Indications for Bone Marrow Aspiration or Biopsy

Bone marrow evaluation provides important diagnostic information on the hematopoietic status of an animal; it is typically performed to evaluate hematopoiesis and to detect evidence of neoplastic or infectious disease (Box 28-1). The complete blood cell count (CBC) is the most common method used to evaluate the hematopoietic system, but a more comprehensive evaluation includes bone marrow aspiration and/or biopsy. Bone marrow aspiration is commonly used to evaluate the cause of cytopenia. Unexplained nonregenerative anemia, neutropenia, thrombocytopenia, and pancytopenia are indications for bone marrow aspiration. In these cases, bone marrow evaluation is used to determine if the cytopenia is more likely a result of lack of production or of consumption or destruction of the cell in question. If an animal’s clinical presentation and other laboratory data provide a reasonable explanation for cytopenia, a decision to perform the procedure may not be justified. For example, if another disease process is present that could explain nonregenerative anemia, such as a chronic inflammatory disease or chronic renal failure, bone marrow aspiration is not indicated in most cases.

Even though complication rates are low, bone marrow aspiration should not be performed indiscriminately; as for all testing, the clinician should consider what question is being asked and if bone marrow evaluation is the appropriate diagnostic test for answering that question. Examination of a peripheral blood film often provides clues as to the necessity of bone marrow evaluation. In general, cytopenia should be persistent and confirmed before bone marrow evaluation is performed. Rechecking a low cell count by drawing a new blood sample, especially if initial results do not fit with the clinical presentation of the animal, is recommended. An ethylenediamine tetraacetic acid (EDTA)-associated pseudothrombocytopenia has been reported in horses,⁵ which can be ruled out by drawing blood into a heparin or sodium citrate tube for platelet counts. A low platelet count can also be artifactual if platelet aggregation occurs during blood collection. The presence of clumped platelets on a peripheral blood film is an indication that platelet aggregation has occurred and that a low platelet count should be viewed with skepticism. Repeated CBCs can also be used to assess persistence of an abnormality. It can take up to 5 days for the bone marrow to respond to acute anemia and for regeneration to be evident in the peripheral blood. Bone marrow aspiration is typically not indicated when there is evidence of regeneration in the peripheral blood. Evidence of erythroid regeneration in the peripheral blood includes an increase in the number of reticulocytes or polychromatophilic cells, basophilic stippling, and macrocytosis. Because the changes typically associated with regenerative anemia are rarely seen in horses, bone marrow aspiration is often the only way to assess erythropoiesis in this species. (See Color Plate 28-1, A-D, for an example of erythroid hyperplasia in a horse.) Evidence of regeneration in granulocytes includes the presence of a left shift and toxic changes.

Cytologic evaluation of a bone marrow aspirate is more commonly performed than core biopsy because results can be attained more quickly and the morphology of the cells is superior, allowing a more accurate assessment of cell types. The disadvantage of an aspirate versus biopsy is that architecture cannot be assessed with an aspirate and it can be impossible to confirm whether a poorly cellular sample is a result of a pathologic process or an unsatisfactory sampling. The core biopsy provides a better assessment of bone marrow cellularity and is necessary for the confirmation of myelofibrosis, generalized bone marrow suppression, or necrosis. (See Color Plate 28-2, A-D, for core biopsy samples of generalized bone marrow hypoplasia from a bovine.) The core biopsy also provides a more accurate assessment of metastatic neoplasia because architecture can be assessed. When collecting a bone marrow sample for cytologic evaluation, some clinicians will also collect a core biopsy to store in formalin in the event that histologic evaluation is later recommended.

If a bone marrow sample is being submitted for evaluation, it is recommended to always submit a concurrent peripheral blood sample for a CBC because interpretation of the bone marrow is dependent on CBC results and changes can occur quickly in the blood.

Hematopoietically active bone marrow is most consistently found in the flat bones (sternum, ribs, pelvis, vertebrae) and proximal ends of long bones (humerus, femur). The most commonly described sites for bone marrow aspiration in large animals are the sternum, ribs, and iliac crest (Figs. 28-1 and 28-2). Which location is chosen may depend on species, age, and temperament of the animal as well as available facilities.

The ventral sternum is the preferred site in adult horses, small ruminants, and camelids (Figs. 28-3 to 28-6). It is one of the preferred sites in cattle (Fig. 28-7). The advantages of the sternum are that it is covered by only a thin layer of bone, it has areas not covered by thick muscles, and samples can be reliably obtained from the site. The disadvantages of the sternum are that it is near vital organs and that the operator is in an awkward position when working on a standing animal. A procedure for sampling the cranial aspect of the sternum has also been described in horses⁶ (see Fig. 28-3). This approach involves going through more muscle.

