Collection of bone marrow may prove valuable in diseases of the blood in which examination of the peripheral blood reveals abnormal cells or cell counts. Conditions such as leukopenia, thrombocytopenia, nonregenerative anemia, agranulocytosis, pancytopenia, leukemias, other bone marrow cancers, and infectious diseases (e.g., histoplasmosis, ehrlichiosis) may be confirmed only by assessment of bone marrow cytology.
Bone marrow in the young animal is cellular and exists in the flat bones (sternum, ribs, pelvic bones, and vertebrae) and in the long bones (humerus and femur). As the animal ages, the cellular content of the marrow decreases, especially in the long bones. In older animals, bone marrow cells still exist in the flat bones; however, in conditions of stress in which new blood cells must be produced in large numbers, primitive cells in the bone marrow of the long bones again become active. Interpretation of the bone marrow smear may be limited by (1) technique used to obtain a bone marrow specimen or (2) the specialized knowledge necessary to interpret bone marrow cells.
Bone marrow aspiration is much underused in clinical practice. The procedure does require some degree of skill if high-quality samples are to be obtained, but the procedure is of low risk to the patient and can be highly valuable in establishing a diagnosis or prognosis.
A short-acting anesthetic occasionally may be needed, but tranquilization together with infusion of local anesthetic is usually sufficient. The site selected for aspiration or biopsy must be shaved and surgically prepared.
Bone marrow aspiration or biopsy is a percutaneous procedure conducted using sterile technique.
The techniques involved include marrow aspiration and bone marrow core biopsy alone or in combination. When aspiration biopsy fails to produce adequate cytology (as in advanced myelofibrosis, neoplasia, or marrow aplasia), a core biopsy of bone marrow is indicated. The bone marrow aspiration needle may be a 16-gauge Rosenthal needle or Illinois needle for a medium-sized dog; an 18-gauge Rosenthal needle for a small dog or a cat; or a Jamshidi (pronounced yam-she-dee) bone marrow biopsy needle, 12 gauge for most adult dogs and 14 gauge for small dogs and cats.
The selection of needles for aspiration biopsy of bone marrow is based on the biopsy site, the depth of the biopsy site, and the density of cortical bone. For bone marrow aspiration, the modified disposable Illinois sternal-iliac bone marrow aspiration needle works well (). For a core biopsy of bone marrow, the Jamshidi bone marrow biopsy-aspiration needle (pediatric, 3.5 inches, 13 gauge) can be used ().
The iliac crest is a commonly used site for marrow aspiration in dogs. Place the animal in lateral recumbency, and prepare the aspiration site. To aspirate marrow, have the needle enter the widest part of the iliac crest and stop the needle just after penetration of the bone. Remove the stylet, place a 12-mL syringe on the needle, and aspirate 0.2 mL of marrow.
Alternatively, the head of the humerus offers easy access to abundant bone marrow. Sedation may be required. With the patient in lateral recumbency and the humerus flexed (the humerus is positioned parallel to the patient’s thorax), instill local anesthetic into the skin and subcutaneous tissues to the level of the head of the humerus. The site of needle insertion is on the most proximal facet of the humoral head (). Direct the needle into the bone toward the elbow and parallel to the humeral shaft. If the needle is positioned too far medially over the humeral head, it is easy to penetrate the joint capsule. Although this is a common occurrence, it does not pose a risk of injury to the patient (assuming the skin was surgically prepared). If joint fluid contaminates the bone marrow aspirate, the sample will be rendered useless.
Contamination of the bone marrow with peripheral blood results if (1) the marrow is not aspirated immediately after the needle enters the marrow cavity or (2) if aspiration time is sustained and a large volume of blood enters the syringe subsequent to the rupture of small blood vessels in the bone marrow.
Perhaps the least desired technique is to obtain marrow from the proximal end of the femur by insertion of the bone marrow needle into the trochanteric fossa. Make a small skin incision over the trochanteric fossa just medial to the summit of the trochanter major. Insert the bone marrow aspiration needle medial to the trochanter major, and place the long axis of the needle parallel to the long axis of the femur.
Once the site has been selected, grasp the needle firmly. Apply steady, slight pressure while alternately rotating the needle tip against the bone (fast, 180-degree clockwise and then counterclockwise movements). Begin with gentle pressure until the needle begins to seat into the bone. Gradually increase the pressure as the needle penetrates into the bone. Insert the bone marrow needle ½ inch into the femoral canal. Remove the stylet from the needle, and aspirate using a 12- or 20-mL syringe that contains a small volume (approximately 0.1 mL) of 4% EDTA. Use significant negative pressure, for example, by withdrawing the plunger of a 12-mL syringe to the 8- or 9-mL mark. Collection of more than 1 mL of bone marrow is unnecessary. Collection of larger volumes may cause greater amounts of peripheral blood to enter the syringe, leading to hemodilution of the sample. Once collection is complete, immediately transfer the aspirate to a watch glass containing approximately 0.25 mL of 4% EDTA. Immediately mix the sample well using the end of the syringe. This is also a good time to remove the bone marrow needle from the patient.
Prepare slides in a manner similar to that used for peripheral blood smears. Preparation of five to eight high-quality slides for submission is customary. Smears are air-dried. Slides may be stained using the same stains used for peripheral blood smears.
Bone marrow biopsy samples, usually obtained as a core, should be placed directly into 10% buffered formalin. It is generally recommended not to roll the core across a microscope slide (exfoliative cytology), as this may significantly disrupt the architecture of the sample and influence histopathologic interpretation.
When submitting a bone marrow aspirate or bone biopsy, a complete blood count (CBC) should also be collected from that patient on the same day. The bone marrow sample and the CBC should be submitted together in order to obtain maximum diagnostic information. A thorough patient history should accompany the submitted samples.
Depending on the volume of bone marrow aspirate obtained, any additional aspirated bone marrow remaining after slides have been made can be mixed with EDTA in the same type of tube used to collect whole blood for a CBC. Tubes may be refrigerated for short periods but never frozen. Prompt shipping and processing of liquid samples of bone marrow is encouraged, as these cells tend to rapidly undergo degeneration.
Bone marrow biopsy core samples, after fixation in 10% buffered formalin, require decalcification before processing and interpretation.
When feasible, a short-acting anesthetic administered to a cat before bone marrow aspiration or biopsy is recommended owing to the difficulty of adequately restraining a cat, even if sedated. The site selected for aspiration or biopsy must be shaved and surgically prepared. Infusion of local anesthetic at the aspiration or biopsy site is appropriate. Supplemental oxygen may be indicated.
Accessible sites for bone marrow sampling and biopsy in the cat are the iliac crest, the head of the humerus, and the proximal end of the femur via the trochanteric fossa. The techniques described for the dog can be used.
Smears of bone marrow are made immediately after aspiration. Extrinsic thromboplastin present in bone marrow tissue will cause the marrow to clot within 30 seconds. Unstained slides should be submitted. A core of bone marrow can be fixed in 10% buffered formalin before submission for decalcification and histologic preparation.
Another method is to aspirate the sample of bone marrow into a syringe containing 0.25 mL of 4% EDTA solution. Expel the aspirate, up to 0.5 mL, into a sterile Petri dish, from which the marrow particles can be isolated easily by aspirating an aliquot with a glass pipette, placing an appropriate volume onto several glass slides, making the appropriate number of smears.
Slides prepared from bone marrow aspirates should be allowed to air-dry and then labeled appropriately. Slides should never be refrigerated, as moisture from condensation can alter or destroy the appearance of individual cells.