Hematopoietic cells such as red blood cells, white blood cells, and platelets are produced and mature within the bone marrow, a semi-solid tissue that is found within cancellous or “spongy” bone. Bone marrow testing is used to diagnose and monitor bone marrow diseases and peripheral blood abnormalities. Indications for bone marrow examination could include persistent non-regenerative anemia, atypical/immature cells on standard blood smear examination, persistent or unexplained leukocytosis or thrombocytosis, further evaluation for multiple myeloma, or detection of metastasis to bone marrow (lymphoma, carcinoma, etc.).
Complete bone marrow evaluation requires patient history, a CBC collected at the time of bone marrow collection, a bone marrow aspirate, and a bone marrow core biopsy. Important clinical history includes patient signalment (age, intact/altered), medications, possible drug exposure, current illnesses, and imaging results, where available. A concurrent CBC is necessary to determine if findings within the bone marrow are effective (peripheral blood matches cellularity of the bone marrow), or ineffective (peripheral blood does not match cellularity of the bone marrow). Previous CBCs are also very helpful to identify patient trends over time.
A bone marrow aspirate has the advantage of providing individual cell detail, allowing estimation of a myeloid to erythroid (M:E) ratio, and evaluating for dysplasia. Aspirates also facilitate evaluation of infectious agents or neoplastic cells that have invaded bone marrow. A bone marrow core biopsy is superior for cases with significant fibrosis of the bone marrow (“dry tap”), and to evaluate overall bone marrow cellularity, cell distribution, and architecture. Ideally, both a bone marrow aspirate and bone marrow core biopsy are collected as their synergy provides the most information. Depending on clinical concerns, a bone marrow aspirate or bone marrow core biopsy may be performed independently of one another, though both are heavily reliant on comparison with a current CBC for interpretation.
Given the collection procedure and critical nature of the specimen, it is important to collect and prepare bone marrow following specific protocols. This will ensure the best diagnostic results.
Bone marrow collection/preparation is outlined on the Texas A&M Veterinary Medical Diagnostic Laboratory’s (TVMDL) online education library (https://tvmdl.tamu.edu/education-library/bone-marrow-collection-and-preparation/). If there is pathology directly related to the bone (cortical or “compact” bone), such as with osteolytic lesions, boney masses, or fracture sites, a bone aspirate or bone biopsy is the appropriate test for further evaluation as opposed to bone marrow evaluation, which focuses on the cancellous or spongy bone. The TVMDL website also has a page dedicated to bone biopsy.