The ribs are one of the preferred sites for bone marrow aspirates in cattle. The dorsal ends of ribs 8 to 11 can be accessed for sequential aspirations (see Fig. 28-2).⁷ Ribs can also be used in the horse, but the needle slips off the bone more easily. In calves, sheep, and goats, the marrow cavity of the rib is small and more difficult to hit,⁸ and in camelids the rib marrow is less consistent in location than the sternum.⁹ Successful collection from ribs has been described in camels, with the fifth to eighth ribs as the best sites.¹⁰

The iliac crest is the site often used in foals for bone marrow sampling and can also be used in young adult horses up to a couple of years in age¹¹ (Fig. 28-8). Samples can also be obtained from older horses at this site, but the needle must be inserted more deeply, needle placement is more critical, and it becomes difficult to obtain a successful sample in horses more than 9 years old.¹² The wing of the ilium is generally considered too thin for bone marrow aspiration in camelids,⁹ although successful bone marrow aspiration from the iliac crest of a 2-year-old alpaca has been reported¹³ (Fig. 28-9). The iliac crest can also be used in sheep (Fig. 28-10).

Another site informally described for horses is the spinal process of the first lumbar vertebra (Fig. 28-11). It is usually vertical, whereas the adjacent processes tend to be more angled. The tip of the process is wider than the shaft and has a thin layer of bone. The needle is advanced straight down into the marrow cavity, which is narrow, but if the needle is close to correct placement, the walls of the process help guide the needle into the marrow cavity. The advantage of this site is that the operator is not working under the horse. The spinal process is reported to be a poor site to sample in the cow.¹⁴

Approach to the Sternum.

In the horse, bone marrow aspiration is typically performed in the standing animal. Sedation or a twitch may be used if necessary. The preferred site is on the ventral midline in the cleavage between the deep pectoral muscles where a line connecting the points of the elbows would cross¹⁵ (Figs. 28-4 and 28-12). The manubrium of the sternum can also be sampled in horses by finding the cranial end of the sternum and going one hand width below and one hand width laterally from the end (see Fig. 28-3). From this spot the needle is advanced through the muscle in a horizontal plane at a 45-degree angle to the longitudinal axis of the horse.⁶

Sternal bone marrow can be sampled from adult cattle standing in stocks or a squeeze chute. It may be necessary to use techniques to prevent kicking, such as jacking the tail, or sedation. Unruly adult cattle and large calves can be cast and held in lateral recumbency. The upper front leg of the cast animal is held extended along the neck, exposing the sternum, after the other legs are secured with a rope.¹⁶ Calves that weigh less than 300 lb can be placed in dorsal recumbency and secured by assistants or with ropes.⁸ The needle should be placed on the midline and inserted perpendicular to the third or fourth sternebra. The appropriate sternebra is located by palpating the third or fourth rib and following it to the sternum.

For the sternum of a small ruminant to be sampled, the animal is placed in dorsal recumbency with legs secured, or it can be restrained in a sitting position with an assistant standing behind, supporting the animal and holding a foreleg in each hand.⁸ The location of the appropriate site is midline, between the front legs. The second through fourth sternebrae can be used in sheep (see Fig. 28-5). A prominence may be felt between the first and second sternebrae. The appropriate sternebrae can also be located by palpating ribs and following them to their articulations.¹⁷ In sheep, the needle will advance approximately 0.5 cm into the bone (2.5 to 3.8 cm from the skin) before entering the marrow cavity.¹⁷

The sternum of llamas can be sampled with the animal either standing in a chute or lying in left lateral recumbency, usually while sedated¹⁸ (see Fig. 28-6). The sample can be taken by inserting the needle perpendicularly through the callosity on the ventrum of the sternum or with a lateral approach, approximately 3 to 4 cm dorsal to the callosity with the needle directed medially and slightly dorsally to engage bone.⁹ Marrow will be approximately 2.5 cm deep in an adult. The sample is normally more dilute with blood than in other species.⁹

When the ventral sternum is sampled, unless a lateral approach is used, the needle should be placed on the midline, as near to the center of the bone as possible, and advanced perpendicular to the bone. A sudden reduction of resistance may be felt when the marrow cavity is entered, but because the sternal cortex is so thin, especially in horses, there may be no obvious change. Care needs to be taken to not enter the thoracic cavity. Because a change in resistance may not be felt, once the tip of the needle is firmly seated in the bone, an aspiration attempt should be made.

Approach to the Rib.

The vertebral end of the rib more consistently contains marrow than the sternal end of the bone^19,20 (Fig. 28-13). Palpation can be used to find the rib that has the least amount of fascia covering it. In the horse, the eighth to eighteenth ribs can be used, going as high as possible but below the latissimus dorsi and serratus posticus muscles¹⁹ (see Fig. 28-1). In cattle, place the needle approximately 3 inches ventral to the ninth or tenth costovertebral junction¹⁴ (Figs. 28-2 and 28-14). The needle should be inserted perpendicularly at the middle of the rib, midway between the anterior and posterior borders (see Fig. 28-14, A). To help keep the needle from sliding off the bone, a scalpel can be inserted first until it touches the bone to be used as a guide.¹¹