Diary of the “mad” med-lab techs

Blood cultures — in a nutshell

The blood culture is the single most important test performed in the clinical microbiology laboratory. Studies are conducted every year to find newer and better methods for the detection and recovery of the most fastidious microorganisms in the shortest period of time. This column revisits guidelines based on those studies and is sprinkled with suggestions extracted from my 40 years’ experience in the clinical microbiology laboratory!

• Site preparation: Adopt the FDA-approved one-step method utilizing ChloraPrep Sepp applicator. Look up the publications.

• Number of blood cultures: For adults, two blood specimens (20 mL to 30 mL) should be drawn simultaneously from different venipuncture sites as soon as possible following the clinical events that prompted performance of cultures.
  Studies done using completely different blood-culture monitoring systems found basically the same thing. If you draw two 20-mL venipunctures from adults, what is the value of a third blood culture? The answer is none.
  In addition, single blood cultures for the detection of pathogens should never be done. Single blood cultures that have become positive with an organism that is normal skin flora are extremely difficult to interpret.

• Volume of blood cultures: Optimal recovery of microorganisms causing bacteremia is clearly best with 20 mL to 30 mL per puncture. For infants and small children, the number of organisms in the blood can be — but is not always — higher. Therefore, bacteremia can be reliably detected when smaller volumes are drawn. Suggested policy: 1 cc of blood per year of age up to 10 years.

• Timing of cultures: Many papers have been published on this subject. Studies have shown that blood cultures either drawn simultaneously or at intervals over a 24-hour period resulted in similar microbial recovery. Since there is no benefit in obtaining blood samples at intervals, and it is more practical to draw blood for a set of cultures at the same time, there seems to be little reason to continue the practice. Moreover, drawing blood specimens simultaneously from different venipuncture sites may help ensure that the blood is drawn before antibiotics are administered.

Keep in mind that any volume of blood taken from one needle-puncture site is considered a single blood-culture specimen. Failure to adhere to this policy should be subject to immediate disciplinary action. If only one site is available for phlebotomy, the same site can be used for the second draw as long as a completely separate venipuncture procedure is performed. If there is nothing written on the patient’s chart, when do you draw the second blood-culture set? The answer is, immediately after drawing the first set.

• Timing of cultures: Many papers have been published on this subject. Studies have shown that blood cultures either drawn simultaneously or at intervals over a 24-hour period resulted in similar microbial recovery. Since there is no benefit in obtaining blood samples at intervals, and it is more practical to draw blood for a set of cultures at the same time, there seems to be little reason to continue the practice. Moreover, drawing blood specimens simultaneously from different venipuncture sites may help ensure that the blood is drawn before antibiotics are administered.

• Length of incubation: Studies have shown that incubating blood-culture bottles past the accepted five days does not improve recovery of even the most fastidious microorganisms, most notably the rarely isolated HACEK group — Haemophilus, Actinobacillus, Cardiobacterium, Eikenella,and Kingella. HACEK bacteria can be recovered well within the standard incubation period of five days.¹

Automated blood-culture systems have improved recovery rates but even the best systems are at the mercy of poor laboratory-management practices:

1. Blood-culture bottles left sitting for hours before being placed on the automated instrument.
2. Flagged positive bottles that are ignored for prolonged periods (e.g., not addressed on the off-shifts).
3. Relying on blood cultures rather than serology or nucleic-acid amplification for the detection of rare and unusual microorganisms (e.g., systemic fungi, Bartonella).
4. Using culture bottles with antimicrobial-removal devices. What does a positive mean — inappropriate antibiotic therapy?
5. Accepting bottles with inadequate blood volumes without a comment (over-inoculated as well as under-inoculated).
6. Giving the infectious-disease specialists more decision-making clout in the microbiology department than they deserve. Who is in charge?

The guidelines above may seem basic and without question — if you are fortunate enough to be a part of a well-managed and well-informed microbiology laboratory. Having participated in many official laboratory inspections and having worked as a temporary travel employee in several institutions, my experience tells me that this is not always the case.

—By Colleen K. Gannon, MT(AMT) HEW,
the “Nancy Grace” for labs

Reference

1. Petti CA, Bhally HS, Weinstein MP, Joho K, Wakefield T, Reller LB, Carroll KC. Utility of Extended Blood Culture Incubation for Isolation of Haemophilus, Actinobacillus, Cardiobacterium, Eikenella and Kingella Organisms: a Retrospective Multicenter Evaluation. J Clin Microbiol. 2006;1:257-259.