Leukemia is cancer of bone marrow stem cells – immature cells that normally would develop into either red blood cells, white blood cells or platelets.

When someone has leukemia, the bone marrow produces abnormal blood cells that do not die when they become old, as normal, healthy cells do. These abnormal cells build up in the bone marrow and prevent normal blood cells from developing. Almost all leukemia begins in the bone marrow, and it usually spreads to the blood. Leukemia may also be found in other tissues, such as lymph nodes and the spleen.

Diagnosing leukemia in its earliest stages can improve a patient’s prognosis, so it is important to be tested as soon as possible if leukemia is suspected.

Just as there are several forms of blood cells, there are different types of leukemia.

Leukemia is classified as either acute or chronic, according to which type of blood cell is improperly reproduced.

“Acute leukemia is made of blasts, immature cells, while chronic leukemia is made of more mature cells,” says Yale Medicine pathologist Alexa Siddon, MD. “Chronic leukemias tend to proliferate slower than acute leukemias, just because the blasts multiply very quickly. People with acute leukemia will become symptomatic more rapidly, in general.”

Leukemia that affects myeloid cells, which would normally develop into red blood cells, platelets or granulocytes (a specific type of white blood cell), is called myeloid leukemia or myelogenous or myeloblastic leukemia.

Leukemia that affects lymphoid cells, which would develop into lymphocytes, is called lymphoid leukemia or lymphoblastic or lymphocytic leukemia.

"A patient may be tested for leukemia if he or she has unexplained weight loss, night sweats or fatigue, or if he or she bruises or bleeds easily," Dr. Siddon says. "Sometimes routine blood work shows an unexplained elevated number of white blood cells."

People with chronic leukemia are more likely than those with acute forms to be asymptomatic before they are diagnosed.

People with leukemia may feel tired because they don't have the normal amount of red blood cells, which carry oxygen around the body. They may also bruise or bleed easily because they don't have enough platelets to facilitate blood clotting, or they may have frequent infections because they aren’t producing normal white blood cells, which mount an immune response.

Other symptoms, such as night sweats, are more general and less specific to leukemia, but they can be a sign that the body is directing its resources to fighting an illness.

People who were exposed to specific chemotherapy agents for treatment of previous cancers may be at risk for developing certain forms of leukemia.

“Also, there are some inherited risk factors,” Dr. Siddon says. “For example, chronic lymphocytic leukemia can run in families.”

A diagnosis of leukemia is usually made by analyzing a patient’s blood sample through a complete blood count (CBC) or microscopic evaluation of the blood, or by using flow cytometry.

"Flow cytometry is a very sensitive technology that uses a laser beam to identify the very specific types of cells found in a sample," Dr. Siddon says. "We can often diagnose the particular type of leukemia using flow cytometry because the technology allows us to differentiate between lymphoid or myeloblastic leukemia, which is important because they are treated with different chemotherapy regimens."

Flow cytometry that is done using blood is relatively noninvasive because it uses blood drawn from the arm. But in some patients, the leukemia stays in the bone marrow and is not found in blood. In that case, flow cytometry may be performed on bone marrow (usually taken from the hip bone) or other tissue, such as a piece of a lymph node.

Bone marrow testing is usually done in conjunction with blood tests because it adds information that can aid with diagnosis and treatment decisions.

"For a bone marrow biopsy, a core of bone is removed with a long needle and the bone marrow is examined under a microscope. At the same time, we also remove the liquid portion of the bone marrow,” Dr. Siddon says. “From the liquid portion, we can run flow cytometry and molecular tests."

Molecular testing evaluates the cytogenetics (the chromosomes within the cell) to look for chromosomal variations or abnormalities in the cells.

"There are some specific cytogenetic abnormalities that are very specific to leukemia, and the presence or absence of these abnormalities can help us decide if a patient should have a particular chemotherapy regimen or possibly a bone marrow transplant," Dr. Siddon says.

In the age of precision medicine, it's important to know whether a patient's leukemia has certain cytogenetic features so doctors can determine the absolute best treatment for the patient.

"We have known for some years about some important cytogenetic abnormalities, but we are now starting to learn just how important some additional abnormalities are," Dr. Siddon.

"We're on the forefront of medicine, when some of these cytogenetic findings make patients eligible for clinical trials for emerging treatments that Yale Medicine is offering, says Dr. Siddon. "We're doing a lot more cytogenetic testing now that we know that these variations have something to do with the patient's disease and how well they may respond to treatment."