Triaging of pleural effusion cytology specimens for ancillary flow cytometric analysis

There are no established criteria in selecting pleural effusion (PE) specimens for flow cytometric analysis (FCA). FCA on effusion specimens may be ordered by a clinician or a cytopathologist. In an effort to improve lab test utilization, this retrospective study aims to identify characteristics of PE specimens on which the addition of FCA has high diagnostic yield.

We identified consecutive cases of PE cytology specimens on which FCA was performed over a 5-year period (2014-2019). Patient demographic data and history, FCA diagnosis, cytologic diagnosis, cellular quantity and composition, and peripheral blood cell counts were collected. Chi-square, Mann-Whitney U, and t tests were used when appropriate with a significance level of P < 0.05.

We identified 164 FCA cases corresponding to 142 patients (age: 19-90 years; male:female 2:1). The majority of cases had no abnormality by cytologic examination, whereas others were obviously malignant due to non-hematologic malignancy. Most (119 of 164, 73%) had negative immunophenotypic studies by FCA. Forty-five of 164 (27%) FCA cases were positive for a monoclonal myeloid or lymphoid population. Clinicopathologic features associated with positive FCA results included a history of hematologic malignancy, peripheral blood lymphocytes of ≥20%, the presence of a monomorphic lymphoid population, large atypical cells, and mitoses.

This study identifies features that are associated with positive FCA in PE cytology specimens. Using these features by cytopathologists to order FCA on PE specimens as a reflex test would significantly reduce unnecessary testing and improve FCA utilization.