Follow-up Blood Culture in Gram-Negative Bacteremia Should Be Avoided

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Persistent bacteremia was far more likely to be a result of Gram-positive cocci than polymicrobial infection or Gram-negative bacilli.
Persistent bacteremia was far more likely to be a result of Gram-positive cocci than polymicrobial infection or Gram-negative bacilli.

More treatment is not necessarily better treatment in Gram-negative bacteremia, according to a new study by Gabriel M. Aisenberg, MD, and colleagues at the University of Texas Health Science Center and Medical School in Houston, recently published online in Clinical Infectious Diseases.1 False-positive results are common in follow-up blood cultures (FUBCs) conducted to determine the duration of antibiotic treatment for Gram-negative bacteremia, often incurring high costs for unnecessary hospitalizations and antibiotic overuse.                      

In a retrospective analysis of 500 cases of bacteremia occurring between January 1, 2015, and December 31, 2015, at the Lyndon B. Johnson Hospital in Houston, Texas, FUBCs were ordered in 77% of cases (n=383), including 54% of Gram-positive cocci (GPC), 37% Gram-negative bacilli (GNB), and 8% polymicrobes. Positive results were found in 14% of cases (55/383), the majority of which (78%; n=43) were GPC. Only 8 cases of bacteremia (15%) tested positive for GNB. Persistent bacteremia was far more likely to be a result of GPC than polymicrobial infection or GNB (21%, 10%, and 6%, respectively).

Current management of bacteremia is left to clinical judgment, for which the investigators could find no single rationale. "The medical records examined did not offer any explanation of why the FUBC was ordered," they wrote. They questioned whether disease severity influenced physicians' decisions to order FUBCs, which in turn may have increased the rate of positive results. Higher positive rates on FUBCs were observed in patients who had fever at the time of blood draw. Likewise, the presence of diabetes mellitus, end-stage renal disease, and/or a central intravenous may have inflated the FUBC positive rates.

The otherwise low yields indicated that FUBCs were largely inconclusive, particularly for GNB. The authors calculated that although it took 5 FUBCs to yield 1 positive result in the entire cohort, when considering GNB alone, 17 FUBCs were performed to obtain a single positive result. Previous studies have shown that up to 90% of all cultures fail to grow any organisms, and more than half of the small percentage that do are likely to be falsely positive.2,3

Repeating FUBCs in GNB is only likely to increase false-positive results, the authors determined, leading to "increased costs, longer hospital stays, unnecessary consultations, and inappropriate use of antibiotics."4 To avoid these consequences, the investigators warned clinicians to avoid drawing FUBCs for GNB bacteremia.

The study had several limitations, including elimination of contaminants in cultures, which may have redistributed positive results. The lack of understanding by clinicians in the cohort of when and why to order FUBCs pointed to a significant need for more comprehensive guidelines and better education to reduce overtreatment of GNB.


  1. Canzoneri CN, Akhavan BJ, Tosur Z, Andrade PEA, Aisenberg GM. Follow-up blood cultures in Gram-negative bactermia: are they needed? [published online July 26, 2017]. Clin Infect Dis. doi: 10.1093/cid/cix648 
  2. Wilson ML. Clinically relevant, cost-effective clinical microbiology. Strategies to decrease unnecessary testing. Am J Clin Pathol. 1997;107:154-167.
  3. Weinstein MP, Towns ML, Quartey SM, et al. The clinical significance of positive blood cultures in the 1990s: a prospective comprehensive evaluation of the microbiology, epidemiology, and outcome of bacteremia and fungemia in adults. Clin Infect Dis. 1997;24:584-602.
  4. Bates DW, Goldman L, Lee TH. Contaminant blood cultures and resource utilization. The true consequences of false positive results. JAMA. 1991;265:365-369. 
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