Chronic myeloid leukaemia: have basophils gone the way of the Dodo bird?

For over 40 years, percentage blood basophils has been used to estimate survival and to define the accelerated phase [1][2][3][4][5][6].Recently, this use has declined or disappeared, especially since the introduction of tyrosine kinase inhibitor (TKI) therapy.No widely used CML survival prediction model includes percentage blood basophils [7].Putting a nail in the coffin, the 2022 WHO classification of myeloid neoplasms, for good reason, eliminated accelerated phase CML making percentage blood basophils irrelevant [8].In contrast, the 2022 International Consensus Classification (ICC) retained accelerated phase CML.The 2025 European LeukemiaNet classification seems undecided [9, 10] (reviewed in refs.[11, 12]).It's interesting to consider how and why percentage blood basophils came into prominence in CML, and why percentage and not concentration was the unit of quantification, like haemoglobin (g/L) and WBCs (x10 9 /L).An explanation is that automated particle counters able to estimate basophil concentration became widely used beginning in the late 1980s.Previously, basophil quantity was assessed only by a visual blood cell differential estimate resulting in percentage.But now, if percentage blood basophils has gone the way of the Dodo bird, last sighted in 1662, might there be residual value were blood basophils to be quantified as a concentration?The units of how a quantity is expressed are quirky.Consider a shrewdness of apes, a congregation of alligators, a murder of crows, a mob of kangaroos, a parliament of owls, etc.So it is reasonable to ask: Why was the quantity of blood basophils previously expressed as a percentage, and is this quantification important??Today, four methods are widely used to quantify blood basophils: (1) visual differential analyses; (2) automated particle counting; (3) multi-parameter flow cytometry (MPFC); and (4) artificial intelligence (AI) and machine learning.Visual differential analyses of WBCs are the reference standard for basophil enumeration despite well-known limitations including non-uniform distribution, observer variability and statistical