Today we’re looking at the first of our medical procedures – the bone marrow aspirate and core biopsy. These are commonly performed in patients with blood cell disorders, such as anaemia, thrombocytopenia or leukopenia. The veterinary nurse plays an important role in this procedure, and in caring for the patient afterwards.

What is a bone marrow biopsy?

A bone marrow aspirate/core biopsy is performed to collect bone marrow samples for analysis. Usually, we aspirate a bone marrow sample (which is similar in appearance to blood, and is liquid) for cytology (+/- additional tests e.g. infectious diseases), and remove a core sample of bone for histology.

Why is a bone marrow biopsy performed?

Performing a bone marrow aspirate/biopsy provides us with vital information about the production of the patient’s blood cells. Remember in our haematology posts we discussed that anaemia, thrombocytopenia and leukopenias can be caused by destruction, consumption/sequestration or a lack of production? Well as all of these cells are produced from precursor cells within the bone marrow, a bone marrow aspirate/biopsy is required to confirm a lack of cell production.

What equipment is required?

You will need the following equipment for a bone marrow aspirate/biopsy:

• Anaesthesia equipment

• Chlorhexidine skin scrub solution

• Sterile chlorhexidine-in-alcohol final prep applicator

• Local anaesthetic agent (e.g. lidocaine or bupivacaine)

• 1 or 2ml syringe x 1 (depending on local anaesthetic dose)

• 23g x 5/8" or 1" needle (depending on patient size)

• Sterile gloves for clinician

• Sterile fenestrated drape

• Scalpel blade #15

• Small surgical kit

• Jamshidi needle of appropriate size (8-11g: giant-large dogs, 13g: medium dogs, 15-16g: small dogs-cats)

• CPDA or ACD anticoagulant (e.g. from a blood collection bag)

• Needle-free bag spike

• 20ml and 10ml syringes x 2-3 of each

• Microscope slides (positioned near-vertically against a tray or box)

• Slide holders x 3

• EDTA tubes x 2 (one for marrow, one for blood)

• 2ml syringe and 23g x 5/8" needle

• Small formalin histology pot

• Pencil (for labelling slides)

• Permanent marker (for labelling sample tubes)

• Non-absorbable suture material with a swaged-on cutting needle

• Adhesive sterile dressing

Which sites are used?

Three different sites are used most commonly for bone marrow aspiration/biopsy. These are the iliac crest (wing of the ilium), the proximal humerus (greater tubercle), or the proximal femur (trochanteric fossa). Generally, any of the three sites can be used for dogs; with cats, the pelvis is not used.

In my personal experience, either the humerus or ilium is used most commonly in dogs, and the humerus used most commonly in cats.

How is the patient prepared?

Following anaesthetic induction, the patient is positioned appropriately for the sampling site used. Where the iliac crest is to be sampled, the patient is positioned in sternal recumbency with the legs extended forwards underneath the body; this makes the iliac wings more obvious and accessible. For femoral and humoral sampling sites, the patient should be positioned in lateral recumbency with the limb to be sampled facing upwards.

The veterinary nurse should clip a wide area over the sampling site (5-10 cm x 5-10cm, depending on the size of the patient). After clipping, the site is prepared with a pre-surgical scrub, as with any other surgical procedure. In order to provide pre-emptive and multimodal analgesia, local anaesthetic can be infiltrated at the sampling site, down to the periosteum and in the subcutaneous tissues. Generally, half the patient’s calculated local anaesthetic dose is used at the site; that way, in the event of a poor bone marrow yield, an additional site can be used and analgised without concern for overdose.

Following local anaesthetic administration (if directed by the veterinary surgeon), a final skin scrub is performed.

How is the procedure performed?

The clinician makes a small stab incision at the site, before inserting the Jamshidi needle. The needle is ‘drilled’ into place using steady, firm pressure and repeated clockwise and anticlockwise rotation. The veterinary nurse assisting should hold the site steady at this point, and push back against the pressure of the needle being inserted, in order to provide resistance for the clinician to work against. This can take quite a bit of force from the VN! If the femur or humerus is used, rotating the limb slightly medially can make needle insertion easier for the clinician, as the insertion point is easier to access.

