Tests for Childhood Leukemia

The first tests done to look for leukemia are blood tests. The blood samples are often taken from a vein in the arm, but in infants and younger children they may be taken from other veins (such as in the foot or scalp) or from a “fingerstick.”

Blood counts and blood smears are the usual tests done on these samples. A complete blood count (CBC) is done to determine how many blood cells of each type are in the blood. For a blood smear, a small sample of blood is spread on a glass slide and looked at under a microscope. Abnormal numbers of blood cells and changes in the way these cells look may make the doctor suspect leukemia.

When doctors look at the blood smear of children with leukemia, they may see abnormal white blood cells and not enough red blood cells and/or platelets. White blood cells in the blood may be blasts, an early type of blood cell normally found only in the bone marrow. In some cases, a blood smear only shows too few or too many white blood cells. Even though these findings may make a doctor suspect that a child has leukemia, they may not be able to be diagnosed for sure without looking at a sample of bone marrow cells.

Testing of the bone marrow includes a bone marrow aspiration and biopsy. These tests are helpful to diagnose leukemia and identify the leukemia type. A bone marrow aspiration and bone marrow biopsy are usually done at the same time. The samples are usually taken from the hip bones.

Before the tests, the skin over the hip bone is cleaned and numbed by injecting a local anesthetic or applying a numbing cream. In most cases, the child is also given other medicines to make them drowsy or go to sleep during the tests.

For a bone marrow aspiration, a thin, hollow needle is inserted into the bone and a syringe is used to suck out a small amount of liquid bone marrow.

For a bone marrow biopsy, a small piece of bone and marrow is removed by placing a slightly larger needle in the bone. Once the biopsy is done, pressure is applied to the site to help stop any bleeding.

The bone marrow samples are then sent to a lab for other tests and looked at under a microscope.

Bone marrow tests may also be repeated after a leukemia diagnosis to find out if treatment is working.

This test is used to look for cancer cells in the cerebrospinal fluid (CSF), which is the liquid that bathes the brain and spinal cord.

It is important for the child to be still for the procedure. The doctor may do the test at the same time as other tests where the child will be under general anesthesia (where the child is asleep). If the test is done by itself, the doctor may recommend that the child be given something to make them sleep so the spinal tap can be done with minimal difficulty or chance of causing harm. Some older children may be able to have the test done while awake.

After cleaning the skin, the doctor will numb an area in the lower part of the back near the spine. A small, hollow needle is inserted between the bones of the spine to remove some of the fluid, which is then sent to the lab for testing.

In children already diagnosed with leukemia, lumbar punctures might also be used to give chemotherapy drugs into the CSF to try to prevent or treat the spread of leukemia to the spinal cord and brain. This is known as intrathecal chemotherapy.

Leukemia in children is most often diagnosed based on blood or bone marrow results. In rare cases, a large lymph node may be the main symptom of a new leukemia. In these cases, a lymph node biopsy may be done to make a diagnosis.

During this procedure, a surgeon cuts through the skin to remove an entire lymph node (known as an excisional biopsy). If the node is near the skin surface, this is a simple operation.  It is more complex if the node is inside the chest or abdomen. Most often, the child will need general anesthesia (where the child is asleep).

All the samples taken (blood, bone marrow, lymph node tissue, or CSF) are looked at with a microscope. The samples might be exposed to chemical stains (dyes) that can cause color changes in some types of leukemia cells.

Doctors will look at the size, shape, and staining patterns of the blood cells in the samples to describe the type of leukemia.

A key element is whether the cells look mature or immature (look like normal blood cells or not). The most immature cells are called blasts. Having too many blasts, especially in the blood sample, is a common sign of acute leukemia.

An important feature of a bone marrow sample is its cellularity. Normal bone marrow contains a certain number of blood-forming cells and fat cells. Marrow with too many blood-forming cells is said to be hypercellular. If too few blood-forming cells are found, the marrow is called hypocellular.

These tests are used to determine the type of leukemia based on certain proteins in or on the cancer cells (known as immunophenotyping). It is most often done on cells from bone marrow, but it can also be done on cells from the blood, lymph nodes, and other body fluids.

For both flow cytometry and immunohistochemistry, samples of cells are treated with antibodies, which are proteins that stick to other proteins, called antigens, on cells. For immunohistochemistry, the cells are then examined under a microscope to see if the antibodies stuck to them (meaning they have these proteins), while flow cytometry uses a special machine to look for these proteins.

Flow cytometry can also be used to estimate the amount of DNA in the leukemia cells. This is important to know, especially in ALL, because cells with more DNA than normal are often more sensitive to chemotherapy and these leukemias have a better prognosis (outlook).

