Diagnostic Yield of a Direct Quantitative Smear of Lower Respiratory Tract Secretions in Patients with Suspected Pneumonia Compared to a Semi-quantitative Culture


Infectious Diseases and Tropical Medicine Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran


Background: Microorganism isolation from respiratory tract specimens is the standard of care in patients with suspected nosocomial and ventilator associated pneumonia. However, these methods are time-consuming and are influenced by several factors. A direct quantitative smear (DQS) with proper staining may be an easy, cost-effective, rapid method. We evaluated the diagnostic yield of direct smears compared to semi-quantitative culture methods. Materials and Methods: Hospitalized, intubated patients with clinically suspected pneumonia and patients who underwent diagnostic bronchoscopic alveolar lavage (BAL) and trans-endotracheal aspiration (TEA) were enrolled in a prospective study. The obtained specimens were Gram stained and microorganisms were computed per 10 high-power fields (HPFs) of light microscopy. All samples were cultured by a standard semi-quantitative method. Colony-forming units (CFU) >104/mL and >105 CFU/mL were reported as culture-positive for BAL and TEA, respectively. Results: A total of 331 respiratory specimens were analyzed. Based on culture results, the best cut-off point was 35 microorganisms in 10 HPFs of microscopy and provided 90.4% sensitivity and 90.8% specificity. The best cut-off point for 25 microorganisms in 10 fields of light microscopy provided 95.2% sensitivity and 85.7% specificity. Conclusion: A DQS obtained by BAL and TEA may be a reliable and rapid method to diagnose pneumonia and anticipate semi-quantitative culture results. The sensitivity and specificity of a direct smear have adequate diagnostic yield to recommend it as an adjunct to microorganism-isolation methods.