Qualitative cultures usually identify pathogenic organisms found by invasive tests, suggesting high sensitivity, but, frequently, they also identify nonpathogenic organisms, reducing the positive predictive value of the procedure. If the culture results are negative for pathogens, VAP is very unlikely to be present, unless the patient has been treated with antibiotics. The results of quantitative cultures on specimens obtained by aspiration vary with the bacterial load, the duration of mechanical ventilation, and prior use of antimicrobial therapy. Role of BAL Bronchoscopic BAL has been used in the diagnosis of VAP since 1988, but bronchoscopic and bacteriologic methods have not been standardized. Only 2 of 23 published studies on this topic have investigated the quality of BAL specimens.
Sensitivity varies inversely with the cutoff point.
Similar problems exist in calculating specificity. The major risk is the reduction of arterial oxygenation, as oxygenation may not be fully reestablished for several hours after injury. 25 Role of PSB Sampling PSB sampling has been used for almost 20 years, but the technique has not been standardized. Most studies do not report the quality of the samples, or state whether secretions were cleared out by using a separate bronchoscope before the test. One study has examined the reproducibility of PSB sampling.