Title:  The diagnostic implications of the pleural fluid cell differentiation

Introduction: Pleural fluid cell differentiation is recommended along with routine biochemical, microbiological, and cytological investigations of undiagnosed unilateral pleural effusion. However, the sensitivity and specificity of cell differentiation from initial pleural aspiration have not been reported.

Objectives: We aimed to establish the value of pleural fluid cell differential in the investigation of undiagnosed pleural effusion

Methods: Patients presenting to a single UK pleural centre were enrolled into this observational study between December 2008 and December 2016. Their final 12-month diagnosis was confirmed by two respiratory consultants. Cell differentiation was defined according to the BTS guidelines. 26 patients were excluded owing to incomplete data and unclear final diagnoses. 

Results: Of the 997 patients, 424 were mixed (42.5%), 349 were lymphocytic (35%), 95 were neutrophilic (9.5%), 73 were eosinophilic (7.3%), and 56 were blood-stained (5.6%). Over half of the lymphocytic effusions, (54.2% (n=189)), were found to be of malignant origin with a wide variety of non-malignant aetiologies making up the remainer (e.g. CCF, BAPE, PPE, etc). Lymphocytic effusion was seen in 75% (15/20) of patients with tuberculosis. Of 95 neutrophilic effusions, 80% (n=76) were secondary to infection, 13% (n=13) were found to have a malignant aetiology, representing 2.4% (13 out of 532) of the entire malignant group. 

Conclusions: This study demonstrated cell counts in pleural aspirates have limited diagnostic value. While they might provide a guide to the likely diagnosis, they lack diagnostic accuracy as a rule-in/out test.