What is being tested?
There are two methods for sampling bone marrow, an aspirate and a biopsy. The bone marrow, a soft fatty tissue found inside the body’s larger bones, is often described as being like a honeycomb – it is a fibrous network filled with a liquid containing cells in various stages of maturation, and “raw materials” such as iron, vitamin B12, and folate that are required for cell production. The bone marrow aspirate provides a liquid sample of cells that can be studied individually, and the biopsy collects a cylindrical core that preserves the marrow’s structure and shows the relationships of bone marrow cells to one another and the overall cellularity (relative amount of marrow cells compared to fat).

Red blood cells (RBCs), platelets, and five different types of white blood cells (WBCs) are produced in the marrow as needed, with the number and type of cell being created at any one time based on the use of cells, loss, and a continual replacement of old cells. For instance, RBCs, which carry oxygen throughout the body, have a lifespan of about 120 days. The marrow paces RBC production at such a rate as to replace old RBCs that are taken out of circulation and maintain a relatively constant number in the blood. The marrow increases the rate of RBC production whenever the patient’s number of RBCs decreases, due to such things as bleeding or hemolysis. The increased rate of production continues until there is a sufficient number of RBCs in the blood stream or until marrow production capacity is reached. If the need approaches this capacity, then an increased number of reticulocytes (immature RBCs) will be released into circulation as the marrow tries to keep up. If the need exceeds capacity then the number of RBCs in the blood stream will decrease and the patient will become increasingly anemic (symptoms of which include pallor, fatigue, and shortness of breath due to decreased oxygen in the blood).

There are a variety of bone marrow disorders, cancers such as leukemia, vitamin and mineral deficiencies, inherited conditions, and diseases such as aplastic anemia that can affect the marrow’s ability to produce an adequate number of each of the different blood cell types and release them into circulation. These diseases may affect the overall number of cells produced, the proportion of cells produced, and/or the function of the cells. Some bone marrow disorders may lead to a deficiency of one or more cell types while others result in excess production of a specific type or of a specific clone of a cell (a single cell that reproduces without regulation).

Leukemia, for example, is a cancer of the blood cells. It results in the excessive production of one WBC type at the expense of other cell types, and can lead to the release of large quantities of abnormal immature WBCs into the blood stream. These WBCs do not fight infection as other WBCs do and they leave the patient more vulnerable to illness. When leukemic WBCs crowd out RBC production, the patient becomes anemic; when they decrease the number of platelets produced, they leave the patient vulnerable to excessive bruising and bleeding. Other conditions, such as vitamin B and folate or iron deficiency, lead to the creation of large or small or abnormally shaped RBCs and result in specific types of anemia. Another disorder, myelofibrosis, is characterized by the overgrowth of the fibrous network found in the marrow, compressing cells and leading to changes in red cell shape and changes in the cell counts..

Bone marrow aspiration and/or biopsy as a “test” includes both the collection of marrow and bone samples and the evaluation of it under the microscope. A pathologist or hematologist microscopically examines slides containing stained smears of marrow samples (bone and/or fluid). The number, size, and shape of each of the cell types present are examined, as are the proportions of mature and immature cells. If leukemia, or another cancer that has spread to the marrow, is present, it can be diagnosed through this examination and the type and severity of the disease (the stage) can be established.

There are other tests that can also be ordered on the marrow sample, depending on what the doctor requests, and what he suspects may be occurring. Additional stains to look for things such as excess iron storage in the marrow, culturing of the marrow to look for viral, bacterial or fungal infections that may be causing a “fever of unknown origin,” and tests for chromosomal abnormalities are among some of the other analyses the physician may request. In the case of leukemia, tests to determine the type of leukemia may be done. These include special stains or determining antigenic markers (immunophenotype) to show just what type of leukemia is present.

How is the sample collected for testing?
The bone marrow aspiration and/or biopsy procedure is performed by a doctor or other trained specialist. Both types of samples may be collected from the hip bone (pelvis), and marrow aspirations may be collected from the sternum (breastbone). In children, samples may also be collected from a vertebrae in the back or from the femur (thigh bone).

The most common collection site is the iliac crest (top ridge) of the hip bone. Before the procedure the patient’s blood pressure, heart rate, and temperature are measured and evaluated to make sure that they are within normal limits and some patients are given a mild sedative. The patient is then asked to lie down on his stomach or side for the collection and his lower body is draped with cloths so that only the area surrounding the site is exposed.

The site is cleaned with an antiseptic such as iodine, and injected with a local anesthetic. When the site has numbed the doctor inserts a needle through the skin and into the bone. For an aspiration, the doctor attaches a syringe to the needle and pulls back on the plunger. This creates vacuum pressure and pulls a small amount of marrow into the syringe. For a bone marrow biopsy, the doctor uses a special needle that allows the collection of a core (a cylindrical sample) of bone and marrow.

Even though the patient’s skin has been numbed, the patient may feel brief but uncomfortable pressure (pulling and/or pushing) sensations during these procedures. After the needle has been withdrawn, a sterile bandage is placed over the site and pressure is applied. The patient is then usually instructed to lie quietly until his blood pressure, heart rate, and temperature are normal, and then to keep the collection site dry and covered for about 48 hours.