Pleural fluid adenosine deaminase/serum C-reactive protein ratio for the differentiation of tuberculous and parapneumonic effusions with neutrophilic predominance and high adenosine deaminase levels

Purpose: Tuberculous pleural effusion (TPE) and parapneumonic effusion (PPE) are usually distinguished by cellular predominance and pleural fluid adenosine deaminase (ADA) levels. However, both diseases may occasionally show similar neutrophilic predominance and high ADA levels. In such cases, the differential diagnosis between TPE and PPE is challenging and has been rarely investigated. Methods: A retrospective study was conducted on TPE and PPE patients with neutrophilic exudate and pleural fluid ADA levels ≥40 U/L. Individual and combined parameters of routine blood and pleural fluid tests were compared between the two groups, and receiver operating characteristic (ROC) curves were constructed for identifying TPE. Results: Thirty-six TPE and 41 PPE patients were included. White blood cell counts, serum C-reactive protein (S-CRP), and pleural fluid pH, lactate dehydrogenase, and ADA levels showed significant difference between the two groups (p < 0.001). Among multiple parameters, pleural fluid ADA/S-CRP ratio, which best reflected different local and systemic characteristics between TPE and PPE, provided the highest diagnostic accuracy with an area under the ROC curve of 0.93. At a cutoff value of 5.62, ADA/S-CRP ratio had a sensitivity of 89 %, specificity of 88 %, positive likelihood ratio of 7.29, and negative likelihood ratio of 0.13 for identifying TPE. Additionally, more than half of TPE patients had a ratio above 15.82, while none of PPE patients showed such findings. Conclusions: Pleural fluid ADA/S-CRP ratio, as a simple method using routine laboratory tests, may be helpful in discriminating between TPE and PPE patients with neutrophilic predominance and ADA ≥40 U/L.