To compare the performance of the rabbit monoclonal antihuman CD246 antibody (D5F3 clone) with the established ALK1 clone for immunohistochemical assessment of anaplastic large cell lymphoma (ALCL).
Archival cases of ALCL (n = 27) were assessed immunohistochemically by use of ALK1 and D5F3 clones under standard Clinical Laboratory Improvement Amendments-compliant conditions. The intensity of cytoplasmic staining (0 = none; 1 = faint; 2 = moderate; 3+ = strong) and proportion of neoplastic cells (0%, 50%) were evaluated and compared with clinical ALK break-apart fluorescence in situ hybridization (FISH) assays.
Nine ALCL specimens were positive for ALK expression by ALK1 staining (33%; 1 = 1+; 0 = 2+; 8 = 3+), while 14 were positive by D5F3 staining (48%; 3 = 1+; 1 = 2+; 10 = 3+). Across the cohort, D5F3 staining showed a significantly greater proportion of cells staining positive (P = .02) and greater intensity (P = .03). Of 3 cases positive for D5F3 only with FISH results, none showed rearrangements, although 1 showed copy number gains at the ALK locus in a subset of cells.
Overall, D5F3 showed greater stain intensity and proportion staining than ALK1 in ALK-positive ALCL cases, which is especially helpful in limited samples. Caution and consideration of orthogonal ALK testing types is recommended, especially for cases with weak or focal staining.

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