The "CD43 only" phenotype. An aberrant, nonspecific immunophenotype requiring comprehensive analysis for lineage resolution.

Paraffin section immunohistology of leukocytic proliferations is a routine method of immunophenotyping in many clinical laboratories. Furthermore, a relatively standard antibody screening panel that includes L26 (CD20), Leu22 (CD43), and UCHL1 (CD45RO) appears to be widely used. Although paraffin section immunophenotyping in general, and this panel in particular, have been shown to be very reliable in defining B-cell lineage, characterization of T-cell lineage is less definitive. This is related primarily to the relatively poor specificity of the commercially available T-cell-associated reagents. CD43 (Leu22) in particular has a broad immunoreactivity profile that has not been stressed adequately in some reports. Seventeen cases with a "CD43 only" phenotype were identified during the last several years while using the relatively standard screening panel mentioned above. These cases were quite heterogeneous with respect to cellular differentiation and most were not T-cell proliferations. Specifically, eight cases were extramedullary leukemic infiltrates (five myeloid, two monocytic, one mixed lineage), four cases were T-cell lymphomas, three cases were B-cell lymphomas and two cases were plasmacytomas. Although CD43 has demonstrable utility in a leukocyte screening panel, this report stresses the aberrancy and lack of specificity of the "CD43 only" phenotype. Caution is recommended in assigning a specific lineage to such cellular proliferations without additional immunologic or genotypic analysis. Recommendations for comprehensive diagnostic evaluation of these proliferations are provided.