The Test Sample
What is being tested?
Bone marrow aspiration and biopsy are procedures used to collect and test bone marrow. Bone marrow is a soft fatty tissue found inside the body’s larger bones. It has a honeycomb or sponge-like structure, consisting of a network of fibres that is filled with liquid. The liquid contains 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. A bone marrow aspirate collects a liquid sample of cells that can be studied individually. A biopsy collects a cylindrical core sample that preserves the marrow’s structure. The biopsy shows the relationships of bone marrow cells to one another and the overall cellularity – the ratio of marrow cells to fat and other material present in the sample.
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 produced at any one time being dependent 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 alters RBC production so that it can 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 person’s number of RBCs decreases, due to such things as bleeding or haemolysis. 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 anaemic, with symptoms such as pale skin (pallor), tiredness, and difficulty breathing due to reduced oxygen in the blood.
A variety of bone marrow diseases, cancers such as leukaemia, vitamin and mineral deficiencies, inherited conditions, and diseases such as aplastic anaemia 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 total number of cells produced, the proportion of different 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.
Leukaemia, for example, is a cancer of the blood cells. It results in the excessive production of one (or a few) WBC types 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 may not fight infection as other WBCs do and leave the patient more vulnerable to infections. When leukaemic WBCs exceed RBC production in the bone marrow, the patient becomes anaemic; when they decrease the number of platelets produced, they leave the patient vulnerable to excessive bruising and bleeding. Other conditions, such as vitamin B or folate deficiency lead to anaemia with large red blood cells and sometimes changes in white blood cells and platelets. Iron deficiency causes anaemia with small red blood cells, sometimes abnormally shaped.. Another disorder, myelofibrosis, is characterised by the overgrowth of fibres 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 samples and the evaluation of it under the microscope. Specialists, a pathologist and/or haematologist, examine under a microscope glass slides containing stained smears of marrow samples – the fibrous network and fluid from a biopsy or the fluid from an aspiration. The number, size, and shape of each of the cell types present are examined, as are the proportions of mature and immature cells. If leukaemia is present, or another cancer has spread into the marrow, it can be diagnosed through this examination, and the type and severity of the disease (the stage) can be worked out.
Depending on what the doctor thinks may be wrong, additional tests can be done on the marrow sample. In the case of leukaemia, tests to work out the type of leukaemia may be done. These include special stains or the determination of antigenic markers (immunophenotype or “cell markers”) to show just what type of leukaemia is present. Special stains may also be used to look for things such as how much iron is stored in the marrow, and tests are sometimes used to detect chromosomal or genetic abnormalities. Rarely, marrow may be used to look for infections that can cause a “fever of unknown origin”.
How is the sample collected for testing?
The bone marrow aspiration and/or biopsy procedure is done by a doctor or other trained specialist. Both types of samples may be collected from the pelvis. Marrow aspirations are sometimes collected from the sternum (breastbone) of adults. In infants, samples may be collected from the tibia (shin bone).
The most common collection site is the iliac crest (top ridge) of the back of the pelvis. Before the procedure, patients are given a local anaesthetic (lignocaine) and/or nitrous oxide gas. A mild sedative may be prescribed depending on the local policy of the hospital.
The patient is usually asked to lie down on their side curled up for the procedure. The site is cleaned with an antiseptic and injected with a local anaesthetic. When the site is numb, 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, pulling a small amount of marrow into the syringe – usually less than a teaspoonful. For a bone marrow biopsy, the doctor uses a special needle that allows the collection of a core (a cylindrical sample) of marrow.
Even though the patient’s skin has been numbed, it is usual to 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 usually instructed to lie quietly for a few minutes, and then 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.