Why it matters
When hematologic changes are evident and a blood smear alone does not provide enough information to make a diagnosis, a complete bone marrow evaluation is often indicated.
What they did
Specialists at the Department of Comparative Pathobiology at Purdue University's School of Veterinary Medicine have put together a clear and comprehensive overview of the indications, procedures, and findings of bone marrow evaluation.
What they found
Bone marrow aspiration is inexpensive and relatively simple to perform, and the results are rapid. However, while it gives outstanding information about cell morphology, only histologic examination of a bone marrow biopsy sample can provide information about both the cells and their relationship to the structural components, fat, and connective tissue. A complete bone marrow evaluation should consist of a complete blood cell count (CBC), bone marrow aspiration, and bone marrow biopsy.
Indications for performing a complete bone marrow evaluation include (when there is no identifiable underlying cause and a nonmarrow source is unlikely) cytopenia of one or more of the hematopoietic cell lines, hematocytosis, dysplastic cells, or blast cells seen on peripheral blood examination. Patients receiving chemotherapy may be monitored via bone marrow evaluation, and in cases of suspected iron sequestration (not useful in cats since they do not accumulate iron stores), a complete bone marrow evaluation is also indicated. Recent research cited in the article suggests that complete bone marrow evaluation is useful in patients that are presented for evaluation of fever of unknown origin as well.
Aspirates and biopsy samples should be collected within 24 hours of each other and acquired from the same location sequentially during the procedure. Performing the aspiration first and obtaining the core biopsy sample second a small distance from the original site will avoid sampling damaged tissue. Sedation along with local anesthesia, pain medication, or both are all that is usually needed to obtain these biopsy samples. When obtaining samples from deceased patients, the biopsies must be performed within 30 minutes of death.
The report outlines in detail the patient preparation and approach required to successfully perform these procedures. Selection of the biopsy site depends on the animal's size and age and the clinician's preference. The guidelines offer descriptions of techniques for various sites.
The proper storage, processing, and staining of bone marrow films are also covered. Refrigeration (not freezing) can be used to slow deterioration. The review provides guidelines for assessing bone marrow aspirates as well as quick guide tables to assessing the cytologic findings, discussion of differences between cell counts in dogs and cats, and a checklist for histologic evaluation.
The final bone marrow examination report should contain the results of a CBC, blood film, and marrow cytology and histology. In many patients, the reticulocyte count and any blood chemistry abnormalities are also reported. Most of the report should consist of information on the cellularity, the M:E ratio (the ratio of myeloid to erythroid precursors in bone marrow), and an assessment of iron stores (in dogs), incorporating any abnormal findings. A complete bone marrow evaluation does not often result in a definitive diagnosis, but it may guide the clinician to appropriate diagnostic testing.
Raskin RE, Messick JB. Bone marrow cytologic and histologic biopsies: indications, technique, and evaluation. Veterinary Clinics of North America: Small Animal Practice 2012;42(1):23-42.