Mutation-specific NPM1 immunohistochemistry is a sensitive method for detecting residual disease in mid- and postinduction bone marrows: comparison of NPM1 immunohistochemistry, NPM1 real-time polymerase chain reaction, and flow cytometric studies

Objectives Per National Comprehensive Cancer Network guidelines for acute myeloid leukemia (AML), bone marrow (BM) evaluation is recommended after starting induction chemotherapy at 14 to 21 days (mid-induction) and 30 to 35 days (postinduction). Timely assessment of residual morphologic and molecular disease is challenging at these time points, given the low cellularity, the lack of CD34 or CD117 expression by blasts in most NPM1-mutated (NPM1mut) AML, and the longer turnaround time for molecular testing. Methods NPM1 mut AML undergoing BM evaluation at mid- and postinduction with concurrent flow cytometry and NPM1 real-time polymerase chain reaction or next-generation sequencing was evaluated. Different cell types positive by NPM1 immunohistochemistry (IHC) were enumerated, and the results were compared with flow cytometry and molecular findings. Results NPM1 IHC demonstrated a sensitivity of 78% and 41% for detecting residual mutation at mid- and postinduction, respectively, with a positive predictive value ranging from 88% to 100%. Of 16 IHC-negative cases, 3 had no residual NPM1 mutation, 12 had very low-level mutation at a variant allele frequency of less than or equal to 21 transcripts, and only 1 had a high-level mutation in a markedly hypocellular BM. Of 24 IHC-positive cases, 22 had residual NPM1 mutation, and 2 showed false-positive staining in only a single cell with no residual NPM1 mutation. The level of IHC staining was mostly proportional to the level of residual mutation. Conclusions Mutation-specific NPM1 IHC is a valuable tool for a rapid assessment of residual morphologic and molecular disease in mid- and postinduction NPM1mut AML.