Bone Marrow Biopsy in Cats

The bone marrow is responsible for producing cell lines vital to life including red blood cells, white cells, and platelets. Conditions resulting in severe anemia (lack of red blood cells), or abnormal white cell or platelet readings may need a bone marrow biopsy in order to identify the root cause. Biopsy is a diagnostic tool which enables the clinician to reach a diagnosis and start appropriate treatment. 

Bone marrow biopsy may be undertaken in first opinion practice, when the clinician feels confident to do so, or at a referral center. The procedure can often be undertaken under sedation and local anesthetic, with the patient feeling no more than a momentary discomfort.

The patient will already have had diagnostic tests such as blood samples and an analysis of fresh blood smears. It is entirely appropriate to first screen the cat for problems outside of the bone marrow which can affect cell line production. Thus ultrasound or radiographic imaging of the chest and abdomen may have already been performed and found normal. 

There are three locations to choose from to harvest the sample. The cat is sedated and the fur clipped over one of the collection sites:

1. The wing of the ilium on the pelvis
2. The greater trochanter of the femur (thigh bone)
3. The proximal humerus 

Local anesthetic is injected into the skin and the membrane (periosteum) overlying the bone. A scalpel is used to make a small stab incision in the skin. A bone marrow biopsy needle is placed on a bony landmark and, with a twisting pushing motion, pushed through the bone into the marrow. 

Different techniques are used depending on the type of biopsy needle used. One method involves collecting a 'plug' of marrow in the needle, which is then withdrawn and the plug spread over a microscope slide. Alternatively, a small volume of fluid is injected into the marrow cavity and the resulting mix sucked back into a syringe, and then smeared on a slide. 

The biopsy tool is removed from the site and a single suture placed over the stab incision. The slides are sent to a laboratory where a histologist examines the types of cells present. 

Bone marrow biopsy is a diagnostic tool used to identify the nature of the underlying bone marrow problem. It is not a treatment in itself, but a means to give the clinician the information necessary to decide on a course of treatment. 

The information gained by examining the bone marrow cannot be obtained in other ways. In those cases where a bone marrow biopsy is indicated, the only options are to either perform a bone marrow biopsy or go without the information it would give. 

It should be said that bone marrow biopsy is a targeted test, which is only performed when the results of other investigations point strongly to bone marrow disease. 

The collection of bone marrow is considered a minor procedure and in good natured cats is often performed under sedation and local anesthetic. This means there is relatively little for the cat to recover from, except to fully wake from the sedation. The single suture placed in the skin is usually removed after 10 to 14 days. 

Once harvested, the sample is sent away and the results may not be known for several days. This means that targeted treatment may not start immediately, but is held pending the histology results. 

The typical cost of a bone marrow biopsy or aspirate, including sedation, taking the sample, and submitting it to a histologist is around $500.

Bone marrow biopsy is a relatively low risk procedure. It is not usually associated with a high degree of hemorrhage, so unless the patient is known to have a bleeding disorder, then it is considered relatively complication free. 

The biggest factor to consider is that diagnostic samples are not always obtained. This can be down to the nature of the bone marrow disease or the sampling technique. For example, some types of bone marrow fibrosis (where active marrow is replaced by inactive scar tissue) exfoliates cells poorly, which means that cells do not yield up to the sampling. Alternatively hemodilution can occur, where blood accidentally contaminates the sample and makes it harder to interpret. 

The chances of using a negative sample can be reduced by staining the slide in-house, and checking for good cellularity before submitting it to the histologist. This then offers the opportunity to take a second sample should the first one be inadequate. 

With the exception of bone marrow suppression as a result of exogenous estrogen administration, the majority of causes of bone marrow suppression are not predictable or preventable.