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.

Of 16 immunohistochemistry-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 bone marrow.

Of 24 immunohistochemistry-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 immunohistochemistry staining was mostly proportional to the level of residual mutation.