Chest
Volume 116, Issue 4, October 1999, Pages 1075-1084
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Critical Care
Blood Cultures Have Limited Value in Predicting Severity of Illness and as a Diagnostic Tool in Ventilator-Associated Pneumonia

https://doi.org/10.1378/chest.116.4.1075Get rights and content

Study objectives

To define the usefulness of blood cultures for confirming the pathogenic microorganism and severity of illness in patients with ventilator-associated pneumonia (VAP).

Design

Prospective observational study using BAL and blood cultures collected within 24 h of establishing a clinical diagnosis of VAP.

Setting

A 15-bed medical and surgical ICU.

Patients

One hundred and sixty-two patients receiving mechanical ventilation hospitalized for > 72 h who had new or progressive lung infiltrate plus at least two of three clinical criteria for VAP.

Interventions

BAL and blood culture performed within 24 h of establishing a clinical diagnosis of VAP.

Measurements and results

Ninety patients were BAL positive (BAL+), satisfying a microbiological definition of VAP (≥ 104 cfu/mL), 72 patients were BAL negative (BAL−). Bacteremia was diagnosed when at least two sets of blood cultures yielded a microorganism or when only one set was positive, but the same bacteria was present at a concentration≥ 104 cfu/mL in the BAL fluid. Bacteremia was significantly more frequent in the BAL+ than in the BAL− group (22/90 patients vs 5/72 patients; p = 0.006). In 6 of 22 BAL+ patients with bacteremia, an extrapulmonary site of infection was the source of bacteremia. Sensitivity of blood culture for disclosing the pathogenic microorganism in BAL+ patients was 26%, and the positive predictive value to detect the pathogen was 73%. Factors associated with mortality were age > 50 years, simplified acute physiology score> 14, prior inadequate antibiotic therapy, Pao2/fraction of inspired oxygen < 205, and use of H2 blockers. By multivariate analysis, only the use of prior inadequate antimicrobial therapy (odds ratio [OR], 6.47) and age > 50 years (OR, 5.12) were independently associated with higher mortality. The rate of complications was not different in patients with bacteremia.

Conclusions

Blood cultures have a low sensitivity for detecting the same pathogenic microorganism as BAL culture in patients with VAP. The presence of bacteremia does not predict complications, it is not related to the length of stay, and it does not identify patients with more severe illness. Inadequacy of prior antimicrobial therapy and age > 50 years were the only factors associated with mortality in a multivariate analysis. Blood cultures in patients with VAP are clearly useful if there is suspicion of another probable infectious condition, but the isolation of a microorganism in the blood does not confirm that microorganism as the pathogen causing VAP.

Section snippets

Materials and Methods

The study was conducted in the Critical Care Unit of the Hospital de Clínicas José de San Martín, University of Buenos Aires, Argentina from April 1, 1992 to July 31, 1997. This is a 15-bed medical (noncoronary) and surgical unit in a 500-bed teaching hospital that serves as both a referral center and a first-line hospital. Intubated patients receiving mechanical ventilation who developed a new or progressive infiltrate on chest radiograph after being in the hospital for > 72 h were eligible

Results

During the 64-month period of study, 162 consecutive episodes of clinically defined VAP were studied. BAL fluid was collected and blood cultures performed in all cases; however, the BAL technique was able to confirm infection microbiologically in 90 episodes. Ten episodes occurred in patients who had had ≥ 1 prior episodes of VAP; these subsequent episodes involved other areas and developed at least 9 days after the patient had improved from the prior episode. Each pneumonia episode in a single

Discussion

Pneumonia is a common complication of mechanical ventilation, but uncertainties about diagnosis and how to identify the likely etiologic pathogen remain a common clinical problem. Several authors have used the results of the culture of bronchoscopically obtained specimens (protected specimen brush and BAL cultures) both to confirm the diagnosis and to identify the etiologic pathogenic microorganisms, based on the data from experimental and clinical studies.251011121314 However, several recent

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    For editorial comment see page 859.

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