Ret-He

Ret-He, or reticulocyte haemoglobin, is a measurement of the iron content of reticulocytes. 

Reticulocytes are immature red blood cells that circulate for 1-2 days before becoming mature erythrocytes. They have the appearance of polychromatophils on blood film examination. The iron content of these cells therefore reflects how much iron was available for erythropoiesis over the preceding 2-4 days. 

Iron deficiency anaemia is typically characterised by microcytosis (low MCV) and hypochromasia (low MCHC). However, because these values represent the mean size/colour density of the entire red blood cell population, it can take many weeks to months of low iron availability for MCV and MCHC to fall below the reference interval. Ret-He is a more sensitive indicator of decreased iron availability than traditional parameters. 

Interpretation

Ret-He values below the reference interval indicate iron-deficient erythropiesis (IDE), however Ret-He cannot distinguish between the causes of reduced iron availability. 

Considerations include:

  • Absolute iron deficiency
    • Due to chronic external haemorrhage, most commonly via the GIT or parasitism
  • Functional iron deficiency 
    • Iron sequestration: total body iron is adequate, but iron is sequestered and is not available for erythropoiesis, most commonly due to inflammation/infection
    • Portosystemic shunt

Ret-He is also useful as a monitoring tool for patients being treated for IDE. 

Fuchs J, Moritz A, Grußendorf E, Lechner J, Neuerer F, Nickel R, Rieker T, Schwedes C, DeNicola DB, Russell J, Bauer N. Evaluation of reticulocyte hemoglobin content (RET-He) in the diagnosis of iron-deficient erythropoiesis in dogs. Vet Clin Pathol. 2017 Dec;46(4):558-568. doi: 10.1111/vcp.12547. Epub 2017 Oct 9. PMID: 28991391.

Keiner M, Fuchs J, Bauer N, Moritz A. Evaluation of reticulocyte hemoglobin content (RETIC-HGB) for the diagnosis of iron-limited erythropoiesis in cats. Vet Clin Pathol. 2020 Dec;49(4):557-566. doi: 10.1111/vcp.12925. PMID: 33617045.

NRBC

Nucleated red blood cells are immature erythroid cells that are present in low numbers in health (<1%). Reticulocytes (identified as polychromatophils on Romanowsky-stained smears) and erythrocytes are the non-nucleated and more mature cells of the erythroid lineage. 

Analysis

At low levels (≤1 NRBC ×109/L, i.e. most healthy animals), analyser NRBC enumeration is precise and has good agreement with manual counts in both dogs and cats. At higher levels, and if earlier erythroid precursors are present, analyser enumeration is typically not reliable. In this case a manual count is recommended. 

Interpretation

An increase in NRBCs is referred to as a metarubricytosis or rubricytosis, and may be appropriate or inappropriate. Appropriate rubricytosis is a consequence of erythropoietin-induced stimulation of haematopoiesis but does not, on its own, indicate regeneration.

Causes of metarubricytosis:

  • Appropriate
    • Associated with a regenerative anaemia (associated with reticulocytosis)
    • Associated with conditions causing hypoxaemia, e.g. high altitude, cardiac shunts, chronic respiratory tract disease
  • Inappropriate
    • Drugs/toxicities
      • Chemotherapy, vincristine, Adderall
      • Lead toxicity
    • Bone marrow injury
      • Heat stroke
      • Sepsis, SIRS
      • Severe trauma
      • Hypotension
    • Altered splenic function
      • Splenic contraction
      • Splenic neoplasia
      • Normal in splenectomised animals 
    • Bone marrow diseases
      • MDS
      • Leukaemia
      • Metastatic neoplasia
      • Myelofibrosis
      • Poodle macrocytosis

In dogs, an inappropriate rubricytosis may also provide prognostic information. The presence of NRBCs is strongly associated with increased mortality in dogs when the rubricytosis occurs in the absence of anaemia. In dogs with heatstroke, the presence of NRBCs is a sensitive and specific predictor of death.

Pierini A, Gori E, Lippi I, Ceccherini G, Lubas G, Marchetti V. Neutrophil-to-lymphocyte ratio, nucleated red blood cells and erythrocyte abnormalities in canine systemic inflammatory response syndrome. Res Vet Sci. 2019; 126: 150-154.

Müller M, Dörfelt R, Hamacher L, Wess G. Association of nucleated red blood cells with mortality in critically ill dogs. Vet Rec. 2014; 175(20): 508.

Wilcox A, Russell KE. Hematologic changes associated with Adderall toxicity in a dog. Vet Clin Path. 2008; 37(2): 184-189.

Wada H, Sakuragawa N, Mori Y, et al. Hemostatic molecular markers before the onset of disseminated intravascular coagulation. Am J Hematol. 1999; 60(4): 273-278.

Brown JE, Oscos-Snowball A, Courtman NF. Analytical errors in nucleated red blood cell enumeration. Vet Clin Pathol. 2023; 52: 554-568.