Bone Marrow Collection and Examination

What is bone marrow and why is it important?

Bone marrow is the soft material found in the central core of many bones. Bone marrow is vitally important for the production of blood cells, specifically red blood cells, white blood cells, and platelets.

A healthy bone marrow is essential for life.

Why is bone marrow collected and examined?

Bone marrow is commonly collected and examined when abnormalities are found in the circulating blood. The most common abnormality is a persistent shortage of one of the blood cell types. This is a serious situation and may be due to a problem in the bone marrow. Examination of marrow can often provide information about the underlying cause, and may help to predict the outcome.

Bone marrow is also collected and examined to look for certain types of cancer. Some cancers start right in the cells of the bone marrow and other cancers spread to the bone marrow from elsewhere in the body. Cancer that starts in the bone marrow is sometimes called leukemia. Examination of the bone marrow helps to identify the cancer and reveals how seriously the marrow is affected.

Occasionally, bone marrow is collected and examined to investigate other problems such as persistent fever, unexplained weight loss, high blood calcium levels (see handout "Hypercalcemia"), and high serum protein level (see handout "Serum Protein Electrophoresis").

Can marrow be collected from any bone?

Although many bones contain marrow, samples are collected from three main sites in cats and dogs. These are:

• the hip bone
• the top of the thigh bone
• the forearm below the shoulder. This site is used especially in young animals and small sized pets such as cats and toy breed dogs.

How is a bone marrow sample collected?

1. The procedure can be done with just local freezing, but sedation or light general anesthesia is typically used to reduce a pet’s stress or discomfort. The actual collection procedure includes the following steps:

• The skin is shaved, cleaned, and disinfected.
• A sterile scalpel blade is used to cut a small nick in the skin, making a small opening.
• A special bone marrow needle is passed through this opening and pushed firmly through the hard outer layer of the bone and into the marrow.
• A syringe is attached to the needle and a small amount of liquid marrow material is sucked up into the syringe. This is called a bone marrow aspirate.
• The collected material is spread in a thin layer on a glass slide and allowed to dry completely.
• The sample is stained with special dyes and studied under a microscope.

2.  Often, a tiny piece of solid bone marrow is collected at the same time as the marrow aspirate. This solid piece of marrow is a tiny cylinder of tissue about ¼” long, and is called a core biopsy.  It is placed in preservative and sent to a veterinary reference laboratory where it is examined under a microscope.

3. To finish, one or two small stitches are used to close the opening in the skin.

Is there anything else that should be done at the same time?

Your veterinarian may also want to collect a blood sample to do a complete blood count or CBC (see handout "Complete Blood Count" for more information). Doing a CBC and bone marrow examination at the same time provides the most complete picture of what is happening in the marrow at the time of collection.

What is involved in examining bone marrow?

Bone marrow examination is complicated and should be done by a specialist. The samples (bone marrow aspirate, core biopsy, and blood sample) are usually sent to a veterinary reference laboratory where they are examined by a veterinary pathologist who has the training and experience to find the answers. The pathologist’s report typically provides information about the health of the marrow, what types of cells are present, whether abnormal cells are found, and other details that may help to explain the patient’s illness.

Does bone marrow evaluation always provide an answer?

Unfortunately, not always. The expectation is that bone marrow examination will pinpoint the cause of the problem and provide information to help bring about a cure. However, in some cases, bone marrow examination may do nothing more than confirm that there is a problem, without providing any additional information about the cause, likely treatment, or expected outcome.