**6.1. Culture**

Semiquantitative culture of endotracheal aspirates (ETA) is the recommended microbiological procedure to diagnose VAP, since it is more sensitive and can be done more rapidly. Other biological specimens have been used, including the ones obtained by invasive sampling, such as: bronchoalveolar lavage (BAL), blind bronchial sampling (mini-BAL), and protected specimen brush (PSB). Blood cultures should also be performed for all patients with suspected VAP. In all cases, samples should be obtained before the patients initiate antibiotic therapy [61].

The main problem with the semiquantitative culture of ETA is that its high sensitivity promotes the unnecessary prescription of antibiotics to some patients. In the case of quantitative cultures of lower respiratory tract secretions, the following threshold cut-offs are usually applied to diagnosis true infection: ETA 105 –106 , BAL 104 , and PSB 103 CFU/mL. This strategy may lead to false-negative results and worse clinical outcomes in some patients [61].
