Aims Chronic lymphocytic B cell leukaemia (CLL)/small lymphocytic B cell lymphoma (SLL) has proven to be not a uniform entity but to consist of various disease subtypes. CLL might also pose diagnostic challenges by demonstrating an uncommon immunohistochemical profile. Recently, the role of lymphocyte enhancer-binding factor 1 (LEF1) in CLL was elucidated being highly expressed and seeming to have a prognostic value. Our aim was to test the applicability of LEF1 as marker for CLL in a diagnostic setting.

Methods We investigated LEF1 expression in lymphomas by immunohistochemistry on tissue microarrays containing several lymphoma entities (altogether 720 cases, including 61 CLL cases). We also separated CLL cases by zeta-chain-associated protein kinase 70 (ZAP70) and CD38 stainings and fluorescence in situ hybridisation analyses for TP53 deletions and trisomy 12 into respective groups and correlated data with LEF1 expression.

Results The area under the receiver operating characteristic curve for LEF1 as a diagnostic marker for CLL was 0.815 (95% CI 0.742 to 0.888). The relevant diagnostic cut-off value for LEF1 positivity determined by the Youden's index was 10% (specificity 92%, sensitivity 70%). The majority of CLL cases (70%) expressed LEF1. Eighteen per cent of (transformed) diffuse large B cell lymphoma cases also expressed LEF1. In most other lymphoma entities, LEF1 was negative. There was a positive correlation of LEF1 staining with ZAP70 expression (Spearman's rho: 0.438, p<0.001), but not with CD38 expression, TP53 deletions or trisomy 12.

Conclusions LEF1 is a useful marker in the differential diagnosis of CLL in difficult cases. It shows a high specificity (92%) and a reasonable sensitivity (70%) for this entity.