Role of quantitative cultures and microscopic examinations of endotracheal aspirates in the diagnosis of pulmonary infections in ventilated patients

Endotracheal aspirates (EA) from 20 intubated patients in a surgical intensive care unit (mean ventilation time/patient = 16.5 days) were investigated serially by performing quantitative cultures using growth of 10(5) cfu/mL as a cut-off point. microscopic examinations were made using Giemsa's stain for polymorphonuclear neutrophils (PMN). The spectrum of pathogens encountered was determined and compared with clinical data to distinguish colonization from infection of the lower respiratory tract. Out of 301 EA cultures, 156 (51.8%) were positive and 145 (48.2%) were below the cut-off point. Counts of PMN were significantly higher in samples which gave positive cultures. Seventy-five different bacterial strains were isolated (64% were Gram-negative bacilli). Seventeen patients (85%) were colonized with Gram-negative bacteria. Nine patients (45%) developed nosocomial pneumonia (NP), five (25%) had no signs of pneumonia, and six (30%) had an uncertain status. Main causative agents for NP were Pseudomonas aeruginosa, Enterobacteriaceae and Staphylococcus aureus. Quantitative EA cultures had a sensitivity of 81.5%, a specificity of 64.8%, a positive predictive value of 55% and a negative predictive value of 87%. Our results suggest that EA quantitative cultures (cut-off value 10(5) cfu/mL), species identification and microscopic examination of EA may help to differentiate tracheobronchial colonization and infection, especially when bronchoscopic techniques are not available.