Bone marrow cytology in cats and dogs

What it shows

Bone marrow is where red cells, white cells and platelets are made. When a blood test shows a problem the marrow may explain, the vet collects a small marrow sample under sedation or anaesthesia with pain relief. Two kinds of sample are common, often taken at the same visit:

• Aspirate — liquid marrow drawn into a syringe and spread on slides. This is the cytology: a pathologist looks at the individual cells.
• Core biopsy — a tiny cylinder of marrow in formalin for histology; it shows how full the marrow is and its structure better than an aspirate.

The report describes how full of cells the marrow is, the balance and maturation of each cell line, iron stores, and any abnormal cells or organisms — a description with a conclusion, not a single number.

The usual reasons are a persistent or unexplained shortage of blood cells (for example non-regenerative anaemia or low platelets), abnormal cells in the blood, staging of lymphoma or mast cell tumours, and the search for organisms such as Leishmania or Histoplasma.

Higher than reference — what it may be a sign of

• Hyperplasia (more precursors of one line) — the expected response of healthy marrow to losses or destruction in the blood. It takes about 3–5 days to develop after the drop in blood counts begins.
• Many blasts (very young cells) — acute leukaemia is defined by blasts making up more than 20% (myeloid) or more than 25% (lymphoid) of marrow cells; their type is then clarified with special stains or cell-surface markers.
• Sheets or large groups of plasma cells — characteristic of multiple myeloma in dogs, once infections are excluded.
• Lymphoma or other tumour cells — lymphoma is one of the most common tumours found in the marrow of small animals.

Lower than reference

• Hypoplasia — fewer precursors of a line despite low blood counts, meaning the marrow makes too little.
• Aplasia — precursors almost absent (usually less than 5%), the sample made mostly of fat; in the blood all cell lines fall (aplastic anaemia). Reported causes include drugs, infections such as Ehrlichia canis, FeLV and parvovirus, immune-mediated and unexplained cases.
• Few particles or a “dry tap” — repeated failure to draw marrow may suggest myelofibrosis (marrow replaced by fibrous tissue), confirmed on core biopsy.

The examination is not always conclusive: sometimes it only confirms that a problem exists without naming the cause.

Look at together with

• A complete blood count taken at the same time — white cells, red cells and platelets. The marrow is read only against the blood: a recent result (within 24–48 hours) or at least a good blood smear is needed, otherwise interpretation is quite limited.
• Reticulocytes — show whether anaemia is regenerative; a non-regenerative anaemia is one of the main reasons to examine the marrow.
• Flow cytometry — helps identify the type of blasts or lymphoid cells.
• FeLV test in cats — the virus is a known cause of marrow failure.

Units and conversion

There are no units: the result is the pathologist’s description. Some parts are written as proportions: cellularity as a ratio of cells to fat, the M:E ratio of white-cell to red-cell precursors (for example 1:1 or 2:1), and cell types as percentages of marrow cells. In the app the result is kept as text, the way it is written on the form.

Reference range

The reference range is on your lab’s form; the pathologist compares the sample with what is expected for the species. In most species the M:E ratio is about 1:1 to 2:1, there are 2–3 megakaryocytes per marrow particle when the platelet count is normal, myeloblasts are under 1%, and plasma cells are under about 2%.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Bone marrow: indications and methods
2. eClinPath (Cornell University) — Bone marrow: cytologic evaluation
3. eClinPath (Cornell University) — Bone marrow: general interpretation
4. VCA Animal Hospitals — Bone marrow collection and examination
5. Merck Veterinary Manual — Nonregenerative anemias in animals