Practical Application of Bone Marrow Aspiration and Its Interpretation
Harold Tvedten Sweden
INDICATIONS FOR BONE MARROW EVALUATION
The basic indication for performing a bone marrow evaluation is to answer questions that a routine hematology examination of a blood sample does not answer. One need not take the additional effort to take a bone marrow aspirate and biopsy, if, for example, the blood already clearly indicated an immune mediated hemolytic anemia, a typical inflammatory response, or even a leukemia with clearly diagnostic features in EDTA blood is present.
The most common indications for bone marrow analysis are a deficiency of cells from one, two or all three cell lines. Cytopenias suggest decreased bone marrow function so one should check for various bone marrow diseases. These would be a nonregenerative anemia, thrombocytopenia and/or leukopenia. Additionally, leukemia may be hidden in the marrow—blast cells may be numerous in the bone marrow but few or no blast cells are seen in the blood (aleukemic leukemia). Suggestions of leukemia are dysplastic changes such as megaloblastic rubricytes, hypersegmentation, or rare blast cells in the blood. Hypercalcemia may be caused by lymphosarcoma, which may be located in the bone marrow. Plasma cell myeloma may be suggested by hyperproteinemia or by lytic lesions in the spine.
BONE MARROW PROFILE
A profile of tests allows the most complete evaluation and best conclusions. The profile should include a complete blood count (CBC), bone marrow aspirate, and bone marrow biopsy. The biopsy is often neglected but is especially important when evaluating cytopenia and when one expects low cellularity of the bone marrow. All too often, one has a poorly cellular aspirate of bone marrow where it is uncertain if the low cellularity of the sample reflects low cellularity of the bone marrow or a poor aspirate. A CBC gives excellent quantitative and morphologic information at the time of the bone marrow evaluation. An aspirate allows excellent morphologic evaluation of cells, differential count and myeloid:erythroid (M:E) ratio. A histologic section of a biopsy sample gives the best quantitative information on the cellularity of the marrow and reveals meylofibrosis and architectural patterns. All three together usually provide the best information possible. Neglecting one or two parts often leaves unanswered questions. Performing a test several days after the other may also leave some questions.
ANSWERS FROM BONE MARROW EVALUATION
The usual answers one gets from bone marrow aspirates and biopsy are quantitative, with morphologic information on the cell lines in the bone marrow. Typical answers include hypoplasia, hyperplasia or normal numbers of myeloid, erythroid, megakaryocytic and lymphoid cells. These changes are then interpreted in conjunction with the cell numbers seen in blood. Additional answers/conclusions/diagnoses are based on the maturity and appearance of the cells examined. Increased immaturity usually indicates a hyperplastic/reactive change unless extreme. Presence of over 30% blast cells indicates an acute leukemia. Hemosiderin amount helps diagnose iron deficiency anemia (absence) or anemia of inflammation (increased).
When interpreting a bone marrow report or reading a smear oneself, the first decision should be whether the sample provided adequate cells of sufficient quality to be representative of the bone marrow. Always realize that one is interpreting changes from the norm in a sample and one must first determine if the sample represents the bone marrow adequately. The next factor is predicting the cellularity of the bone marrow. Cellularity is best predicted by particles on an aspirate or by a histologic section. Cellularity of individual cells may be so dense as to also indicate normal to increased cellularity of the bone marrow. A differential count gives the percentage of various cell types, which when compared to the estimate of total cellularity, is used to predict hyperplasia or hypoplasia of a cell line. The M:E ratio is the percentage of myeloid cells divided by the percentage of erythroid cells. The M:E ratio is usually slightly greater than 1:1 in dogs and cats. Lymphocytes and plasma cells usually represent about 5% of the marrow cells.
Morphologic changes seen usually are increasing percentages of immature cells. The presence of atypical morphology indicates a dysplastic process. Dysplasia may be induced by drugs, infectious agents, toxins, nutritional problems, or myelodysplastic or leukemic diseases. Feline leukemia virus (FeLV) causes a wide range of dysplastic changes in cell types or alteration in the number of various cells: from nonregenerative anemia to erythremic myelosis, neutropenia to leukemia, and thrombocytopenia to megakaryocytic leukemia. Thus FeLV and Feline immune deficiency virus (FIV) testing is indicated in any hematologic disorder of cats.
SPECIFIC BONE MARROW DISEASES
There are too many diseases to list all, but a few examples follow. Aplastic pancytopenia (aplastic anemia, fatty marrow) is when a disease process has damaged hematopoietic cells so those cells are absent and increased fat remains in the marrow. Infectious causes include Ehrlichia canis, parvovirus, FeLV, FIV, septicemia and endotoxemia. Toxic causes include estrogen, phenylbutazone, meclofenamic acid, trimethoprim-sulfadiazine, quinidine, griesofulvin, thiascetarsamide, and some chemotherapeutic agents. Often the diagnosis is idiopathic. Myelofibrosis is somewhat similar in that hematopoietic cells have been damaged and fibrosis has replaced normal tissue.
Canine pure red cell aplasia can be immune mediated (primary) or secondary to parvovirus infection or vaccine in dogs or FeLV of FW in cats. The bone marrow has very few erythroid cells remaining, while other cell lines are normal.
Ineffective hematopoiesis may be difficult to understand. The pattern seen is a deficiency of a cell line in blood (e.g., nonregenerative anemia) yet normal to increased numbers of that cell line are found in the bone marrow (e.g., erythroid hyperplasia). Bone marrow cellularity and morphology in cytologic and histologic samples do not directly measure function. Various chemical, infectious, nutritional, and other causes prevent even increased numbers of cells in the marrow from effectively producing cells that are released from the marrow. Ineffective erythropoiesis may be caused by inflammatory diseases or iron deficiency; ineffective neutropoiesis may be caused by drugs (e.g., phenobarbital) or viruses (e.g., FeLV).
Leukemia is a neoplastic process of hematopoietic cells in the bone marrow. The neoplastic cells are often seen in the blood, giving the name “white blood” which one can appreciate by looking at buffy coats in microhematocrit tubes from leukemic patients. The blast cells may be trapped to a great extent in the bone marrow and one may be surprised to see a bone marrow filled with blast cells where the blood had so few blast cells that one could not rule out only the presence of reactive lymphoid cells. The diagnosis of leukemia is a broad topic but most commonly there are over 30% blast cells in the bone marrow to allow an easy basic diagnosis. Specific diagnosis of types of leukemia is useful in countries where treatment of leukemia in dogs and cats is well accepted.
1. Freeman K: Bone marrow evaluation. in Schalm’s Veterinary Hematology 5th ed., Feldman, Zinkl and Jain, Lippincott Wiliams and Wilkins 2000, 29-32.
2. Tvedten HW, Weiss D: The complete blood count and bone marrow examination: General comments and selected techniques. in Small Animal Clinical Diagnosis by Laboratory Methods 3rd ed., Willard, Tvedten and Turnwald, WB Saunders 1999, 11-30.
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