FL5 Through the Years: A Summary of Interpretation-Only Leukemia/Lymphoma Flow Cytometry Proficiency Testing From the College of American Pathologists Diagnostic Immunology and Flow Cytometry Committee

The Diagnostic Immunology and Flow Cytometry Committee of the College of American Pathologists implemented the FL5 interpretation-only leukemia and lymphoma program in 2009 to survey laboratories for interpretative agreement of flow cytometric data.

To report the results of the FL5 program from 2009 to 2024.

Program participants received 2 sets of paper challenges per year consisting of clinical summaries, laboratory findings, and flow cytometric data. Summary responses were provided for all participants. Supplemental questions were included in 2024.

Subscribing laboratories increased from 38 in 2009 to 368 in 2024. The 16 years of the program have seen 32 FL5 surveys and 80 challenges. Diagnostic categories included 34 B-cell cases (42.5%; 27 mature [33.75%], 7 immature [8.75%]); 23 T/NK (natural killer)-cell cases (28.75%; 16 mature [20%], 7 immature [8.75%]); 18 myeloid cases (22.5%); and 5 others (6.25%). Cases were varied, covering routine and rare entities. There were 34 high-agreement cases, most common within the mature B-lymphoproliferative disorder category (15). Supplemental question responses included 123 of 290 (42%) that performed both testing and interpretation, while 168 (58%) performed interpretation only. The majority (182 of 275; 66%) used FL5 to assess integration of all data to arrive at a specific hematopathologic diagnosis, while some (89; 32%) assessed flow cytometry-based diagnosis. Few (4; 1%) assessed discernment of antigen positivity in a defined population of interest.

Diagnostic agreement was high across a broad range of diagnostic flow cytometry cases encountered in clinical practice. The FL5 survey offers laboratories valuable insight for diagnostic accuracy and comparison of interpretation to that of other laboratories.