Introduction: In patients with lymphocytic exudative pleural effusions (PE), the diagnosis is often secured by invasive tests, such as pleural biopsy.Our study aims to examine peripheral blood monocyte values in these patients and whether the monocyte count can differentiate different PE aetiologies.

Methods: clinical and laboratory information was prospectively collected from patients with PE admitted to a tertiary hospital including differential blood cell count. The final aetiology of PE was recorded after full investigations were completed. The absolute and relative monocyte and lymphocyte counts as well as Monocye:Lymphocyte ratio (MLR) was calculated for all patients. 

Results: A total of 65 patients were included(30 (46%) males, Mean age 52.3+16.9 years). The PE aetiology was: 38 malignant & 14 TB.(plus 13 Transudate PE as control).The absolute values of lymphocytes and monocytes as well as MLR did not differ between PE groups. Both lymphocyte and monocyte ratios were higher in TB effusion. The median lymphocyte ratio was 16.8, 19.2 and 24.6 in transudate, malignant and TB effusions respectively (p=0.041).

The median monocyte ratio was 7.3 [5.9-8.9] for transudate PE,7.4 [4.9-8.6] for malignant PE and 9.9 [8.1 ? 11.2] for TB (P = 0.025, figure)

Conclusion: peripheral monocyte ratio and other blood counts,are potentially useful biomarkers in the work up of exudative PE, particularly when TB is a differential.