Nucleated red blood cells (nRBC) in cats and dogs

What it shows

Red cells mature in the bone marrow and lose their nucleus before release. The nucleated form seen most often in blood is the most mature one, the metarubricyte (normoblast); earlier forms appear less often. An increase is called normoblastosis or erythroblastosis.

nRBC are counted during the manual differential on a blood smear, as the number per 100 white blood cells. Analyzers and counting chambers cannot tell them from white cells, so the reported “WBC” is really a count of all nucleated cells, and analyzers often count nRBC as lymphocytes. When there are many nRBC, the white cell count is corrected: corrected WBC = measured count × 100 / (nRBC + 100). One laboratory corrects from about 5 nRBC per 100 WBC; a fully automated count without a smear is not corrected at all.

Higher than reference — what it may be a sign of

• Regenerative anemia, especially from red cell destruction (hemolysis, including immune-mediated hemolytic anemia): the marrow releases young cells early. If the number of nRBC matches the degree of polychromasia, no other explanation is usually sought. Still, nRBC alone are not used to decide whether an anemia is regenerative.
• Bone marrow injury — with sepsis or endotoxemia; in dogs heat stroke is a frequent example.
• Changed spleen function — after removal of the spleen, with splenic hemangiosarcoma, or in dogs receiving corticosteroids.
• Lead poisoning — many nRBC (5–40 per 100 WBC) without severe anemia suggest it, together with basophilic stippling of red cells. These findings point toward lead but do not confirm it.
• Disordered red cell production — inherited in some breeds (miniature and toy poodles with hereditary macrocytosis) or acquired, with bone marrow tumours or with medicines that interfere with folate or vitamin B12.

Many nRBC with only mild polychromasia are a reason for the vet to look for one of these causes.

Lower than reference

Zero is the expected result in most healthy animals, so there is no “too low” for nRBC. Their absence in an anemic animal does not mean the anemia is non-regenerative — that question is answered by the reticulocyte count.

Look at together with

• Reticulocytes and polychromasia — show whether the marrow is truly regenerating; the number of nRBC is compared against them.
• WBC and lymphocytes — can be inflated by nRBC; it is worth checking whether the form says the count was corrected.
• Hematocrit — many nRBC without anemia tell a different story from nRBC in a strongly regenerative anemia.
• Red cell morphology — basophilic stippling (lead) helps explain extra nRBC.

Units and conversion

nRBC are reported as a number per 100 white blood cells (/100 WBC) — a count from a smear, not a concentration in blood. Some labs also print an absolute nRBC count in ×10⁹/L (the same number as ×10³/µL), which equals the measured nucleated count minus the corrected WBC. The app stores the count per 100 WBC.

Reference range

The reference range is on your lab’s form. In most healthy animals nRBC are absent; for dogs one laboratory allows up to 1 per 100 WBC. Anecdotal reports suggest that miniature schnauzers and dachshunds may have more, but this has not been confirmed. What matters is whether the number fits the situation — the hematocrit, the signs of regeneration and the animal’s condition — and whether the WBC on the same form was corrected.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). nRBC — hematology / tests.
2. eClinPath (Cornell University College of Veterinary Medicine). Nucleated RBC — hematology / morphologic features.
3. Merck Veterinary Manual. Lead Poisoning in Animals.
4. Merck Veterinary Manual. Clinical Hematology — diagnostic procedures for the private practice laboratory.