Reticulocytes in cats and dogs

What it shows

When a red cell leaves the bone marrow, it still contains some RNA. A special stain, or an analyzer’s dye, makes this RNA visible, and such cells are counted as reticulocytes. In dogs they mature into ordinary red cells within a few days.

After blood loss or destruction of red cells (hemolysis) a healthy marrow speeds up production, and reticulocytes rise. If the count is adequate for how severe the anemia is, the anemia is called regenerative; if not, non-regenerative — production is the problem. The marrow needs 3–5 days to respond, so in the first days after sudden bleeding or hemolysis any anemia looks non-regenerative; a repeat test answers the question.

Cats have two types. Aggregate reticulocytes, with clumps of RNA, are the youngest; they mature into punctate ones within 12–24 hours. Punctate reticulocytes, with only small dots of RNA, stay in the blood for many days. Only aggregate reticulocytes reflect the marrow’s current response, so in cats it is the aggregate count that is used.

Higher than reference — what it may be a sign of

• Response to blood loss — injury, bleeding in the gut, parasites such as fleas — usually visible after 3–4 days.
• Response to hemolysis: immune-mediated hemolytic anemia, blood parasites, toxins.
• Without anemia: eClinPath describes reticulocytosis in some non-anemic dogs and cats; it can reflect, for example, blood loss, heart or lung disease, or inflammation, and the vet looks for the reason.

Lower than reference

Few reticulocytes in an anemic animal mean that the marrow is not answering adequately — a non-regenerative anemia. Common reasons are chronic kidney disease (the kidneys make erythropoietin, the hormone that drives red cell production), chronic inflammation, bone marrow disease and, in cats, FeLV. The other possibility is timing: a very recent anemia, within the first days, has not had time to become regenerative. In an animal without anemia, a low reticulocyte count on its own usually means little.

Look at together with

• Hematocrit and hemoglobin — show how severe the anemia is; the reticulocyte count is judged against that.
• MCV and RDW — many young, larger cells raise the average size and its spread; small cells point toward iron deficiency.
• Creatinine — a non-regenerative anemia in an older animal raises the question of kidney disease.

Units and conversion

The absolute count is reported as ×10⁹/L (×10^9/л) or K/µL — the same number. The percentage (Retic%) is the share of reticulocytes among red cells. Absolute count (K/µL) = percentage × red cell count (×10¹²/L) × 10. A vet relies on the absolute count: the percentage alone does not account for how severe the anemia is. Cat forms may list aggregate and punctate reticulocytes separately, in ×10⁹/L.

Reticulocyte hemoglobin (RET-He, CHr, RETIC-HGB) is reported in picograms (pg). It shows how much hemoglobin young cells carry — the quality of new red cells. Because reticulocytes are only a few days old, it reflects lack of available iron earlier than indices of all red cells. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form. More important than the range is whether the count is adequate for the degree of anemia; laboratories and textbooks use somewhat different thresholds for calling a response regenerative, so the vet reads the result together with the hematocrit. A low reticulocyte hemoglobin can also be caused by inflammation, not only by true iron deficiency. A repeat test after a few days often says more than one result.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Regeneration
2. eClinPath (Cornell University) — Reticulocyte %
3. eClinPath (Cornell University) — Absolute reticulocyte count
4. eClinPath (Cornell University) — Reticulocyte indices
5. Kendall A. Overview of Anemia in Animals. Merck Veterinary Manual
6. IDEXX — CBC testing at IDEXX Reference Laboratories