Acute Myeloid Leukemia (AML) involves the rapid growth of abnormal cells in the bone marrow called “blasts.” These are immature cells that, in a healthy person, are programmed to mature into various functional blood cells, including red blood cells, white blood cells, and platelets. AML develops when this process goes awry.
The blast cells become cancerous, failing to mature and instead multiplying uncontrollably. These malignant cells, known as myeloblasts, accumulate in the bone marrow and crowd out healthy blood cells. This proliferation leads to bone marrow failure and the characteristic symptoms of AML, such as anemia, infections, and bleeding.
The Role of Blasts in AML Diagnosis
The percentage of blast cells in the bone marrow is a primary factor in diagnosing Acute Myeloid Leukemia. In a healthy individual, blast cells constitute less than 5% of the cells within the bone marrow and are mostly absent from circulating blood. A higher number indicates a disruption in normal blood cell production.
The World Health Organization (WHO) sets the diagnostic threshold for AML at 20% or more blast cells in the bone marrow or peripheral blood. Reaching this mark distinguishes AML from less aggressive conditions like myelodysplastic syndromes (MDS), where blast counts are elevated but remain below 20%. This distinction separates a rapidly progressing leukemia from a condition that may evolve more slowly.
While the 20% blast count is the standard criterion, there are exceptions. The presence of certain genetic abnormalities can secure an AML diagnosis even if the blast percentage is below the 20% threshold. These genetic markers indicate the disease is present regardless of the blast count, allowing for a precise diagnosis.
Identifying and Counting Blast Cells
Determining the precise percentage of blast cells requires specific medical procedures. The process begins with obtaining a bone marrow sample through two complementary procedures: a bone marrow aspiration and a biopsy. During an aspiration, a needle withdraws a liquid sample of the marrow, while a biopsy collects a small, solid piece of marrow tissue.
Once collected, samples undergo a morphological examination where a pathologist reviews them under a microscope to visually identify and count the leukemic blasts. The pathologist looks for features like large size and an immature appearance. The presence of Auer rods, needle-like structures found within some AML blasts, is a definitive microscopic sign of the disease.
A more advanced method for counting and characterizing these cells is flow cytometry. This technique uses lasers to analyze cells treated with fluorescent markers that bind to specific proteins, or antigens, on the cell surface. By identifying the unique combination of antigens, flow cytometry can precisely quantify the blasts and help differentiate AML from other leukemias.
Blast Count in Treatment and Remission
Following a diagnosis, the blast count is used to monitor treatment response. The primary objective of therapies like chemotherapy is to eliminate leukemic blasts from the bone marrow, thereby lowering the blast percentage. This reduction allows normal blood cell production to resume.
A primary goal of treatment is achieving “complete remission.” This is defined by the recovery of normal blood cell counts and a blast cell percentage in the bone marrow that has fallen below 5%. At this stage, signs and symptoms of the disease are no longer present.
Even after complete remission, a small number of cancer cells may persist, a condition known as Minimal Residual Disease (MRD). These cells are not detectable by standard microscopy but can be identified with sensitive tests like flow cytometry or polymerase chain reaction (PCR). The presence of MRD is a factor in patient outcomes, as it may indicate a higher likelihood of the cancer returning.