How to read your bone marrow report

What is a bone marrow pathology report?

A bone marrow pathology report is a medical document prepared for you by a pathologist. It includes information about what the pathologist saw when they examined the tissue from your bone marrow sample under the microscope. It may also include information about additional tests that were performed on the tissue sample such as flow or immunohistochemistry.

Normal bone marrow

The bone marrow is a specialized type of tissue found at the centre of a bone. Unlike the outside of a bone, which is very hard, the bone marrow is soft. In children, bone marrow can be found at the centre of most bones. However, as adults, the bone marrow is typically found in the ribs, sternum, pelvis (hip bones), and vertebra (backbones).

The bone marrow is where most of your blood cells are produced. These cells include white blood cells (WBC), red blood cells (RBC), and platelets. The normal bone marrow is full of developing blood cells, which are surrounded by connective tissue and fat.

Reasons for examining bone marrow

The best way to see if your bone marrow is healthy and producing normal blood cells is to examine a sample of tissue under the microscope. Your doctor may also request a bone marrow examination if you have symptoms that can be caused by a bone marrow disease or if there is an unexplained change seen in your blood cells.

There are many types of bone marrow diseases, such as leukemia or diseases that affect the production of blood cells or platelets. Your bone marrow can also be involved by lymphoma or plasma cell disorders. Cancers from other parts of the body can also spread to the bone (this is called a metastasis).

Your bone marrow may also be affected by problems that involve your entire body such as deficits in nutrients like iron or vitamin B12, infections, and renal disease. In some cases, your doctor may request a bone marrow examination if abnormal changes in your blood persist despite treatment or if they suspect there may be a separate issue involving the bone marrow.

How is a bone marrow examined?

In order to see what is happening inside your bone marrow, your doctor will remove a small sample of bone marrow. The sample is usually taken from a bone in the hip. Most samples are taken from an area of the hip bone called the posterior iliac crest, because it is large and easy to reach with a needle.

There are two types of tests that can be performed to examine bone marrow. Your doctor may perform one or both types at the same time.

1. Aspirate – An aspirate uses a needle and suction to remove a small amount of bone marrow. The tissue sample is then spread on a slide so it can be examined. Spreading the tissue allows your pathologist to examine the size, shape, and colour of individual cells and count them. Because the sample is spread onto the slide, it is not possible to see how the cells were organized inside the bone marrow.
2. Core needle biopsy – A core needle biopsy also uses a needle to remove a small amount of bone marrow. However, unlike an aspirate, the tissue sample in a core biopsy is a solid piece of tissue that needs to be cut into thin sections before it can be examined under a microscope. A core needle biopsy is better at looking at the organization of the bone marrow and how the cells stick together. In addition, some diseases cause fibrosis in the bone marrow which can make it very difficult to aspirate cells. In this situation, a core biopsy is crucial to examine the bone marrow.

When a pathologist examines a bone marrow tissue sample, they first determine if enough tissue is available to make a diagnosis. They then look for some basic features that allow them to decide if the tissue is normal or abnormal.

Below you will find the basic information pathologists usually look for when examining a bone marrow tissue sample.

Length and quality of the core biopsy

Your pathologist will measure the length of the tissue sample in the core needle biopsy. Because certain types of diseases may only involve part of the bone marrow, small tissue samples may miss the area of disease.

Your pathologist will also comment on the quality of the core needle biopsy. For example, some tissue samples can be crushed during the procedure which will limit the pathologist’s ability to analyze the cells. Your pathologist may recommend repeating the biopsy if the tissue sample is too small or of low quality.

Bone trabeculae

Trabeculae are thin pieces of hard bone that run through the bone marrow. Some types of disease cause the trabeculae to become thicker or thinner than normal. For that reason, your pathologist will describe any trabeculae seen in the biopsy and if they look normal or abnormal.

Quality of the aspirate

A bone marrow aspirate is usually made up of multiple small pieces of tissue and the quality of the aspirate depends on the number of pieces on the slide. Too few pieces may prevent your pathologist from making a diagnosis. The aspirate can also contain a lot of blood (this is called a hemodiluted aspirate) and in some cases may not be trusted to represent the bone marrow.

Your pathologist may recommend repeating the biopsy if there are not enough pieces to examine or if there is too much blood.

Cellularity

The bone marrow is made up of both developing blood cells and fat. The number of cells in the bone marrow relative to fat changes as we age. Younger people normally have more cells in their bone marrow compared to older people who have more fat. Your pathologist will look to see if the number of cells relative to fat in your bone marrow is normal for your age or if there is a change in the total amount of cells.

A change in the total number of cells in your bone marrow can be a sign of cancer, a sign that your bone marrow cells are not functioning normally, or a reaction of the bone marrow to something happening in a different part of your body. Your pathologist will carefully examine the cells to determine the cause and may order additional tests if required.

Types of cells normally seen in the bone marrow

Hematopoietic cells

The developing blood cells in the bone marrow are called hematopoietic cells. There are three main types of hematopoietic cells and each produces a different group of blood cells. All of the blood cells that come from a single type of hematopoietic cell are called a “lineage”.

The three lineages of hematopoietic cells are:

1. Erythroid: This is the lineage that produces red blood cells (RBC). Developing red blood cells are called erythroblasts.
2. Granulocytic: This lineage produces white blood cells such as neutrophils. Immature granulocytes are called myeloblasts.
3. Megakaryocytic: This lineage produces platelets. Platelets come from large cells called megakaryocytes.

Cells from all three lineages are found in the normal, healthy bone marrow. Your pathologist will examine the tissue sample to see if all three lineages are present. They will also look to see if there is any change in the number of cells from anyone lineage or if any of the developing cells show an abnormal shape or size. Pathologists use the word dysplasia to describe abnormal-looking cells.

Blasts

Normal bone marrow shows a mixture of both developing blood cells and mature blood cells ready to be released into the bloodstream. The most immature cells are called blasts, and they should only be seen in very small numbers. If your pathologist sees more developing cells than normal, this is called a “left shift”. If no mature cells are seen this is called a “maturation arrest”. Both a left shift and maturation arrest are abnormal, but a left shift can sometimes be a reaction of your bone marrow to something else happening in your body such as an infection.

Other types of cells

Normal bone marrow also contains small numbers of other cells like lymphocytes and plasma cells. Developing lymphocytes are called lymphoblasts.

Abnormal cells that may be seen in the bone marrow

If your pathologist sees other types of cells not normally found in the bone marrow, they will be described in your report. Cancers that start in another part of the body can spread to the bone. This is called a metastasis. Additional tests may be ordered to determine where the abnormal cells come from. Lymphomas can also involve the bone marrow and your doctor may do a bone marrow examination as part of your staging.

Iron stain

Iron is stored in the bone marrow. Your pathologist can use a special stain for iron on an aspirate slide to determine if there is a normal amount of iron is present in the bone marrow.

The iron stain also helps your pathologist see abnormal cells called ring sideroblasts. These cells can be seen in various circumstances such as exposure to toxins, some medications, copper deficiency, but also in some types of bone marrow diseases such as myelodysplastic syndromes.

Fibrosis

Fibrosis is a word pathologists use to describe the look of a scar under a microscope. Your pathologist may order special stains such as reticulin and Masson Trichrome to look for areas of fibrosis and to determine the severity.

Certain types of diseases can cause fibrosis in your bone marrow. If there is too much fibrosis, this can affect the function of your bone marrow. In certain types of diseases, such as myeloproliferative neoplasms, the amount of fibrosis is related to the severity of the disease.

by Rosemarie Tremblay-LeMay MD MSc FRCPC (updated on January 17, 2022)