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Objective: To determine whether the diagnostic accuracy of bronchoscopy samples in patients with suspected ventilator-associated pneumonia is affected by prior antibiotic treatment given for a previous infection, and/or by antibiotic treatment recently started to treat suspected ventilator-associated pneumonia.Design: Study of critically ill patients.Setting: Intensive care unit in a university hospital.Patients: Sixty-three episodes of suspected ventilator-associated pneumonia were prospectively evaluated. Based on prior antibiotic treatment, three groups were defined: no antibiotic group (no previous antibiotic treatments), n = 12; current antibiotic group (antibiotic treatment initiated >72 hrs earlier), n = 31; and recent antibiotic group (new antibiotic treatment class started within the last 24 hrs), n = 20.Interventions: Fiberoptic bronchoscopy with quantitative protected specimen brush cultures, bronchoalveolar lavage cultures, and intracellular organism counts of bronchoalveolar lavage cells.Measurements and Main Results: The diagnosis of ventilator-associated pneumonia was made in 35 cases, based on histology (n = 2), cavitation (n = 2), blood cultures (n = 4), or outcome under appropriate antibiotic treatment (n = 27). The discriminative value of the tests, based on the area under the receiver operating characteristic curve, was high (>or=to0.85) in both current antibiotic treatment and recent antibiotic treatment patients. Sensitivities for a 5% intracellular organism count of bronchoalveolar lavage cells, a protected specimen brush culture threshold of 103 colony-forming units (cfu)/mL, and a bronchoalveolar lavage culture threshold of 105 cfu/mL were as follows, respectively, in the three groups: 0.71, 0.88, and 0.71 (no antibiotic treatment group); 0.5, 0.77, and 0.83 (current antibiotic group); and 0.67, 0.40, and 0.38 (recent antibiotic group). Specificity was consistently >or=to0.9. In the recent antibiotic group, protected specimen brush and bronchoalveolar lavage cultures had lower sensitivities (p < .05), and the best threshold values for these two tests were 102 cfu/mL and 103 cfu/mL, respectively.Conclusions: After recent introduction of an antibiotic treatment for suspected ventilator-associated pneumonia, protected specimen brush and bronchoalveolar lavage culture thresholds must be decreased to maintain good accuracy. In contrast, current antibiotic treatment prescribed for a prior infectious disease does not modify the diagnostic accuracy of protected specimen brush or bronchoalveolar lavage. (Crit Care Med 1998; 26:236-244)Nosocomial pneumonia in intensive care unit (ICU) patients receiving mechanical ventilation is a severe, common complication that increases morbidity, mortality, and ICU stay duration [1-3]. Quantitative protected specimen brush cultures with a 103 cfu/mL threshold and bronchoalveolar lavage fluid cultures with a 104 cfu/mL threshold are both sensitive and specific for the diagnosis of ventilator-associated pneumonia. However, these thresholds were selected mainly on the basis of data from patients who had not received antimicrobial agents before specimen collection [4,5]. There is general agreement that a count of intracellular organisms of >5% in bronchoalveolar lavage fluid is also strongly predictive of ventilator-associated pneumonia [5,6].The effectiveness of these three tests for the diagnosis of ventilator-associated pneumonia in patients who are receiving antibiotic treatments has been debated [7-13]. Previous antibiotic therapy is known to affect the results of bacterial culture tests [4,7,9,14-17] and may preclude the recovery of organisms from respiratory secretions [18,19]. Many ICU patients develop ventilator-associated pneumonia while receiving antibiotic treatments for a previous infection. In this situation, the diagnostic accuracy of the microbiological results may be increased if antibiotic treatments are discontinued 48 hrs before sample collection . However, antibiotic treatment discontinuation may pose undue risks. Moreover, for technical reasons, fiberoptic bronchoscopy and bacteriologic analysis are not performed on a 24-hr-a-day basis in all institutions, and, in some instances, antibiotic therapy for suspected ventilator-associated pneumonia must be initiated without delay because of a severe clinical presentation.Only a few studies [7,14,16,20] have focused on the diagnostic consequences of the timing of bronchoscopy with regard to previous antibiotic therapy. Studies have not considered how measurements derived from interventions such as protected specimen brush and bronchoalveolar lavage would be affected by the prior use of antibiotics and by the use of antibiotics as a prophylactic measure for the treatment of suspected ventilator-associated pneumonia.The aims of this study were to prospectively evaluate the diagnostic yields of bronchoalveolar lavage fluid intracellular organism counts, protected specimen brush cultures, and bronchoalveolar lavage cultures, according to the presence and timing of previous antibiotic therapy.