Morphologic features of binucleated lymphocytes to assess the diagnosis of persistent B-cell polyclonal lymphocytosis or other mature B-cell neoplasms.

The observation of binucleated lymphocytes (BNLs) on a peripheral blood smear is essential to the diagnosis of persistent polyclonal B-cell lymphocytosis (PPBL). Only a few case reports have mentioned their presence in other contexts, mainly mature B-cell neoplasms such as chronic lymphocytic leukemia or reactive circumstances such as multiple sclerosis undergoing natalizumab treatment. We sought to investigate whether any particular morphologic features of BNL were more specific to PPBL than other diseases. We reviewed peripheral blood smears of a series of patients affected by PPBL or harboring BNLs whatever the diagnosis. We found that BNLs in PPBL were heterogeneous, but mostly medium-sized with a moderately abundant basophilic cytoplasm, sometimes vacuolated. The chromatin was mature in an asymmetrically bilobed nucleus showing one to two nucleoli. Though mainly observed in patients with PPBL, all these criteria remained non-specific. Conversely, the presence of either monocytoid or hyperbasophilic cells had clinical value to confirm PPBL. We conclude that a substantial percentage of BNLs possessing the morphologic features described and observed in a context of monocytoid and/or hyperbasophilic lymphocytes is predictive of the diagnosis of PPBL.