What Does MDS/AML Mean? Blast Percentage Explained

MDS/AML refers to a newer overlap diagnostic category used to describe patients whose disease sits on the borderline between myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Historically, doctors used a conventional boundary of 20% blasts (immature blood-forming cells) to separate the two conditions: having fewer than 20% blasts generally supported an MDS diagnosis, while having 20% or more (≥20%) supported an AML diagnosis ^[1][2]. Today, modern classification systems recognize a “gray area” of 10% to 19% blasts. The International Consensus Classification (ICC) designates this 10–19% group as MDS/AML ^[3][4]. Furthermore, the presence of certain specific genetic abnormalities can result in an AML diagnosis even if your blast count is below 20% ^[1][2].

What Are Blasts?

In healthy bone marrow, blasts are early, immature blood-forming cells that normally develop into fully functioning red blood cells, white blood cells, or platelets. In MDS and AML, the bone marrow produces abnormal blasts (often called myeloblasts or immature myeloid cells) that do not mature properly and instead build up in the bone marrow and bloodstream ^[1][2].

Doctors determine your blast percentage by taking samples of your peripheral blood (a standard blood draw) and your bone marrow (via an aspirate and biopsy). They count the cells under a microscope (morphology) and use specialized laboratory techniques like flow cytometry to identify and characterize the abnormal cells ^[1][5]. It is common for the blast percentage in your blood to differ from the percentage in your bone marrow, and doctors use an integrated assessment of both ^[2].

The Conventional 20% Boundary

For decades, the dividing line between MDS and AML was based heavily on a numerical cutoff (when no other AML-defining genetic abnormalities were present):

• MDS: Generally associated with fewer than 20% blasts.
• AML: 20% or more (≥20%) blasts in the bone marrow or peripheral blood ^[1][2].

While this 20% rule remains a useful conventional boundary, experts now understand that the biological difference between MDS and AML is a continuum ^[3][4]. The 20% cutoff is no longer a standalone diagnostic test.

The “MDS/AML” Overlap Category (10–19% Blasts)

Because disease characteristics can overlap whether a patient has 19% blasts or 21% blasts, the International Consensus Classification (ICC) updated its guidelines in 2022 to introduce a specific bridging category called MDS/AML ^[3][4].

This classification applies to adults who have 10% to 19% blasts in their blood or bone marrow and lack specific AML-defining genetic abnormalities ^[3][4]. It highlights that the disease sits in an overlap area. Being diagnosed with MDS/AML does not necessarily mean you have two separate diseases, nor is it a guarantee of how your disease will behave; rather, it is a classification label that helps your clinical team discuss treatment options and may allow you to be considered for clinical trials that were previously restricted by the strict 20% cutoff ^[2][6].

How Genetics Change the Rules

Today, the genetic makeup of your disease—determined through molecular testing or next-generation sequencing (NGS)—is just as important as your blast percentage. Certain genetic features are so strongly associated with AML that they can override the 20% boundary:

• AML-Defining Genetic Abnormalities: If your abnormal cells contain specific features—such as a gene mutation (like NPM1) or a gene fusion/rearrangement (like PML::RARA)—you can be diagnosed with AML even if your blast count is below 20% ^[7][6].
• Different Blast Requirements: Depending on the exact genetic abnormality and the diagnostic system used, the minimum blast requirement varies. For example, the ICC requires at least 10% blasts for some AML-defining abnormalities, while other guidelines may eliminate the blast cutoff entirely for certain mutations ^[7][8].
• The ≥20% Requirement: Alternatively, certain features—such as the BCR::ABL1 gene fusion or specific TP53 mutations (under the ICC)—still strictly require a blast count of 20% or more (≥20%) to be classified as AML ^[7][9].

Because of these genetic factors, two people with the same blast percentage could receive different diagnoses depending on their molecular test results.

Differing Classification Systems: WHO vs. ICC

It can be emotionally jarring to seek a second opinion and see a completely different disease name on your pathology report. This happens because the two major diagnostic frameworks updated in 2022 handle the 10–19% “gray area” differently:

Consequently, a patient with 15% blasts in their bone marrow might be told they have “MDS/AML” under the ICC system, but “MDS-IB2” under the WHO system ^[10][11]. Both are accurate ways of describing the exact same biology.

Moving Beyond Just Percentages

Treatment decisions are not dictated entirely by the name of your diagnosis or which side of the 20% line your blast count falls on. Your doctor will look at the entire picture—including your exact blood and marrow blast percentages, your cytogenetic and molecular testing results, your overall health, and your clinical history—to recommend the most appropriate care plan ^[1][2][7].

A Note on Symptoms and Safety:
While tracking how you feel is important, symptoms like fatigue, easy bruising, or frequent infections are common with both MDS and AML due to low blood counts. You should not try to self-diagnose disease progression based on symptoms alone. Always follow your team’s specific safety plan. Contact your hematology team or seek urgent medical care immediately if you experience a fever of 100.4°F (38.0°C) or higher, uncontrolled bleeding, severe shortness of breath, or sudden confusion.