BRONCHOALVEOLAR LAVAGE FOR DIAGNOSING BACTERIAL PNEUMONIA IN MECHANICALLY VENTILATED PATIENTS

To assess the usefulness of BAL in diagnosing bacterial pneumonia in mechanically ventilated patients, 80 BAL fluid samples obtained from 72 patients with lung infiltrates were studied using the following parameters: infected cell count (polymorphonuclear leukocytes or macrophages with intracellular organisms), microscopic examination of stained smears, and quantitative culture with the determination of the simplified bacterial index (SBI) and the predominant species index (PSI). Of the 80 BAL samples studied, 56 were performed under antibiotic therapy. Bacterial pneumonia was the final diagnosis in 28 cases. The SBI is the sum of the whole numbers of each bacterial concentration expressed as a common logarithm. The PSI is the whole number of the predominant microorganism's concentration expressed as a common logarithm. The discriminative 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 pneumonia 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)), but no cutoff value could be proposed. Under microscopic examination, the presence of bacteria was noted with a significantly greater frequency in the pneumonia group (sensitivity 67.8% and specificity 82.7%). A total of 58 BAL samples were positive when cultured. The SBI was significantly 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 equal to, 4 as the cutoff value, the test sensitivity was 89.3% and specificity 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 the ROC curve was 0.92. With a PSI greater than or equal to 3 as the cutoff value, the test sensitivity was 89.3% and specificity was 85.5%. These results demonstrate that quantitative BAL fluid culturing, expressed as either SBI or PSI, is a good discriminative test for the diagnosis of bacterial pneumonia in mechanically ventilated patients, even if antibiotic therapy is present.