After the needle is inserted into the bone, the clinician often asks the nurse to move the leg (if the humerus or femur is being sampled); if the needle is in the correct location, the needle should move in line with the limb. The central stylet of the needle is then removed, and a syringe pre-flushed with anticoagulant is attached. Multiple short, sharp bursts of negative pressure are placed on the needle until the marrow is seen entering the syringe.

After bone marrow is seen, the veterinary nurse needs to act quickly to prepare the cytology samples. This is vitally important because bone marrow clots rapidly – within 10-20 seconds! The samples are prepared using this procedure:

1. Place a drop of bone marrow at the top of each microscope slide ( these are positioned at an angle, against a tray or box – the drop should be placed near the frosted section of the slide)

2. Allow the excess blood to run down the slides. Spicules of bone marrow should remain near the top of the slide.

3. Using the squash preparation technique, smear the bone marrow spicules across the slide. To do this, take a clean microscope slide at 90 degrees to the bone marrow preparation, and slide this across the bone marrow slide using even force and pressure.

4. Air-dry the slides to prevent drying artefact and label with a pencil.

The slide preparation procedure should be completed within the 10-20 seconds it takes for the marrow to clot. After the slides have been made, place any spare marrow within the syringe into an EDTA tube so that additional testing can be performed where necessary. Agitate this thoroughly to prevent the marrow clotting.

Following the marrow aspiration, a bone ‘core’ biopsy is obtained for histology. This is performed by advancing the Jamshidi needle by 1-2cm further into the bone marrow cavity, and vigorously rotating the needle to detach the sample from the surrounding bone. A blunt probe or stylet (suppled with the Jamshidi needle) is used to remove the sample from the needle; the nurse can then use a hypodermic needle to gently guide the sample into a formalin pot.

After the core sample is obtained, the skin incision is closed using 1-2 skin sutures and a blood sample is collected for paired haematology analysis (to compare the cells in the blood and bone marrow).

Post-procedure care

After the procedure is complete, the veterinary nurse should clean the surgical site as necessary and apply a small, sterile adhesive dressing to protect the incision from hospital-acquired infection or self-trauma. Following anaesthetic recovery, the nurse should consider the following areas in addition to general nursing care: 

• Pain: Though considered a minor procedure, bone marrow aspirates can be painful. These patients should have regular pain assessments, and analgesia should be provided at an appropriate dosage and frequency, depending on these assessments.

• Cardiovascular stability: Bone marrow aspirates/biopsies are often performed to investigate anaemia. These patients may have significant anaemia requiring blood product transfusion to maintain perfusion to tissues; if this is the case, cardiovascular compromise and a reduced oxygen-carrying capacity may be present. Blood product transfusions may be required to improve patient stability prior to and/or during general anaesthesia.

•  Infection control considerations: Bone marrow aspirates/biopsies may also be performed to investigate neutropenia, or immunosuppressive agents may be given to patients with immune-mediated anaemia or thrombocytopenia after samples are obtained. If this is the case, patients should be barrier nursed to reduce their exposure to hospital-acquired infection and to protect the team from exposure to immunosuppressive medications.

Sample considerations

After the samples have been collected, the histology sample should be stored and packaged away from any microscope slides. This is vitally important because fumes from the formalin will fix the cells on the microscope slides, preventing them from taking up any stain at the laboratory, and therefore making them non-diagnostic. To avoid this, have 2 trays or dishes to put your samples in – one for histology samples and one for slides. When packaging them, ensure your fixed samples are submitted in a separate bag (photocopy the lab form to ensure the lab have the necessary information with each sample) to avoid any interaction between fixed and non-fixed samples.

So that’s bone marrow biopsies in a nutshell! Because of the specific sample preparation requirements, having a nurse who is familiar with the procedure is a huge benefit for these patients (and your vets!) – and now you all are. Do your practice perform bone marrow aspirates in practice? Let me know below!

References:

1. Merrill, L. 2012. Small Animal Internal Medicine for Veterinary Technicians and Nurses. Iowa: Wiley-Blackwell.

2. Bexfield, N. and Lee, K. 2014. BSAVA Guide to Procedures in Small Animal Practice. 2nd Ed. Gloucester: BSAVA.