Leukemia is a hematologic (blood) cancer that can affect any age group. Though it occurs mostly in adults older than 55, it is also the most common cancer affecting children and young teens. It starts in the blood and bone marrow and occurs when white blood cells transform into leukemia cells and grow uncontrollably.

Understanding Leukemia

Normal white blood cells help the body fight infections, and when they become old or damaged, they die and are replaced by new, healthy cells. However, in leukemia, the leukemic cells cannot fight infections properly and do not die when they should. Large numbers of the leukemic cells accumulate in the bone marrow and/or the blood, which may slow down or prevent normal body functions, including the bone marrow’s normal production of healthy blood cells (see Figure 1).

There are four major types of leukemia: acute lymphocytic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL) and chronic myeloid leukemia (CML).

Leukemia is categorized by how fast the disease progresses (“acute” means it grows quickly, “chronic” means it grows slowly) and by the type of white blood cell it affects.

In acute leukemia, the leukemia cells look similar to immature white blood cells. The number of immature cells increases rapidly, preventing the bone marrow from making normal blood cells. Treatment should begin as soon as possible once a leukemia expert physician has been consulted because these fast-growing cells can quickly become life-threatening.

In chronic leukemia, the leukemia cells look similar to healthy, mature white blood cells, but the cells are unable to mature fully. The leukemia cells grow slowly, and the progression of chronic leukemia varies. Like acute leukemias, chronic leukemias are also classified as lymphocytic or myelogenous (myeloid) based on the type of cells in the bone marrow that become abnormal.

Lymphocytic leukemia begins in cells that become lymphocytes. Lymphocytic leukemias are also sometimes called lymphoid or lymphoblastic leukemias.

Myeloid leukemia begins in early myeloid cells, which become white blood cells (with the exception of lymphocytes), red blood cells or cells that make platelets. Myeloid leukemias are sometimes called myelogenous, myelocytic or myeloblastic leukemias.

Diagnosing Leukemia

People with leukemia often have low numbers of healthy white blood cells, red blood cells and platelets that increase the risk for infection, anemia and bleeding, respectively. To diagnose which form of leukemia you have, your doctor may perform a physical exam and any of the following tests.

  • Blood tests, including a complete blood count (CBC) and a peripheral blood smear.
  • Bone marrow aspiration and biopsy (often done at the same time) to remove bone marrow samples for examination.
  • Lumbar puncture (spinal tap) to see if leukemia cells are in the cerebrospinal fluid, which surrounds the brain and spinal cord.
  • Specialized tests, such as flow cytometry, cytogenetics with fluorescence in situ hybridization (FISH) and polymerase chain reaction (PCR), to help classify the subtype of leukemia. Some doctors are now ordering “sequencing,” where a great number of genes in the leukemic cells’ DNA are studied.
  • Imaging tests, such as computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET) and X-rays, to help determine the extent of disease.