Diagnostic value of neutrophil-lymphocyte ratio versus C-reactive protein in discrimination between different pathogens causing community-acquired pneumonia

Early discrimination of the causative organisms of community-acquired pneumonia (CAP) is important. The aim of the present study is to evaluate neutrophil–lymphocyte ratio (NLR) and derived NLR (dNLR) as adjunct tests to differentiate different types of CAP. NLR, dNLR, and C-reactive protein (CRP) were measured in 187 patients with CAP. Identification of causative organisms was done by direct smears, microbiological cultures, VITEK 2 compact system, and PCR of patient’s sputa together with detection of specific IgM antibodies against Chlamydia pneumoniae. There was a significant increase of NLR and CRP values in patients with bacterial pneumonia as compared to patients with TB and patients with atypical pneumonia. NLR and CRP were significantly higher in atypical than TB pneumonia. Although dNLR was significantly higher in both bacterial and atypical than TB pneumonia, it was insignificantly differ between bacterial and atypical pneumonia. For discriminating bacterial CAP from TB, the area under curve (AUC) for NLR was found to be significantly higher than that of CRP at cutoff values of 7.8 for NLR and 172.9 mg/l for CRP. Also, at cutoff values of 6.6 for NLR and 179.1 mg/l for CRP, NLR distinguished bacterial from atypical pneumonia more accurately than CRP (AUC = 0.92 versus 0.74). There was no significant difference in inflammatory marker levels between subcategories of atypical pneumonia except for CRP. There was a significant positive correlation between NLR and CRP in pneumonic patients (r = 0.71, p = 0.000). NLR is easy to be calculated from daily practice and might be used as a surrogate marker to differentiate bacterial CAP from atypical and TB pneumonia as well as atypical from TB pneumonia but could not discriminate the different pathogens causing atypical bacterial pneumonia. dNLR could discriminate TB from both bacterial and atypical pneumonia but could not differentiate bacterial from atypical pneumonia.