Abstract & Commentary
Synopsis: This study of blood culture draw sites in febrile ICU patients found that it is safe and reliable to obtain at least 1 of the blood cultures from a central venous catheter, irrespective of the type of catheter in place.Source:Beutz M, et al. Clinical utility of blood cultures drawn from central vein catheters and peripheral venipuncture in critically ill medical patients.Chest. 2003;123(3):854-861.
In this article by Kollef’s group at Washington University, the authors explore the usefulness of obtaining blood cultures in febrile ICU patients, using existing central venous catheters as one of the sites for obtaining blood cultures. Beutz and colleagues prospectively collected data from medical ICU patients over 9 months. The goal was to establish the sensitivity and specificity of blood cultures obtained from central venous catheters and peripheral sites using a very reasonable definition of "true bacteremia." The latter was considered to be present when 2 physician investigators blinded to the culture site analyzed the results following these pre-established criteria: (1) Certain pathogens such as Staphylococcus aureus, Gram-negative bacilli, and Candida species identified from any culture sample; or (2) common skin contaminants (coagulase-negative staphylococci, diphtheroids, Propionibacterium species, Bacillus species, or Micrococcus species) or viridans streptococci isolated from 2 or more culture samples from different sites and associated with fever, rigors or hypotension. Similar criteria were applied for polymicrobial infections. Other clinical information was also used to classify polymicrobial bacteremia as true bacteremia. Paired blood cultures drawn by standard technique and equipment were included in the study.
During the study period, 300 blood culture sets met the entry criteria for paired blood cultures. Each paired culture was used as an independent observation, even when obtained from the same patient. In 235 of the paired cultures, both the central venous catheter- and peripherally drawn cultures were negative. Sixty-five paired cultures had at least one positive site. Sixteen were catheter-positive/venipuncture-positive, 17 were catheter-negative/venipuncture-positive, and 32 were catheter-positive/venipuncture-negative. The majority of the catheter specimens (223) came from traditional multi-lumen catheters, with others obtained from a variety of central accesses such as including Hohn catheters, dialysis catheters, and Cordis catheters. Using the investigators’ pre-established criteria, 34 paired culture results were classified as true bacteremia (11.3%; 95% CI, 7.8-14.8%). All 16 catheter-positive/venipuncture-positive patients were classified as having true bacteremia. Additionally, 6 out of 17 catheter-negative/venipuncture-positive and 12 out of 32 catheter-positive/venipuncture-negative pairs were classified as true bacteremia.
Using Beutz et al’s definition of true bacteremia, the sensitivity of catheter drawn specimens was 82.4%, as compared to 64.7 % for peripherally drawn specimens. The specificities for true bacteremia from these specimens were 92.5% and 95.9%, respectively. The positive predictive values were 58.3% for catheter drawn specimens compared to 66.7% for peripherally drawn specimens, whereas the negative predictive values were 97.6% and 95.5%, respectively.
Comment by Uday Nanavaty, MD
Febrile illness in the ICU is a very common problem. There are multiple infectious and noninfectious causes of fever in the ICU patient. When fever is associated with clinical manifestations of sepsis, it is routine to obtain paired blood culture specimens. It has been shown before that multiple cultures (up to 3 sets) increase the yield to identify the existence of bacteremia.
There are many problems with obtaining multiple cultures in ICU patients. Edema may make access to peripheral veins difficult. Peripheral catheters are often inserted prior to the patient’s arrival in ICU, again limiting the sites available for peripheral blood draws. In 1998 guidelines for evaluation of fever in ICU, O’Grady and colleagues1 suggested that blood cultures can be obtained from existing central venous catheters along with a peripheral site, but that care should be taken to obtain cultures within a reasonable interval between the 2 draws, that the drawing of cultures should not delay initiation of antibiotics if clinically indicated, especially in patients with new manifestations of sepsis, and that the interpretation of culture results should take the clinical picture into consideration.
This study further adds to the literature by suggesting that it is safe and reliable to obtain at least one of the blood cultures in febrile ICU patients from a central venous catheter, irrespective of the type of the catheter in place. It further suggests that if the probability of bacteremia is estimated to be 10% or less, the catheter-drawn culture has high negative predictive value. These data and conclusion are similar to those of a study in oncology patients with central venous catheters.2 When interpreting studies involving the usefulness of a test, it is helpful to remember the formulas used to derive the sensitivity, specificity, positive predictive value and negative predictive values:
- Sensitivity (the proportion of people with disease who have a positive test) = the proportion of patients with catheter positive results among true bacteremia patients;
- Specificity (the proportion of people free of disease who have a negative test) = the proportion of patients with catheter negative results among those free of true bacteremia;
- Positive Predictive Value (the proportion of people with positive test who have disease) = the proportion of patients with catheter positive results among true bacteremia; and
- Negative Predictive Value (the proportion of people with negative test who are free of disease) = the proportion of patients with catheter negative results among those who do not have true bacteremia.
In conclusion, this study suggests that clinicians should make case-by-case assessments of blood culture results in medical ICU patients with central venous catheters. It is unclear to me whether it is possible to obtain specimens from 2 different existing catheters, such as 2 central catheters, a central catheter and a dialysis catheter, or a central catheter and an arterial catheter. If such paired cultures were as reliable as peripherally drawn cultures, it would save sticks and a lot of nursing time across all ICUs.
Dr. Nanavaty of Pulmonary and Critical Care Specialists of Northern Virginia, Fairfax, VA.
1. O’Grady NP, et al. Practice guidelines for evaluating new fever in critically ill adult patients. Task Force of the Society of Critical Care Medicine and the Infectious Diseases Society of America. Clin Infect Dis. 1998;26(5):1042-1059.
2. DesJardin JA, et al. Clinical utility of blood cultures drawn from indwelling central venous catheters in hospitalized patients with cancer. Ann Intern Med. 1999;131(9):641-647.