Red blood cells, most white blood cells, and platelets are produced in the bone marrow, the soft fatty tissue inside bone cavities. Sometimes a sample of bone marrow must be examined to determine why blood cells are abnormal or why there are too few or too many of a specific kind of blood cell. A doctor can take two different types of bone marrow samples:
- Bone marrow aspirate: Removes fluid and cells by inserting a needle into the bone marrow and sucking out (aspirating) fluid and cells
- Bone marrow core biopsy: Removes an intact piece of bone marrow using a coring device (similar to a large diameter needle)
The bone marrow aspirate shows what cells, normal and abnormal, are present in the bone marrow and provides information about their size, volume, and other characteristics. Special tests, such as cultures for bacteria, fungi, or viruses, chromosomal analysis, and analysis of cell surface proteins can be done on the sample.
The core biopsy removes an entire piece of bone marrow and shows not only what cells are present but also how full the bone marrow is with cells and where the cells are located within the marrow.
Taking a Bone Marrow Sample
Bone marrow samples are usually taken from the hipbone (iliac crest). The person may lie on one side, facing away from the doctor, with the knee of the top leg bent. After disinfecting the skin and numbing the area over the bone with a local anesthetic, the doctor inserts a needle into the bone and withdraws the marrow.
Although the aspirate often provides enough information for a diagnosis to be made, the process of drawing the marrow into the syringe breaks up the fragile bone marrow. As a result, determining the original arrangement of the cells is difficult.
When the exact anatomic relationships of cells must be determined and the structure of the tissues evaluated, the doctor also does a core biopsy. A small core of intact bone marrow is removed with a special bone marrow biopsy needle and sliced into thin sections that are examined under a microscope.
Both types of samples are usually taken from the hipbone (iliac crest), often during a single procedure. Aspirates are rarely taken from the breastbone (sternum). In very young children, bone marrow samples are occasionally taken from one of the bones in the lower leg (tibia).
A bone marrow sampling begins with cleaning, sterilizing, and anesthetizing the skin over the bone. The procedure generally involves a slight jolt of pain, followed by minimal discomfort. The procedure takes a few minutes and causes no lasting damage to the bone.