Flow cytometry can also be used to measure the response to treatment and the existence of minimal residual disease (MRD), or very small amounts of cancer cells, in some types of leukemias. (See Prognostic Factors and Survival Rates for Childhood Leukemias.)

These tests look at the chromosomes (long strands of DNA) inside the cells. Normal human cells have 23 pairs of chromosomes, each of which is a certain size and looks a certain way under the microscope. But in some types of leukemia, the cells have changes in the structure and/or number of chromosomes inside.

For instance, sometimes 2 chromosomes swap some of their DNA, so that part of one chromosome becomes attached to part of a different chromosome. This structural change, called a translocation, can usually be seen under a microscope. Other types of chromosome changes are also possible. Recognizing these changes can help identify the exact type of leukemia and can help determine the right treatment and prognosis (outlook).

Sometimes the leukemia cells have an abnormal number of chromosomes (instead of the usual 46) – they may be missing some chromosomes or have extra copies of some. This can also affect a child’s outlook.

Finding these types of chromosome changes with lab tests can be very helpful in choosing the right treatment and predicting how well the leukemia may respond to certain treatments.

For more information, see Cancer-related Genomic Testing and Genetic Testing.

Cytogenetics

For this test, leukemia cells are grown in a lab dish and the chromosomes are looked at under a microscope. Doctors look at the number and structure of the chromosomes to find any changes.

Cytogenetic testing usually takes a few weeks because the leukemia cells take time to grow before the chromosomes are ready to be looked at.

Not all chromosome changes can be seen under a microscope. Other lab tests can often help detect these changes.

Fluorescent in situ hybridization (FISH)

This is another way to look at chromosomes and genes. It uses special dyes that only attach to specific parts of certain chromosomes. FISH can find most chromosome changes (such as translocations) that are visible under a microscope in standard cytogenetic tests, as well as some changes too small to be seen with routine cytogenetic testing.

FISH can be used to look for specific changes in chromosomes. It can be used on blood or bone marrow samples. It is very accurate and can usually provide results within a couple of days.

Polymerase chain reaction (PCR)

This is a very sensitive test that can also find chromosome and gene changes too small to be seen under a microscope, even if there are very few leukemia cells in a sample. This test can be very useful in looking for small numbers of leukemia cells especially during or after treatment.

Next generation sequencing (NGS)

This test may also be used to look for gene changes in leukemia cells that might not be seen on routine cytogenetic tests. NGS can tell us more about the best treatment for the leukemia and prognosis (outlook).

If leukemia is suspected or has been found, your child’s doctor will probably order more blood tests, called a chemistry panel, to check liver and kidney function and the balance of salts (electrolytes) in the body. They may also check tests to make sure the blood is clotting properly (coagulation tests) and blood tests for substances that may be high when leukemia cells are present (called lactate dehydrogenase, or LDH, and uric acid).

A chest x-ray can help detect an enlarged thymus or lymph nodes in the chest. If the test result is abnormal, a computed tomography (CT) scan of the chest may be done to get a more detailed view.

Chest x-rays can also help look for pneumonia if your child has symptoms of a lung infection like cough, shortness of breath, or low oxygen levels.

CT scans combine many x-ray pictures to make detailed cross-sectional images of the inside of the body. This test is not usually needed for children with leukemia, but it might be done if the doctor suspects the leukemia is growing in lymph nodes in the chest or in organs like the spleen or liver. It is also sometimes used to look at the brain and spinal cord, but an MRI scan may also be used for this. In rare cases where radiation is needed to treat leukemia, CT scans may be used for radiation planning.

PET/CT scan

Some machines combine the CT scan with a positron emission tomography (PET) scan. A PET scan uses a radioactive substance (usually a type of sugar called FDG) to mark active cells in the body. Because cancer cells multiply quickly, they absorb large amounts of the sugar and show up on the PET scan. This can provide more information about any abnormal areas that appear on the CT scan.

An MRI scan, like a CT scan, makes detailed images of soft tissues in the body. It’s most helpful in looking at the brain and spinal cord, so it’s most likely to be done if the doctor has reason to think the leukemia might have spread there (such as if a child has symptoms like headaches, seizures, or vomiting). This test does not use radiation. A contrast material called gadolinium may be injected into a vein before the scan to better see details. In rare cases where radiation is needed to treat leukemia, MRI scans may be used for radiation planning.

Ultrasound uses sound waves and their echoes to look inside the body. It can be used to look at lymph nodes near the skin surface or to look for enlarged organs inside the abdomen such as the kidneys, liver, and spleen. It cannot be used to look at organs or lymph nodes in the chest because the ribs block the sound waves. There is no radiation with an ultrasound test.