The Test Sample
What is being tested?
Bone marrow is the soft and sponge-like tissue found inside the body's larger bones that produces blood cells. Bone marrow aspiration and biopsy are procedures used to collect and evaluate bone marrow cells and structure.
Bone marrow has a honeycomb or sponge-like structure, consisting of a fibrous network that is filled with liquid. The liquid portion contains blood (hematopoetic) stem cells, blood cells in various stages of maturation, and "raw materials" such as iron, vitamin B12, and folate that are required for cell production.
The primary function of the bone marrow is to produce red blood cells (RBCs), platelets, and white blood cells (WBCs). The number and type of cell being produced at any one time is based on the cell function, blood loss, and a normal, continual replacement of old cells.
A bone marrow aspiration collects a sample of the fluid that contains cells so that they can be examined under a microscope and/or evaluated with other tests. A biopsy collects a cylindrical core sample that preserves the marrow's structure. The biopsy sample is evaluated to determine the relationships of bone marrow cells to one another and the overall cellularity – the relative ratio of marrow cells to fat and other constituents present in the sample.
Red Blood Cells (RBCs)
Red blood cells, also called erythrocytes, transport oxygen throughout the body. RBCs typically make up about 40-45% of the blood volume and usually survive in circulation for about 120 days. The marrow produces RBCs at a rate that replaces old RBCs that age and degrade or are lost through bleeding, striving to maintain a relatively constant number of RBCs in the blood.
White Blood Cells (WBCs)
There are five different types of white blood cells: lymphocytes, neutrophils, eosinophils, basophils, and monocytes. Each plays a different role in protecting the body from infection.
Platelets, also called thrombocytes, are cytoplasmic fragments of very big cells seen in bone marrow called megakaryocytes and are essential for normal blood clotting.
In the bone marrow, a stem cell undergoes development and differentiation to become one of these different types of blood cells. Those that differentiate into lymphoid cells subsequently develop into lymphocytes. Other precursors further develop and differentiate into granulocytes (neutrophils, eosinophils, basophils), monocytes, platelets, or red blood cells (erythrocytes).
The cells are released from the bone marrow into circulation when they are fully mature or near full maturity. Thus, the population of cells within bone marrow will typically include cells in various stages of development, from very immature to almost fully mature.
Bone marrow aspiration and/or biopsy as "tests" include both the collection of marrow samples and the evaluation of the cells and structure under the microscope.
- A specialist microscopically examines slides of stained smears of the fluid from an aspiration. The cells are evaluated according to number, type, maturity, appearance, etc. and compared to those in the blood using results from a complete blood count (CBC) and blood smear.
- The specialist also evaluates the structure and cellularity of the marrow sample from a biopsy.
For example, if leukemia is present, or another cancer has spread into the marrow, it can be diagnosed through these examinations, and the type and severity of the disease (the stage) can be established.
Depending on what condition(s) a health practitioner suspects or is investigating, a number of other tests may be performed on the marrow sample. A few examples include:
- In the case of leukemia, tests to determine the type of leukemia may be done. These include special stains or the determination of antigenic markers (for example, immunophenotyping by flow cytometry) to provide information on the type of leukemia present.
- Special stains may also be used to evaluate iron storage in the marrow and to determine whether an abnormal erythroid precursor with iron particles surrounding its nucleus (so-called ringed sideroblasts) is present.
- A chromosome analysis and/or fluorescence in-situ hybridization (FISH) analysis may be ordered to detect chromosomal abnormalities in the case of leukemia, myelodysplasia, lymphoma, or myeloma.
- Molecular tests such as BCR-ABL1 or JAK2 mutation may be performed on a sample of bone marrow to help establish a diagnosis.
- Bone marrow may be cultured to look for viral, bacterial, or fungal infections that can cause a "fever of unknown origin." Certain bacteria and fungi can also be detected by special stains.
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 (iliac crest). Marrow aspirations are sometimes collected from the sternum (breastbone) of adults. In infants, samples may be collected from the tibia (shin bone). Sometimes, a bilateral iliac procedure is performed.
The most common collection site is the iliac crest (top ridge) of the back 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 or her stomach or side for the collection, and the person's lower body is draped with cloths so that only the area surrounding the site is exposed.
The site is cleaned with an antiseptic and injected with a local anesthetic. When the site has numbed, the health practitioner inserts a needle through the skin and into the bone. For an aspiration, a syringe is attached to the needle to collect a small amount of marrow. For a bone marrow biopsy, a special needle is used that allows the collection of a solid core (a cylindrical sample) of marrow.
Even though the patient's skin has been numbed, the patient may feel brief but uncomfortable pulling and/or pushing pressure 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 asked to keep the collection site dry and covered for about 48 hours.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.