What is the WHO blast threshold for AML?

The WHO classification defines acute myeloid leukaemia as 20% or more blasts of all nucleated cells in the bone marrow or blood. Below 20% the picture usually falls into MDS or another category, subject to the clinical and genetic context.

Why are erythroid cells counted separately?

When erythroid precursors make up 50% or more of the marrow, blasts are also expressed as a percentage of non-erythroid cells, an older FAB rule still used in some schemes. Separating erythroid from other cells lets the tool compute that non-erythroid denominator.

How is the non-erythroid percentage calculated?

Non-erythroid cells equal the total nucleated count minus the erythroid count. Blast percentage of non-erythroid cells is blasts divided by that non-erythroid total, times 100. The tool only shows it when it is meaningful.

How many cells should I count?

A 500-cell differential is standard for marrow and gives a more stable percentage than a 200-cell count, especially near the 20% cut-off. The tool works with any totals you enter; larger counts reduce sampling error.

Does meeting 20% always mean AML?

No. Some defining genetic abnormalities such as t(8;21), inv(16), or PML-RARA establish AML regardless of blast count, and the blast count must be interpreted with morphology, flow cytometry, and genetics. This tool computes percentages only.

Bone Marrow Blast Percentage Calculator

The blast percentage in a bone marrow aspirate is the single number that most often decides whether a case is acute myeloid leukaemia. This calculator takes raw counts from your differential and returns the two denominators that matter — all nucleated cells and non-erythroid cells — against the WHO 20% threshold.

How it works

From the differential counts, the tool computes:

total_nucleated = blasts + erythroid + other
blast_%_total   = blasts / total_nucleated × 100
non_erythroid   = total_nucleated − erythroid
blast_%_NEC     = blasts / non_erythroid × 100      (when non_erythroid > 0)
erythroid_%     = erythroid / total_nucleated × 100

The percentage of all nucleated cells is the primary WHO denominator. When erythroid precursors reach 50% or more of the marrow, the percentage of non-erythroid cells (NEC) becomes relevant and the tool flags it.

Example and notes

In a 200-cell count with 40 blasts, 60 erythroid, and 100 other cells, blasts are 20% of all nucleated cells but 28.6% of non-erythroid cells — the erythroid expansion lowers the total-cell figure. Always interpret the blast percentage alongside morphology, flow cytometry, cytogenetics, and molecular results; certain defining genetic lesions make a case AML regardless of blast count, and this tool computes percentages only.