultures and detection of sepsis

Often called the most critical test in the clinical microbiology lab, the detection of bloodstream infections remains challenging—even though testing methods and paradigms have improved over the past 20 years. Bloodstream infections are one of the most serious problems in patient care, as they are associated with significant morbidity and mortality. Data show that sepsis and septic shock rank as the tenth leading cause of death in the United States, and it is estimated that 750,000 patients develop bacteremia/fungemia each year in the U.S.¹ The ICU can realize the greatest benefit from enhanced management of septic patients, as more than 40% of ICU costs are associated with sepsis.² Managing sepsis more effectively would have important repercussions for the lab, too.

The gold standard for the detection of bacteremia is a blood culture. There are several best practices that should be followed to assure optimal results. This article will review the key variables for a successful blood culture.

The volume of blood tested is the single most important variable for the detection of bacteria and yeast from blood cultures.^3,4 The majority of bacteremias in adults have a low density of microorganisms (often lb1 CFU/mL). Therefore, adequate blood volumes must be tested and 20-30 mL per blood culture set is recommended for adults.⁵

The volume of blood tested is, in fact, so important that there is a CAP recommendation (MIC.22640) that states the laboratory should periodically monitor collected blood volumes and provide feedback to the clinical staff. Labs utilize many methods to meet this requirement. The weighing of filled bottles, use of a manual measurement, or use of automated Blood Volume Monitoring software allows for the collection of data to meet this requirement.

The number of blood cultures drawn is also important. Two to three sets of blood cultures should be drawn per septic episode. Each set should be made up of two bottles: an aerobic and anaerobic bottle. There has been debate over the years about the relevancy of an anaerobic bottle. In 2007, Lassman et al. reported that the mean incidence of anaerobic bactermia at the Mayo Clinic increased by 74% from 1993 to 2004. The authors in this study concluded that anaerobic bacteremia had reemerged as a significant clinical problem, most likely attributable to the increase in patients with complex underlying diseases such as malignancies and associated chemotherapeutic treatment regimens.⁶ As antimicrobial resistance among anaerobic organisms has emerged during the last decade, the ability to accurately detect anaerobic bacteremia has become even more important. A study published in 2011 reconfirms the 2007 Lassman data. Patel et al. documented that the use of an aerobic/anaerobic paired set of bottles resulted in significantly more pathogens recovered as compared to two aerobic bottles.⁷ The number of bottles collected and the volume of blood added to these bottles are the key components of a successful blood culture.

In the quest to assure recovery of the causative agent, sometimes one may need to utilize different media types to optimize the recovery of the organism. Due to the high mortality associated with bacteremia and the use of antibiotics in outpatient settings, 28% to 63% of patients are already receiving antimicrobial therapy at the time that blood cultures are drawn.⁸ There are many media choices available for use on automated blood culture systems, and some have been designed to help remove antimicrobials and other inhibitory substances. Antibiotic inactivating resins are present in the BD BACTEC Plus media, and the BacT/ALERT FA FAN media contain charcoal for antibiotic inactivation. Many publications have demonstrated improved recovery using these media.

Most recently, an oral presentation at the IDSA (Infectious Diseases Society of America) conference reviewed data collected during a nine-month study of two blood culture systems. In this study, Hansen et al. analyzed 3,137 clinical blood culture sets, which is significant because there had not been a clinical head-to-head comparison study completed in more than 15 years. There were 284 positive samples included in the study, of which 187 were deemed clinically significant. The BACTEC Plus media had a statistically significantly higher detection rate, 89% (167/187), as compared to a positivity rate of 55% (102/187) for the BacT/ALERT FAN media. The time to detection favored the BACTEC system by an average of 4.7 hours. In this study, researchers concluded that the BD BACTEC System had higher recovery rates of clinically significant isolates and a faster time to detection.

Other variables, such as collection methods, contamination rates, and rapid, accurate reporting of positive blood culture samples can also have an impact on the usefulness of blood culture testing. By adhering to best practices and actively monitoring quality assurance measures, labs can feel confident that they are providing the best results in supporting clinicians in the diagnosis of sepsis.

Diane Flayhart, MS, MT(ASCP), is Marketing Manager for BD Diagnostics—Diagnostic Systems.

References

1. Martin G, et al. The epidemiology of sepsis in the United States from 1979 through 2000. N Engl J Med. 2003;348:1546-1554. http://www.nejm.org/doi/full/10.1056/NEJMoa022139. Accessed February 1, 2012.
2. Bone RC, et al. Definitions for sepsis and organ failure and guidelines for the use of innovative therapies in sepsis. The ACCP/SCCM Consensus Conference Committee. American College of Chest Physicians/Society of Critical Care Medicine. Chest. 1992;101:1644-1655. http://chestjournal.chestpubs.org/content/101/6/1644.full.pdf. Accessed February 1, 2012.
3. Weinstein MP. Current blood culture methods and systems: clinical concepts, technology, and interpretation of results. Clin Infect Dis. 1996;23(1):40-46. http://cid.oxfordjournals.org/content/23/1/40.full.pdf+html. Accessed February 1, 2012.
4. Mermel LA, Maki DG. Detection of bacteremia in adults: consequences of culturing an inadequate volume of blood. Ann Intern Med. 1993;119(4):270-272. http://www.annals.org/content/119/4/270.full.pdf+html. Accessed February 1, 2012
5. Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures; Approved Guideline. Wayne, PA: Clinical and Laboratory Standards Institute; 2007. CLSI document M47-A
6. Lassman B, et al. Reemergence of anaerobic bacteremia. Clin Infect Dis. 2007;44:895-900. http://cid.oxfordjournals.org/content/44/7/895.full.Accessed February 1, 2012.
7. Patel R, et al. Optimized pathogen detection with 30-compared to 20-milliliter blood culture draws. J Clin Microbiol. 2011;49:4047-4051.
8. Cockerill FR III, et al. Optimal testing parameters for blood cultures. Clin Infect Dis. 2004;38:1724-1730. http://cid.oxfordjournals.org/content/38/12/1724.full.pdf+html. Accessed February 1, 2012.