Comparison between quantitative and qualitative culture of tracheal aspirates in the diagnosis of ventilator associated pneumonia

Objectives

Tracheal aspirates cultures play an important role in the diagnosis of ventilator associated pneumonia, but there are scarce data on the utility of quantification of the bacterial colonies recovered. Our objective was to compare, in the same group of patients, the positive and negative predictive values (PPV and NPV) of qualitative and quantitative (10⁵ and 10⁶cfu/ml) cultures of tracheal aspirates in the diagnosis of VAP.

Patients and methods

Between March and December 2000, ventilated patients (>48 hours on ventilatory support) in a clinical/surgical intensive care unit (ICU) were evaluated every Monday for the presence or absence of VAP based on clinical and radiological criteria (concordance of at least two out of three ICU physicians). In the same day, tracheal aspirates were collected aseptically and assayed for quantitative results with standard techniques. The physicians were unaware of the culture results.

Results

The 76 patients enrolled yielded 154 opportunities for evaluating the presence or absence of VAP. Overall concordance between ICU physicians was 94% and radiological criteria was the major reason for discordance. There were 26 episodes of VAP (85% of concordance) and the major pathogens were P. aeruginosa (27%) and S. aureus (23%). Sensitivity, specificity, positive and negative predictive values of qualitative and quantitative cultures of traqueal aspirates are shown in the table.

Table 1

Conclusions

Based on our preliminary results, quantification of tracheal aspirates for the purpose of improving the diagnosis of VAP is useless and may not be cost-effective.