The significance of bacterial engulfment in Gram-stained sputum in patients with respiratory infections

In general, physicians believe that the presence of bacterial engulfment in white blood cells (WBCs) on Gram-stained sputum is a hallmark of lower respiratory infection. However, no studies have described the significance or diagnostic accuracy of engulfment in lower respiratory tract infections. We prospectively studied sputum samples by Gram staining (Favor method) for their quality and engulfment score in WBCs obtained from patients with respiratory symptoms at inpatient and outpatient settings at Kyorin University Hospital between December 2012 and April 2015. A total of 163 patients were enrolled. The patients were classified into an infection (n = 93) or non-infection (n = 70) group based on clinical or radiological findings prior to the evaluation of sputum samples. The proportion of engulfment-positive cases was equal in the infection and non-infection groups (49.5% vs 35.7%, P = 0.11). In the infection group, the engulfment score (%) for Streptococcus pneumoniae was significantly lower (median 3%, interquartile range [IQR]: 2% to 5%, P = 0.005) than that of the non-S. pneumoniae bacteria (H. influenzae, M. catarrhalis, and methicillin-susceptible Staphylococcus aureus (MSSA))(median 22.5%, IQR: 17% to 35.5%). The engulfment score of S. pneumoniae in the WBC was low in the infection group, and no cases were recognized in the non-infection group. Using a cut-off value of 3%, the diagnostic accuracy for infection was as follows: sensitivity: 50%, specificity: 65.7%, and area under the curve (AUC): 0.579 (95% CI 0.464 to 0.694). For the non-S. pneumoniae bacteria (H. influenzae, M. catarrhalis, and MSSA), the engulfment score was significantly higher in the infection group (median 22.5%, IQR 17 to 35.5%) than in the non-infection group (median 6.0%, IQR: 3 to 13%, P = 0.011), and the diagnostic accuracy for infection was as follows: sensitivity: 75%, specificity: 85.7%, and AUC: 0.902 (95% CI 0.75 to 1.00) when the threshold for the engulfment score was defined as 18%. This study provides the first evidence that the engulfment of bacteria in WBCs is not always indicative of infection and that the engulfment score can fluctuate according to the pathogen.