Microscopic examination of intracellular organisms in bronchoalveolar lavage fluid for the diagnosis of ventilator-associated pneumonia: a prospective multi-center study

Abstract

Background The presence of intracellular organisms (ICOs) in polymorphonuclear leukocytes obtained from bronchoalveolar lavage fluid (BALF) is a possible method for rapid diagnosis of ventilator-associated pneumonia (VAP). However, the validity of this diagnostic method remains controversial and the diagnostic thresholds reported by investigators were different. Our objective was to evaluate the accuracy of quantification of ICOs in BALF for the diagnosis of VAP, and to detect the best cutoff percentage of PMNs containing ICOs (PIC) in the microscopic examination of BALF for the diagnosis of VAP. Methods This was a prospective multi-center study conducted in 4 ICUs in Wuhan, China, which involved 181 patients suspected of first episode of VAP. BALF was obtained from all enrolled patients. The BALF samples underwent quantitative culture, cytological and bacteriological analysis to detect the culture results, PIC values and the morphological features of microorganisms. Definite diagnosis of VAP was based on pre-set criteria. The receiver-operating characteristic curve was used to detect the best cutoff point for PIC to diagnose VAP, and the diagnostic accuracy was calculated. Moreover, quantitative culture and Gram's stain of BALF were adopted to diagnose VAP, and their diagnostic accuracy was evaluated as well. Results There were 102 patients definitely diagnosed with VAP (VAP group), and 60 patients definitely diagnosed without VAP (no VAP group). We found that ICOs were present in 96.08% (98 out of 102) of VAP patients and 20.00% (12 out of 60) of no VAP patients. The PICs were significantly higher ((9.53±6.65)% vs. (0.52±1.33)%, P<0.01) in VAP group. In our study, the best cutoff point for PIC to diagnose VAP was 1.5%,which had a sensitivity of 94.12%, a specificity of 88.33%, a positive predictive value (PPV) of 93.20% and a negative predictive value (NPV) of 89.83%.The area under the receiver-operating characteristic curve was 0.956 (95% confidence interval,0.925-0.986; P<0.01). When the positive quantitative culture results of BALF were used to diagnose VAP, the sensitivity, specificity, PPV and NPV were 65.69%, 95.00%, 95.71% and 61.96%, respectively. Whereas they were 70.59%, 76.67%, 83.72% and 60.53%, respectively, when the positive Gram's stain results of BALF were used to diagnose VAP. The concordance between the results of Gram's stain and quantitative cultures was poor, only 32.10% (52 out of 162) was totally right, and 17.28% (28 out of 162) was partially right. Conclusions PIC>1.5% has good diagnostic performance in the microscopic examination of BALF for the diagnosis of VAP. However, Gram's stain is not reliable for the early application of antibiotic therapy, due to the poor bacteriological predictive value.