Monocytes in cats and dogs
Monocytes are the largest white blood cells. They circulate only briefly before moving into tissues, where they become macrophages that clear away germs and damaged cells. A higher count is common with inflammation and, in dogs, with stress; a low count is hard to document and usually has no clinical significance.
What it shows
Monocytes are made in the bone marrow, spend about 24–36 hours in the blood and then settle in tissues, where they turn into macrophages — cells that engulf microorganisms and cellular debris; some of them also process antigens for other immune cells. In the blood they make up a small share of the white cells.
eClinPath calls the monocyte the most problematic leukocyte to identify: its size and shape vary, and in dogs it can resemble a band neutrophil. That is why analyzers and people sometimes disagree about monocytes more than about other cells, and why a vet often checks a surprising monocyte count on a blood smear.
Higher than reference — what it may be a sign of
- Inflammation — a common cause, infectious or not. Monocytosis often goes with long-lasting inflammation and may accompany the inflammatory pattern on the leukogram.
- Stress and corticosteroids — common in dogs: the stress pattern there includes more neutrophils, fewer lymphocytes and eosinophils, and more monocytes. In cats this part of the pattern is uncommon.
- Recovery of the bone marrow after injury, for example after chemotherapy — uncommon, but the rise can be large.
- Tumours: a reaction to some cancers such as lymphoma, or, rarely, a leukemia of the monocytes themselves.
Lower than reference
A low monocyte count is hard even to document, because the lower limit of many reference ranges goes down to zero or close to it. eClinPath describes monocytopenia as having no clinical significance. A result of 0 or a few tenths on the form is therefore usually not a reason for concern by itself; the vet looks at the other white cells instead.
Look at together with
- Neutrophils and lymphocytes — the stress pattern in dogs (more neutrophils, fewer lymphocytes, more monocytes) is read only from all three together.
- Eosinophils — fewer eosinophils completes the stress pattern.
- Total white cell count (WBC) — needed to turn the percentage into an absolute number.
- Blood smear review — confirms that the cells counted as monocytes really are monocytes.
Units and conversion
The absolute count (MONO, Mono#) is reported as ×10⁹/L (×10^9/л on Russian forms). The percentage (MON%, MONO%) is the share of monocytes among all white cells. Absolute count = percentage × total WBC ÷ 100. A vet relies on the absolute count: with a high total WBC, a “normal” percentage can hide a real monocytosis, and with a low total WBC a high percentage may mean an ordinary number of cells. In the app each row keeps the unit printed on the form.
Reference range
The reference range is on your lab’s form. It differs between species and analyzers, and its lower limit is often zero. Monocytes are a small group of cells, so the count can jump from one test to the next by chance; a lasting rise across several tests says more than a single high value, and it is read together with the rest of the leukogram.
Cats and dogs
In cats a stress response usually does not raise monocytes, so a monocytosis in a cat points more toward inflammation. eClinPath notes no consistent species differences in how monocytes look; in cats, as in dogs, they are usually the largest white cell on the smear.
In dogs monocytosis is a common part of the stress pattern, whether from the dog’s own cortisol in illness or from steroid medicines. So in a dog a mild rise in monocytes together with more neutrophils and fewer lymphocytes usually reflects stress rather than a separate problem. On the smear canine monocytes can be confused with band neutrophils.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University) — Individual WBC
- eClinPath (Cornell University) — Leukogram patterns
- eClinPath (Cornell University) — Normal leukocytes
- Merck Veterinary Manual — Leukogram Abnormalities in Animals
- Merck Veterinary Manual — White Blood Cell Disorders of Dogs (dog owners)
- Merck Veterinary Manual — Physiology of Leukocytes in Animals
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.