Blood cultures have limited value in predicting severity of illness and asa diagnostic tool in ventilator-associated pneumonia

Study objectives: To define the usefulness of blood cultures for confirming the pathogenic microorganism and severity of illness in patients with vent ilator-associated pneumonia (VAP). Design: Prospective observational study using BAL, and blood cultures colle cted 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 ventilati on hospitalized for > 72 h who had new or progressive lung infiltrate plus at least two of three clinical criteria for VAP. Interventions: Brit and blood culture performed within 24 h of establishing a clinical diagnosis of VAP. Measurements and results: Ninety patients were BAL positive (BAL+), satisfy ing a microbiological definition of VAP (greater than or equal to 10(4) 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 on e set was positive, but the same bacteria was present at a concentration gr eater than or equal to 104 cfu/mL in the BAL fluid. Bacteremia was signific antly 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 extr apulmonary site of infection was the source of bacteremia. Sensitivity of b lood culture for disclosing the pathogenic microorganism in BAL+ patients w as 26%, and the positive predictive value to-detect the pathogen was 73%. F actors associated with mortality were age > 50 years, simplified acute phys iology score > 14, prior inadequate antibiotic therapy, PaO2/fraction of in spired oxygen < 205, and use of Ii, blockers, By multivariate analysis, onl y;he use of prior inadequate antimicrobial therapy (odds ratio [OR], 6.47) and age > 50 years (OR, 5.12) were independently associated with higher mor tality. The rate of complications was not different in patients with bacter emia. Conclusions: Blood cultures have a low sensitivity for detecting the same p athogenic microorganism as BAL culture in patients with VAP. The presence o f bacteremia does not predict complications, it is not related to the lengt h of stay, and it does not identify patients with more severe illness, Inad equacy of prior antimicrobial therapy and age > 50 years were the only fact ors associated with mortality in a multivariate analysis. Blood cultures in patients with VAP are clearly useful if there is suspicion of another prob able infectious condition, but the isolation of a microorganism in the bloo d does not confirm that microorganism as the pathogen causing VAP.