BRONCHOALVEOLAR LAVAGE FOR DIAGNOSING BACTERIAL PNEUMONIA IN MECHANICALLY VENTILATED PATIENTS

To assess the usefulness of BAL in diagnosing bacterial pneumonia in m echanically ventilated patients, 80 BAL fluid samples obtained from 72 patients with lung infiltrates were studied using the following param eters: infected cell count (polymorphonuclear leukocytes or macrophage s with intracellular organisms), microscopic examination of stained sm ears, and quantitative culture with the determination of the simplifie d bacterial index (SBI) and the predominant species index (PSI). Of th e 80 BAL samples studied, 56 were performed under antibiotic therapy. Bacterial pneumonia was the final diagnosis in 28 cases. The SBI is th e sum of the whole numbers of each bacterial concentration expressed a s a common logarithm. The PSI is the whole number of the predominant m icroorganism's concentration expressed as a common logarithm. The disc riminative value of each test was assessed using a receiver operating characteristic (ROC) curve, whereby the possibility of establishing a cutoff value used to discriminate between the presence or absence of p neumonia is evaluated. The percentage of infected cells was higher in the pneumonia group (8.8 +/- 18.1 versus 0.4 +/- 1.1%, p < 10(-3)), bu t no cutoff value could be proposed. Under microscopic examination, th e presence of bacteria was noted with a significantly greater frequenc y in the pneumonia group (sensitivity 67.8% and specificity 82.7%). A total of 58 BAL samples were positive when cultured. The SBI was signi ficantly higher in the pneumonia group (6.5 +/- 2.9 versus 1.6 +/- 1.7 , p < 10(-4)). The SBI was found to be a good discriminative test: the area under the ROC curve was 0.944, and with an SBI greater than or e qual to, 4 as the cutoff value, the test sensitivity was 89.3% and spe cificity was 82.7%. The PSI was significantly higher in the pneumonia group (3.5 +/- 1 versus 1.2 +/- 1.2, p < 10(-4)), and the area under t he ROC curve was 0.92. With a PSI greater than or equal to 3 as the cu toff value, the test sensitivity was 89.3% and specificity was 85.5%. These results demonstrate that quantitative BAL fluid culturing, expre ssed as either SBI or PSI, is a good discriminative test for the diagn osis of bacterial pneumonia in mechanically ventilated patients, even if antibiotic therapy is present.