No single set of criteria is reliably diagnostic of pneumonia in a ventilated patient. The inability to accurately identify such patients com promises efforts to prevent and treat VAP and even calls into question estimates of the impact of VAP on mortality rates.

Application of clinical criteria typical for CAP consistently results in overdiagnosis of VAP, largely because of (1) frequent tracheal colonization with pathogenic bacteria in patients with endotracheal tubes, (2) multiple alternative causes of radiographic infiltrates in mechanically ventilated patients, and (3) the high frequency of other sources of fever in critically ill patients. The differential diagnosis of VAP includes atypical pulmonary edema, pulmonary contusion, alveolar hemorrhage, hypersensitivity pneumonitis, acute respiratory distress syndrome, and pulmonary infarction. Findings of fever and/or leukocytosis may have alternative causes, including antibiotic-associated diarrhea, central line–associated infection, sinusitis, urinary tract infection, pancreatitis, and drug fever. Conditions mimicking pneumonia are often documented in patients in whom VAP has been ruled out by accurate diagnostic techniques. Most of these alternative diagnoses do not require antibiotic treatment; require antibiotics different from those used to treat VAP (fungal or viral pneumonia); or require some additional intervention, such as surgical drainage or catheter removal, for optimal management.

This diagnostic dilemma has led to debate and controversy about whether a quantitative-culture approach as a means of eliminating false-positive clinical diagnoses is superior to a clinical approach enhanced by principles learned from quantitative-culture studies. The most recent IDSA/ATS guidelines for HAP/VAP give a weak recommendation for a clinical approach based on semiquantitative cultures, with consideration of the availability of resources, cost, and the avail ability of expertise. The guidelines acknowledge that the use of a quantitative approach may result in less antibiotic use, which may be critical for antibiotic stewardship in the ICU. Therefore, the approach at each institution—or potentially for each patient—should be individualized and based on local colonization rates, local diagnostic expertise, and recent history of antibiotic therapy.

Quantitative-Culture Approach This method uses quantitative cultures of deep respiratory tract samples to distinguish colonization from true infection. The more distal in the respiratory tree the diagnostic sampling, the more specific are the results and therefore the lower the threshold of growth necessary to diagnose pneumonia and exclude colonization. For example, an endotracheal aspirate yields proximal samples, and the diagnostic threshold is 10⁶ cfu/mL. The protected specimen brush method, in contrast, collects distal samples and has a threshold of 10³ cfu/mL. Conversely, sensitivity declines as more distal secretions are obtained, especially when they are collected blindly (i.e., by a technique other than bronchoscopy). Additional tests that may increase the diagnostic yield include Gram staining, differential cell counts, staining for intracellular organisms, and detection of local protein levels elevated in response to infection.

If the quantitative approach is used, therapy decisions should be linked to culture results (no antibiotics if below the diagnostic thresh old), with antibiotics withheld until results are available unless the patient is critically ill. Studies have documented less antibiotic use with this approach than with the clinical approach, but the results are less clear if antibiotic decisions are not directly linked to culture data. One common limitation of the quantitative approach is that the use of a new and effective antibiotic agent in the 24–48 h prior to sampling can lead to false-negative results. With antimicrobial-sensitive microorganisms, a single antibiotic dose can reduce colony counts below the diagnostic threshold. After 3 days, the operating characteristics of the tests improve to the point at which they are equivalent to results obtained when no prior antibiotic therapy has been given. Conversely, colony counts above the diagnostic threshold during antibiotic therapy suggest that the current antibiotics are ineffective. In addition, quantitative cultures may give results below the diagnostic threshold if samples are collected early in the course of infection or if sampling is delayed until after an effective host response has reduced bacterial counts. Ideally, a specimen should be obtained as soon as pneumonia is suspected and before antibiotic therapy is initiated or changed.

Clinical Approach The lack of specificity of a clinical diagnosis of VAP has hampered its utility, but this approach has been improved by the addition of microbiologic and other laboratory data. Tracheal aspirates generally yield at least twice as many potential pathogens as quantitative cultures, but the causative pathogen is almost always present. The absence of bacteria in Gram-stained endotracheal aspirates makes pneumonia an unlikely cause of fever or pulmonary infiltrates. These findings, coupled with a heightened awareness of the alternative diagnoses possible in patients with suspected VAP, can prevent inap propriate antibiotic overtreatment. Furthermore, the absence of an MDR pathogen in tracheal aspirate cultures eliminates the need for MDR coverage, allowing de-escalation of empirical antibiotic therapy. Similarly, with newer and more sensitive molecular diagnostic methods, a suspected MDR pathogen can be eliminated as a therapy target if test results are negative. A clinical approach that focuses on careful antimicrobial use and de-escalation of therapy after culture results become available may have an impact on the avoidance of antimicrobial overuse and the consideration of alternative sites of infection similar to that of a quantitative-culture approach.

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مواضيع ذات صلة

protein C, protein S

prostate-specific antigen (PSA), prostate cancer gene 3 (PCA3), MI-prostate score (MiPS)

prolactin levels (PRLs)

Enzyme-Linked Immunosorbent Assay (ELISA)

Agglutination Reaction

Precipitation Reaction

Heparins and Fondaparinux

Activated Partial Thromboplastin Time (aPTT)

Prothrombin Time (PT)

Diagnosis of Community-Acquired Pneumonia

progesterone receptor assay (PR assay, PRA, PgR)

progesterone assay