Bone Marrow Aspirate
Indications:
Bone marrow aspiration is an important diagnostic tool in the
investigation of many haematological disorders. It is particularly
important in the investigation of non-regenerative anaemias, where
differentials include hypoplastic anaemias, neoplastic disease and
immune-mediated destruction of the red cell precursors.
Examination of marrow aspirates is also indicated in the
investigation of persistent neutropenia and thrombocytopenia. In
addition to allowing investigation of marrow failure, such procedures
also facilitate the confirmation and classification of leukaemic
processes.
Collection of bone marrow per se is rarely contraindicated in the
dog and cat but the sedation or general anaesthesia required for
restraint may be a major risk. The tendency for haemorrhage after
collection is very small, even in animals with severe
thrombocytopenia.
Equipment:
- 18 gauge, Jamshidi Disposable Illinois sternal-iliac bone marrow
needle (Vet Instrumentation, 62 Cemetry Road, Sheffield. Tel:
0845 1309596)
- Petri dish
- Microscope slides
- Pipette
- 2-3% EDTA/isotonic saline solution (Cowell & Tyler 1989)
This may be prepared as follows:
A 2 ml EDTA vacutainer tube (lavender top) is filled with isotonic
(0.9%) saline and mixed well. The 2 ml of the resulting EDTA
solution is added to 5-8 ml of sterile isotonic saline in a serum tube
(red top).
Procedure:
Bone marrow is most commonly collected from the iliac crest in
large dogs and the trochanteric fossa of the femur in small dogs and
cats. Other sites include the proximal humerus.
The animal is anaesthetised or sedated and placed in lateral
recumbency (sternal recumbency may be used when sampling from the
iliac crest). The region is clipped and prepared as for surgery. The
technique must be performed aseptically.
In sedated animals, a small amount of local anaesthetic is
infiltrated into the subcutaneous tissue and the periosteum.
The site of collection from the iliac crest is indicated (Figure
1).
Figure 1: The trochanteric fossa and the iliac
crest are the most commonly used sites for bone marrow aspiration.
The approach to the trochanteric fossa of the femur is as follows:
A stab incision is made and the needle advanced medial to the
trochanter into the trochanteric fossa. The femur should be held in
one hand and the needle advanced with the other. The direction of the
needle must be parallel to the shaft of the femur. A moderate amount
of pressure is needed to rotate the needle through the periosteum and
bone cortex into the marrow cavity.
When the needle is correctly seated in the marrow cavity, it is
usually very secure and need not be held.
The stylet is then removed and a 10 ml syringe containing
1 ml EDTA attached. Marrow is aspirated by applying short bursts
of negative pressure. After a small amount of marrow is collected the
pressure is released. The volume collected should not exceed the
volume of anticoagulant. Many animals show signs of pain when the
marrow is aspirated.
The syringe is disconnected and the stylet replaced. The contents
of the syringe are gently expelled into the Petri dish. Marrow
spicules (light opaque flecks which may appear "granular")
must be differentiated from fat globules. A small number of spicules
are removed and placed on a glass slide. Excess blood should be
removed from the surface of the slide using a pipette. Squash
preparations of the slide (using a cover slip or a second microscope
slide as a "spreader") usually gives the best results
(Figure 2). Care should be taken not to apply excessive pressure. The
preparations should be dried rapidly. Further fixation is not required
prior to dispatch. An EDTA blood sample should be collected
immediately before bone marrow aspiration for a full blood count to
assist in interpretation of the marrow findings.
Figure 2: Squash preparations of slide
Problems:
When sampling from the trochanteric fossa it is important to keep
the needle parallel to the femoral shaft.If no marrow is aspirated on
the first tap then the stylet should be replaced, the needle advanced
and the procedure repeated. If no marrow is removed, a different site
may be selected or a core biopsy collected. Any "dry taps"
should be noted on laboratory submission forms.
The aspiration of excessive amounts of material should be
avoided—this usually results in haemodilution of the marrow.
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