Tracheal colonization of Gram-negatives and S. aureus is not eradicated even though lower airways are sterilized. Thus, post-treatment cultures in the absence of clinical deterioration (fever, rising WBC, new infiltrates, worsening ventilatory status) are not recommended. Tracheal cultures may remain positive and do not imply treatment failure.
Inadequate initial treatment of VAP is associated with higher mortality (even if treatment is changed once culture results are known).
Efforts to reduce the duration of therapy are justified by studies of the natural history of the response to therapy. Data support the premise that most patients with VAP who receive appropriate antimicrobial therapy have a good clinical response within the first 6 days. Prolonged therapy simply leads to colonization with antibiotic resistant bacteria.