Manual white cell differential (blood smear) in cats and dogs
A manual differential is a white cell count done by a person at the microscope: the stained blood smear is scanned, white cells are sorted by type one by one and the result is given in percent. Analysers also produce a differential, but they cannot judge what cells look like and can misclassify them, so the smear remains the check. Percentages mean little on their own — they are turned into absolute numbers using the total white cell count.
What it shows
The smear is first scanned at low magnification for platelet clumps, unusual cells and parasites. Then, usually, 100 white cells are counted in the thin, even part of the smear (the monolayer) and sorted into neutrophils, band (immature) neutrophils, lymphocytes, monocytes, eosinophils and basophils. Cells that do not normally circulate — blasts, mast cells — are noted, or counted as “other cells” when there are enough of them. Nucleated red cells are tallied separately per 100 white cells; when there are more than 5, the total white count is corrected for them, because counters take them for white cells.
The smear also shows what an analyser may miss: band neutrophils (a left shift), toxic change, reactive or abnormal cells. Automated differentials are accurate for neutrophils, lymphocytes and, in some species, eosinophils, but less so for other cells. A smear review is recommended for sick patients, when the analyser flags a result and when results do not fit the clinical picture.
Higher than reference — what it may be a sign of
A high percentage of one cell type is not yet a finding. Percentages are never read alone: they are multiplied by the total white cell count to give the absolute number. For example, 10% eosinophils with a total of 20 thousand per µL gives 2 thousand per µL — an eosinophilia; the same 10% with a total of 6 thousand per µL gives 0.6 thousand, which is usually unremarkable. What a raised absolute count means is covered on each cell’s page: neutrophils, lymphocytes, monocytes, eosinophils, basophils.
A few lines matter as soon as they appear: band neutrophils (normally under 5% of white cells in healthy animals), blasts or other cells not normally found in blood.
Lower than reference
A low percentage, too, is read through the absolute count. When white cells are very low, there may not be 100 cells in the monolayer; the whole smear is used, and even then fewer may be counted, which makes the percentages less precise. Old or stored blood gives more smudged, broken cells; a few do not change the count, but many do.
Look at together with
- Total white cell count — the number every percentage is multiplied by.
- Nucleated red cells — counted on the same smear; they correct the total.
- Toxic change in neutrophils — described from the same smear.
- Platelet clumping — seen during the scan and can disturb counts.
Units and conversion
The manual differential is reported in %: the share of each cell type among the white cells counted, together adding up to 100. Nucleated red cells are given per 100 white cells and are not part of that 100. The absolute count is the percentage × the total white cell count and is reported in the same units as that total; labs usually do this calculation for you.
Reference range
The reference range is on your lab’s form. Labs usually report absolute numbers next to the percentages, and it is the absolute numbers that are compared with the range. Ranges differ between cats and dogs and between labs.
Cats and dogs
Feline platelets clump easily, and on some analysers clumps are counted as white cells, raising the total; the smear shows this. Rarely, when more than half of the red cells carry Heinz bodies, one analyser method overestimates white cells in cats. A few Döhle bodies can be seen in neutrophils of healthy cats, and basophils are harder to recognise with quick aqueous stains.
The smear picks up a left shift and toxic change, which an analyser may miss. These two usually go together with inflammation, but some dogs — for example with immune-mediated haemolytic anaemia — have a left shift without toxic change. Very toxic neutrophils can look like monocytes, which makes the count harder.
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 College of Veterinary Medicine). Smear examination.
- eClinPath (Cornell University College of Veterinary Medicine). WBC counts.
- eClinPath (Cornell University College of Veterinary Medicine). Toxic change.
- Today’s Veterinary Practice. Blood Smear Review: A Step-by-Step Guide to Normal Findings for Cats and Dogs.
- Today’s Veterinary Practice. Blood Smear Review: How to Identify and Interpret Neutrophil Abnormalities.
- Harvey JW. The feline blood film: 2. Leukocyte and platelet morphology. Journal of Feline Medicine and Surgery, 2017.
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.