Bone marrow aspirate evaluation may not be in your primary diagnostic toolbox, but it is essential for diagnosing many disorders. Challenges in obtaining high-quality, diagnostic samples may deter many practitioners from confidently using this technique when the need arises.
In this article, we provide direction in determining when a bone marrow aspirate is indispensable to your workup and outline the techniques for obtaining diagnostic samples. Guidelines for interpretation of bone marrow aspirates are available from other sources.1-10 However, we describe the general cytologic criteria of good-quality aspirates. We also discuss pathologic conditions that render aspirates difficult to collect and provide diagnostic alternatives, such as bone marrow core biopsies.
INDICATIONS FOR BONE MARROW SAMPLING
The primary indication for bone marrow aspiration is abnormal complete blood count results. Once extra-marrow causes have been excluded, abnormalities, such as nonregenerative anemia, neutropenia, thrombocytopenia, or the presence of immature blast cells or mature cells with atypical morphology, should prompt bone marrow evaluation. Findings from the physical examination (fever of unknown origin), radiography (bony lysis), and the serum chemistry profile (hyperproteinemia, hypercalcemia) may also be indications for examining bone marrow. Pathogenic organisms found in the marrow cavity that may lead to fever or hyperglobulinemia include Leishmania, Cytauxzoon, and Histoplasma species. The definitive diagnosis of some neoplasms, such as plasma cell myeloma, and the staging of certain malignancies, such as lymphoma and mast cell tumors, require marrow examination. Finally, bone marrow aspiration is indicated to evaluate marrow iron stores.
There are few contraindications to bone marrow aspiration. Thrombocytopenia is not a contraindication, and, in fact, bone marrow aspiration is often performed to rule out decreased production as the cause of low platelet numbers. However, care should be taken to minimize trauma. Severe bleeding disorders characterized by prolonged coagulation times are more of a concern, and marrow aspiration should be delayed until the coagulopathy is controlled. Risk of bone fracture is ordinarily low, but a fracture may occur if the biopsy needle is inappropriate for the patient's size or if cortical integrity is compromised by disease.
BONE MARROW ASPIRATE VS. BONE MARROW CORE BIOPSY
The advantage of bone marrow aspiration over bone marrow core biopsy is the ability to distinguish individual cellular morphology, which is important in identifying cell lineages; characterizing morphologic abnormalities, especially subtle ones, within a lineage; and calculating ratios, such as myeloid-to-erythroid and maturation ratios. The extensive processing required to prepare bone core biopsy samples alters cellular morphology, rendering individual cellular identification more difficult. On the other hand, bone marrow core biopsies are required to evaluate overall marrow cellularity, architectural relationships, and the presence of myelofibrosis or necrosis. Appreciation of marrow architecture, which maintains spatial relationships, facilitates the diagnosis of osteomyelitis and metastatic disease within the marrow space. Accordingly, bone marrow aspirates and core biopsies are complementary techniques, and both may be required for full and accurate interpretation of the pathophysiologic process.
Selecting appropriate equipment is necessary for acquiring high-quality samples (see boxed text titled "Supply checklist for bone marrow collection"). Bone marrow aspirates are usually obtained with a Rosenthal needle or an Illinois sternal-iliac aspiration needle, whereas bone marrow core biopsies are performed with a Jamshidi biopsy needle. Newer biopsy needles, such as the Goldenberg Snarecoil needle (Ranfac), offer several advantages, but we have no experience with them. Several companies carry both reusable (autoclavable) and disposable instruments, including Becton, Dickinson; Cardinal Health; Dyna Medical; and Jorgensen Laboratories. Disposable needles with plastic handles can be reused after gas sterilization but should be disposed of when they become dull or bent.
Supply checklist for bone marrow collection
Bone marrow aspiration and biopsy needles come in several sizes determined by needle gauge. At least two gauges should be available. A 15- or 16-ga aspiration needle is appropriate for most patients; an 18-ga needle should be available for cats and small dogs. A 1-in needle may be sufficient for many animals, but a 1 5/16-in needle is required for large-breed dogs. Thirteen-gauge Jamshidi biopsy needles are appropriate for most patients; 8-ga needles can be used in larger dogs.