Polychromasia, anisocytosis and hypochromasia on a blood smear

What it shows

• Polychromasia — polychromatophils are young red cells without a nucleus that still contain RNA, which stains blue-purple with the routine stain. They are a subset of reticulocytes, which are counted with a special stain, so reticulocytes always outnumber them. More polychromasia means the marrow is releasing more new cells.
• Anisocytosis — variation in red cell size (diameter). It can come from more large cells, more small cells or both, and some degree of it is normal. It roughly corresponds to the analyser’s RDW.
• Hypochromasia — red cells with less haemoglobin than normal; the word is also used when MCHC is below the reference interval. True hypochromic cells have to be told apart from torocytes, an artefact that only looks similar.

Higher than reference — what it may be a sign of

• More polychromasia — part of a regenerative response: the marrow answers anemia from blood loss or red cell destruction by sending out young cells. It can come with increased anisocytosis, because young cells are larger.
• More anisocytosis — large young cells during regeneration; in cats, a feline leukemia virus (FeLV) infection or a bone marrow disorder (myelodysplasia), where large cells come with a severe non-regenerative anemia; some inherited conditions; cell swelling, for example in a stored sample. A few abnormal cells may not shift MCV at all — the smear can show them first.
• Hypochromasia — in common domestic animals true hypochromasia occurs only with advanced iron deficiency anemia, most often from chronic blood loss from the gut. Rarer causes: chronic lead poisoning and copper deficiency. In very young animals it can be physiological. In developing iron deficiency, hypochromasia on the smear can appear before MCHC drops below the range.

Lower than reference

Low numbers of polychromatophils are normal in healthy dogs and cats, and some anisocytosis is normal too, so “slight” or “not seen” is the expected result in a healthy animal. The finding that matters is little or no polychromasia in an animal that is anemic: it suggests the marrow is not responding (a non-regenerative anemia), and the vet checks the reticulocyte count, which is more precise than a smear impression.

Look at together with

• Reticulocytes — the counted version of polychromasia.
• RDW and MCV — the analyser’s measure of size variation and average cell size.
• MCHC and iron — when hypochromasia raises the question of iron deficiency.
• Hematocrit — whether there is anemia at all.
• Red cell shapes — spherocytes, schistocytes and other findings on the same smear.

Units and conversion

There are no units: these are smear descriptions graded by eye, usually as mild, moderate or marked. The analyser counterparts (RDW, MCV, MCHC) have their own units and articles. In the app the smear result is stored as text, as written on the form.

Reference range

The reference range is on your lab’s form. For a smear description there is no number: in a healthy dog or cat polychromatophils are few and anisocytosis is slight, and hypochromasia is not expected. In healthy dogs the low number of polychromatophils corresponds to reticulocytes under 1.5%. Grades are subjective, so one lab’s “mild” and another’s “moderate” may be closer than they look.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Color changes — polychromasia and hypochromasia.
2. eClinPath (Cornell University College of Veterinary Medicine). Size changes — anisocytosis.
3. eClinPath (Cornell University College of Veterinary Medicine). Red blood cells — quick guide.
4. Merck Veterinary Manual, professional version. Regenerative Anemias in Animals.