Who Needs a Blood Culture? A Prospective Derived and Validated Prediction Rule

Nathan I. Shapiro, MD, MPH; Richard E. Wolfe, MD; Sharon B. Wright, MD, MPH; Richard Moore, MD; David W. Bates, MD, MSC

Disclosures

J Emerg Med. 2008;35(3):255-264. 

In This Article

Abstract and Introduction

Abstract

The study objective was to derive and validate a clinical decision rule for obtaining blood cultures in Emergency Department (ED) patients with suspected infection. This was a prospective, observational cohort study of consecutive adult ED patients with blood cultures obtained. The study ran from February 1, 2000 through February 1, 2001. Patients were randomly assigned to derivation (2/3) or validation (1/3) sets. The outcome was "true bacteremia." Features of the history, co-morbid illness, physical examination, and laboratory testing were used to create a clinical decision rule. Among 3901 patients, 3730 (96%) were enrolled with 305 (8.2%) episodes of true bacteremia. A decision rule was created with "major criteria" defined as: temperature > 39.5°C (103.0°F), indwelling vascular catheter, or clinical suspicion of endocarditis. "Minor criteria" were: temperature 38.3-39.4°C (101-102.9°F), age > 65 years, chills, vomiting, hypotension (systolic blood pressure < 90 mm Hg), neutrophil% > 80, white blood cell count > 18 k, bands > 5%, platelets < 150 k, and creatinine > 2.0. A blood culture is indicated by the rule if at least one major criterion or two minor criteria are present. Otherwise, patients are classified as "low risk" and cultures may be omitted. Only 4 (0.6%) low-risk patients in the derivation set and 3 (0.9%) low-risk patients in the validation set had positive cultures. The sensitivity was 98% (95% confidence interval [CI] 96-100%) (derivation) and 97% (95% CI 94-100%) (validation). We developed and validated a promising clinical decision rule for predicting bacteremia in patients with suspected infection.

Introduction

Background. Bacteremia and sepsis are common problems, with an estimated 751,000 cases of sepsis per year in the United States, or approximately 2000 cases per day.[1] It is common practice to obtain blood cultures during the initial presentation of patients who may harbor an infection. Despite the frequency of this practice, there is sparse evidence relating to when it is appropriate to order a blood culture.

Importance. The general indications for blood culture use are poorly defined, and as a result, overall blood culture yields from hospitalized patients remain remarkably low at 4-8%.[2,3,4,5,6] Furthermore, there has been no large prospective study specifically evaluating indications for blood culture in the Emergency Department (ED). The low yield of blood cultures has significant financial costs, wastes health care worker time, and results in unnecessary needle sticks for patients and risk for health care workers. In addition, cultures that grow contaminants may result in increased hospital charges and unnecessary antibiotic treatment, hospital admission, and resource utilization.[7] Prior studies in hospitalized patients have developed criteria for the rational ordering of blood cultures, but these guidelines are frequently ignored in clinical practice.[2,3,4,6,8,9,10,11,12] Physicians have also been shown to inaccurately predict bacteremia risk, often overestimating the patient's likelihood of bacteremia.[13,14]

Nonetheless, the identification of ED patients at risk for bacteremia is critical. Untreated bacteremia may lead to the development of a sepsis syndrome and septic shock, with mortality rates estimated at 30-50%.[1,15,16,17,18] Early administration of empiric antibiotics has been shown to be associated with lower mortality.[8,19,20,21] Thus, the ability to accurately assess a patient's risk for bacteremia and selectively order blood cultures would be clinically useful.

Goals of this Investigation. We undertook this study to identify independent predictors of bacteremia in ED patients, to derive and validate a clinical decision rule to assess risk of bacteremia, and to identify a low-risk population of patients for whom blood cultures may be safely eliminated.

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