The inflamed sputum in lower respiratory tract infection: l‐lactate levels are correlated to neutrophil accumulation

Lower respiratory tract infections (LRTI) are common, but little is known about the response of biomarkers of inflammation in the lungs. Therefore, our primary aim was to compare the concentration of l‐lactate to the concentration of neutrophils in sputum and systemic markers of infection. Because it is difficult to differentiate viral and bacterial infection on the basis of clinical presentation in LRTI, our secondary aim was to evaluate if l‐ and d‐lactate may serve as markers of local inflammation as representatives of neutrophils and bacteria, respectively. Methods Patients with acute LRTI were prospectively recruited. Sputum samples were collected and analysed for neutrophil count, l‐lactate and d‐lactate. We had data on pathogens from sputum cultures and polymerase chain reaction (PCR) (atypical bacteria, virus) and C‐reactive protein (CRP) from blood. Results In 44 sputum samples from 32 patients, the median (interquartile range (IQR)) sputum neutrophil granulocyte count was 0.615 × 107 cells/mL (0.236–1.575). The sputum neutrophil granulocyte count was associated with sputum l‐lactate (p = 0.011) and CRP (p = 0.018), but not with d‐lactate (p = 0.177). There was a strong association between sputum d‐lactate and l‐lactate (p