Notes supplied by Amanda Boag, MA, VetMB, DACVIM, DACVECC, FHEA, MRCVS.

Bone marrow aspiration is a useful technique that can play an important part in the work-up of several groups of patients. Practically the technique is not challenging, although, as with any skill, a good diagnostic sample is obtained more reliably and quickly with practice. It can be performed in the conscious or lightly sedated canine patient as long as local anaesthesia is used to desensitise the periosteum. Cats and more fractious dogs may require a short general anaesthetic, and if a core biopsy specimen is being taken at the same time anaesthesia is required. In the conscious patient, and with regard to patient restraint the author finds the wing of the ilium to be the easiest site to access; in the anaesthetised patient, and especially in cats, the proximal humerus and trochanteric fossa of the femur are also possible sites. Considering the ease with which samples may be taken, the technique is often under-used or used relatively late in the work-up of a patient.

Patients in which bone marrow aspiration should be considered can be split into three groups:

 Those where there is evidence from peripheral haematology that bone marrow function is impaired or abnormal

 Those where the clinician is suspicious there may be an infiltrative disease of the marrow

 Miscellaneous

Indicated by Peripheral Haematology

The bone marrow contains haemopoeitic stem cells and is the principal site of production of red blood cells, platelets, granulocytes (neutrophils, eosinophils and basophils) and monocytes in the adult animal. Evidence on the haemogram of either inadequate production of any of these cell lines or production of abnormally large quantities or types of these cells is an indication for bone marrow aspiration. Interpretation of the haemogram and decisions on the need for bone marrow aspiration must always be made in light of information about the whole patient and consideration of a prioritised differential diagnosis list.

Red Blood Cells

Patients with a non-regenerative or poorly regenerative anaemia are some of the most common candidates for bone marrow aspiration. An absolute reticulocyte count of less than 60 x 10⁹/l in the face of a moderate to severe anaemia confirms inadequate regeneration. Patients with recent severe haemorrhage may be anaemic with a low reticulocyte count but have a 'preregenerative anaemia'; if this is suspected, then repeat sampling 24–48 hours after the initial sample is recommended prior to making the decision to perform an aspirate. In patients with a mild-moderate non-regenerative anaemia in which 'anaemia of chronic disease' is suspected, bone marrow aspiration may represent over testing; however, if there is any doubt as to the nature of the chronic disease, an aspirate should be considered. Dyserythropoiesis or erythroid neoplasia is much less common; however the finding of abnormal erythroid cells in the circulation is an indication for bone marrow aspiration.

Platelets

Moderate to marked thrombocytopenia most commonly occurs secondary to peripheral immune-mediated destruction of platelets as opposed to megakaryocyte destruction or dysfunction. Unlike with anaemia, where the presence and adequacy of red cell regeneration can be assessed from the peripheral haemogram and used to guide the need for a bone marrow aspirate, this is not generally possible with platelets. Strictly speaking, a bone marrow aspirate is indicated in all cases of severe thrombocytopenia to ensure there are adequate megakaryocytes present in the bone marrow. However, considering that peripheral destruction is so much more common than bone marrow disease as a cause of isolated thrombocytopenia, bone marrow aspiration is often omitted and only performed in patients that are failing to respond to immunosuppressive medication. Concerns over haemorrhage following bone marrow aspiration in thrombocytopenic patients are not warranted. The finding of thrombocytosis should also lead to consideration of a bone marrow aspirate.

Leucocytes

The commonest cause of neutropenia is acute overwhelming inflammation/infection and in this scenario there are usually band neutrophils present in the circulation (a degenerative left shift) and clear clinical evidence of a severe infection/inflammatory focus. Bone marrow aspiration is not generally indicated in these patients. However, in the less common situation where there is a neutropenia of unknown cause, bone marrow aspiration is indicated. Isolated cytopenias of other white cell lines are generally hard to identify and are rarely a reason for bone marrow aspiration. Similarly moderate to marked neutrophilia generally represents severe inflammation and is not an indication for aspiration; however, if any of the white cell lines are present in exceedingly high numbers and/or if there are large numbers of abnormal forms in circulation, bone marrow aspiration is indicated to rule out or aid in the diagnosis or characterisation of myeloid leukaemia.

Pancytopenia

Identification of pancytopenia is an indication for early bone marrow aspiration. Furthermore as both myelodysplasia and myelophthisis are differential diagnoses, it is recommended that a concurrent core biopsy is performed as routine in these patients.

Suspicion of Infiltrative Marrow Disease

In some patients the clinician is suspicious that there may be a disease infiltrating the marrow and yet there are no changes present on the haemogram. Bone marrow aspiration may play a key role in the diagnosis of these patients. Examples include:

 Unexplained hypercalcaemia: Bone marrow aspirate is indicated as part of the work-up of patients with ionised hypercalcaemia if other differentials such as primary hyperparathyroidism and hypoadrenocorticism have been ruled out and if no evidence of neoplasia/granulomatous disease can be found on a thorough physical examination and routine imaging.

 Unexplained hyperproteinaemia/hyperglobulinaemia: Bone marrow aspirate is indicated as part of the work-up of patients with hyperglobulinaemia especially if it is monoclonal.

 Patients with lytic bone lesions: Especially if identified in conjunction with either of the above findings.

Bone marrow aspiration may also be considered as part of the staging of some malignancies. Dependent on the tumour, an oncologist should be consulted for advice as to whether bone marrow aspiration will help with prognostication or treatment planning.

Miscellaneous

Bone marrow aspiration may also be required in the workup of some complex fever of unknown origin cases. Although it is rarely, if ever, one of the first panel of diagnostic tests considered in these patients, it may prove a useful part of the diagnostic work-up when struggling to reach a diagnosis. Occasionally infectious agents (e.g., Mycobacterium spp.) may be identified in marrow and not found elsewhere. Bone marrow aspiration may also be required to evaluate total body iron stores.