Nosocomial Pneumonia: Blood Cultures Remain Useful

Abstract

The toxœmia outweighs all other elements in the prognosis of pneumonia; to it (in a gradual failure of strength or more rarely in a sudden death…) is due in great part the terrible mortality from this common disease, and unhappily against it we have as yet no reliable measures at our disposal. William Osler, 18971Osler W On certain features in the prognosis of pneumonia.Am J Med Sci. 1897; 113: 1-10Crossref Google Scholar Rebuke a wise man, and he will love you. Proverbs 9:8 Without claiming to be wise, I am pleased that Luna and colleagues in this issue of CHEST (see page 1075) challenge my assertion that patients with pneumonia whose blood cultures are positive for the causative microorganism(s) experience higher mortality than patients with sterile blood cultures.2Bryan CS Reynolds KL Bacteremic nosocomial pneumonia: analysis of 172 episodes from a single metropolitan area.Am Rev Respir Dis. 1984; 129: 668-671Crossref PubMed Scopus (156) Google Scholar This assertion is widely accepted for patients with community-acquired pneumonia, and it was recently shown that collecting blood cultures from older patients even improves survival.3Meehan TP Fine MJ Krumholz HM et al.Quality of care, process, and outcomes in elderly patients with pneumonia.JAMA. 1997; 278: 2080-2084Crossref PubMed Google Scholar The study of hospital-acquired pneumonia poses many difficulties, among which, is the huge problem of case definition. Our 1984 study of nosocomial pneumonia was limited to patients with positive blood cultures.2Bryan CS Reynolds KL Bacteremic nosocomial pneumonia: analysis of 172 episodes from a single metropolitan area.Am Rev Respir Dis. 1984; 129: 668-671Crossref PubMed Scopus (156) Google Scholar No attempt was made to determine whether patients with positive blood cultures experienced higher mortality than patients with sterile blood cultures. Like other investigators, we felt the major problem was accurate identification of cases of nosocomial pneumonia. This problem—a traditional source of frustration to those who practice hospital infection control—has been partially rectified by the advent of newer techniques, such as BAL and the protected specimen brush. Using BAL, Luna and colleagues found blood cultures to have a 26% sensitivity and 73% positive predictive value for disclosing the same pathogenic microorganism(s) found in BAL cultures. Blood culture results did not correlate with the outcome. The authors conclude that blood cultures are useful if there is reason to suspect another infectious condition, with the caveat that isolation of a microorganism from blood does not confirm that microorganism as the causative pathogen. This new study from Argentina adds to a growing body of data supporting the use of BAL cultures for diagnosis of nosocomial pneumonia. I suspect that quantitative cultures from a well-done BAL procedure are more accurate than blood cultures in defining the etiology of pneumonia. However, I also believe that it would be premature to discontinue the practice of obtaining blood cultures in suspected cases of nosocomial pneumonia, for at least three reasons. First, Luna and colleagues studied a subset of patients with nosocomial pneumonia—ventilator-associated pneumonia of sufficient severity to prompt their carrying out BAL—known to have a high case-fatality rate. The mortality in their patients with positive BAL cultures was 64% and 74% in those with negative cultures. These data amply confirm the seriousness of ventilator-associated pneumonia. Blood cultures are minimally invasive and are inexpensive, relative to the total cost of caring for these patients. Blood cultures remain worthwhile even if the data are crucially important for only a minority of cases. Osler's conclusion that the prognosis of pneumonia hinges on “the toxæmia” (today, we would use the term “sepsis” or “systemic inflammatory response syndrome”) remains valid. Second, it is still generally agreed that there is no gold standard for the diagnosis of pneumonia in critically ill patients.4Cook DJ Walter SD Cook RJ et al.Incidence and risk factors for ventilator-associated pneumonia in critically ill patients.Ann Intern Med. 1998; 129: 433-440Crossref PubMed Scopus (757) Google Scholar5Singh N, Rogers P, Atwood C, et al. Short-course empiric antibiotic therapy for suspected nosocomial pneumonia: a proposed solution for indiscriminate antibiotic prescription for pulmonary infiltrates in the ICU. Presented at: 36th annual meeting of the Infectious Diseases Society of America; November 12–15, 1998; Denver, COGoogle Scholar As a result, we do not have denominator data for nosocomial pneumonia, just as we do not have denominator data for community-acquired pneumonia. Many processes cause pulmonary infiltrates, and there is no easy way to tell who is infected and who is not, especially in milder cases. In our study of 2,978 consecutive cases of bacteremia in Columbia, SC, we found the mortality rate for patients with positive blood cultures was extremely high relative to the mortality rate for the entire patient population (30% vs 2.5%).6Bryan CS Hornung CA Reynolds KL et al.Endemic bacteremia in Columbia, South Carolina.Am J Epidemiol. 1986; 123: 113-127Crossref PubMed Scopus (43) Google Scholar Positive blood cultures for microorganisms usually considered to be pathogens may not certify the cause of pneumonia, but they indicate a high probability of significant, life-threatening infection.7Weinstein MP Towns ML Quartey SM The clinical significance of positive blood cultures in the 1990s: a prospective comprehensive evaluation of the microbiology, epidemiology, and outcome of bacteremia and fungemia in adults.Clin Infect Dis. 1997; 24: 584-602Crossref PubMed Scopus (1020) Google Scholar Third, blood cultures are indicated because essentially all of these patients have alternative sites of potential infection. It would be a rare ventilator-dependent patient in the ICU who does not have at least one of the following: a vascular access device, an indwelling urinary catheter, a nasogastric tube, a recent operative or traumatic wound, or a drainage tube connected to one or another body cavity. In their comprehensive review of ventilator-associated pneumonia, Louthan and Meduri8Louthan FB Meduri GU Differential diagnosis of fever and pulmonary densities in mechanically ventilated patients.Semin Respir Infect. 1996; 11: 77-95PubMed Google Scholar made it clear that blood cultures are indicated as part of the evaluation to rule out an extrapulmonary source of infection. As they put it, “Until proven otherwise, management of critically ill patients should not be exempted from the rule of “knowing what you are treating.” Despite these reservations about the conclusions, I believe that the Argentine workers are on the cutting edge of the study of nosocomial pneumonia. They confirm, for example, that patients frequently have more than one pulmonary pathogen, even when blood cultures show a single microorganism. Moreover, I suspect that investigators in the next century will be using BAL cultures as the standard by which to evaluate still-newer, less-cumbersome ways of telling who has pneumonia and from what microorganisms.