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Blood Cultures Have Limited Value in Predicting Severity of Illness and as a Diagnostic Tool in Ventilator-Associated Pneumonia^*

^* From the Pulmonary and Critical Care Divisions (Drs. Luna, Videla, and Vujacich), the Clinical Analysis Department, Microbiology Division (Drs. Mattera, Fay, and Famiglietti), Hospital de Clínicas "José de San Martín," University of Buenos Aires, Argentina; and the Division of Pulmonary and Critical Care Medicine (Dr. Niederman), Winthrop University Hospital, Mineola, NY.

Correspondence to: Carlos M. Luna, MD, FCCP, Acevedo 1070, Banfield (CP 1828), Buenos Aires, Argentina; e-mail: cymluna{at}fmed.uba.ar

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.

Key Words: bacteremia • blood cultures • complications • diagnosis • mortality • nosocomial pneumonia • ventilator-associated pneumonia

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