The Argument against Using Quantitative Cultures in Clinical Trials and for the Management of Ventilator-Associated Pneumonia

Quantitative cultures have been proposed as the most accurate way to both establish the presence of ventilator-associated pneumonia (VAP) and define the etiologic pathogen. Although the clinical diagnosis of VAP has been much maligned, it may be very accurate, particularly if it is objectively defined by calculating the Clinical Pulmonary Infection Score and if the score incorporates a Gram stain of a lower respiratory tract sample. After the clinical diagnosis of VAP is made, a culture is needed to identify the etiologic pathogen, but this culture does not need to be quantitative or bronchoscopic. Quantitative culture-based diagnosis may not be more accurate than clinical diagnosis, and quantitative cultures have a number of methodologic limitations that can cause both false-positive and false-negative results. Finally, a number of studies have suggested that clinical management without quantitative cultures may be accurate and that outcomes, such as mortality and change in antibiotics to a focused regimen, are not improved by the use of quantitative cultures. In clinical trials, management using nonquantitative cultures of a tracheal aspirate specimen may be preferable. Reliance on quantitative cultures can complicate enrollment and will ensure that only a subset of patients with VAP is studied because of the relatively high false-negative rate of quantitative culture results, particularly among patients treated with antibiotics before samples are obtained.