Overview
Bone marrow evaluation is indicated when certain peripheral blood abnormalities are detected. The most common indications are persistent neutropenia, unexplained thrombocytopenia, poorly regenerative anemia, or a combination thereof. Examples of proliferative abnormalities in which bone marrow examination may be indicated include persistent thrombocytosis or leukocytosis, abnormal morphologic characteristics of blood cells, or the unexplained presence of immature cells in blood (e.g., nucleated erythroid cells in the absence of polychromasia or neutrophilic left shift in the absence of inflammation). Bone marrow is sometimes examined to stage neoplastic conditions (lymphomas and mast cell tumors), estimate the adequacy of body iron stores, and evaluate lytic bone lesions. Bone marrow examination can also be useful in determining the cause of hyperproteinemia secondary to multiple myeloma, lymphoma, leishmaniasis, and systemic fungal diseases. It may also reveal the cause of a hypercalcemia when associated with lymphoid neoplasms, multiple myeloma, or neoplasms that have metastasized to bone.
Aspiration Versus Core Biopsy Bone marrow aspiration biopsy is done more frequently than bone marrow core biopsy in veterinary patients because it is easier, faster, and less expensive to perform. Bone marrow core biopsy requires special needles that cut a solid core of material that is then placed in fixative, decalcified, embedded, sectioned, stained, and examined microscopically by a pathologist. Core biopsy sections are more accurate for evaluating marrow cellularity and examining for metastatic neoplasia than are aspirate smears, but the morphologic characteristics of cells are more difficult to assess. Bone marrow core biopsy must be done when repeated "dry taps" occur during attempts at aspiration, and it is the only method of definitive diagnosis of myelofibrosis. Core biopsy may also be needed to confirm the presence of inflammation within the marrow.
Bone marrow aspiration biopsy has few contraindications. Postbiopsy hemorrhage may occur in patients with hemostatic diatheses but is rare. It may also occur in animals with monoclonal hyperglobulinemia, but this is easily controlled by placing a suture in the skin incision and applying pressure over the biopsy site. Postbiopsy infection is also a potential complication but is very unlikely if proper techniques are used.
This presentation addresses collection and preparation of smears of bone marrow aspirate. Sterile needles, syringes, and gloves are always used, but the aspiration site does not have to be draped. If general anesthesia is required for other procedures, bone marrow aspiration may be scheduled at the same time to minimize the stress on the animal. Most animals are tranquilized before bone marrow biopsy.
Biopsy Sites
Active marrow is present in the flat bones (vertebrae, sternum, ribs, and pelvis) and proximal ends of the humerus and femur in adults.
Iliac Crest & Femur The iliac crest is often used as a site for marrow biopsy in dogs and cats. The biopsy needle is positioned so that it enters the greatest prominence of the iliac crest parallel to the long axis of the wing of the ilium. The wing of the ilium may also be aspirated at the central depression of the wing caudal and ventral to the iliac crest (Figure A, top left).
For small cats and toy breeds of dogs, in which the ilium is especially thin, the marrow may be aspirated from the head of the proximal femur by way of the trochanteric fossa.
Humerus Aspiration of marrow from the anterior side of the proximal end of the humerus is another popular site, especially in obese patients. The greater tubercle is palpated, and the needle is inserted into the flat area on the craniolateral surface of the proximal humerus distal to the tubercle (Figure B, bottom left).
Sternum In large dogs, biopsy can be done on the third, fourth, or fifth sternebra. Biopsy of the sternum is associated with risk for inadvertent penetration of the thorax and damage to structures in the thoracic cavity. A short biopsy needle (preferably with an adjustable guard) should be used, and care should be taken to remain in the center of these bones to minimize the risk for pneumothorax, uncontrolled hemorrhage, and cardiac laceration. Biopsy samples may be taken from other sites if specific lesions are identified using diagnostic imaging.
How to Perform Aspiration Biopsy
Getting Started It is good practice to set out the equipment close to where the procedure will be done. See below for a list of necessary equipment.