Beginning May 15, 2020, the routine mammalian CBC will now contain three additional values: automated reticulocyte concentration (RCauto) for all species except equines and camelids, mean hemoglobin content of reticulocytes (CHr) for all species except equines and camelids, and plateletcrit (Pct).

Reticulocytes are immature anucleate erythrocytes that contain cytoplasmic RNA that may be detected by automated or manual microscopic staining methods. Measurement of reticulocyte concentration is used to determine if the bone marrow is responding appropriately to an anemia by increasing reticulocyte release via increased erythropoiesis.

The automated ADVIA 2120 hematology analyzer used at the MSU VDL Clinical Pathology Laboratory detects reticulocyte fluorescence following addition of a fluorescent dye. In some cases, an accurate automated reticulocyte concentration cannot be generated by the analyzer; if desired, a manual microscopic reticulocyte concentration may be ordered for an additional charge by contacting the laboratory.

MSU VDL-generated reference intervals are available only for dogs and cats at this time. Although an automated reticulocyte concentration can be measured for most species, validation data are not available for species other than dogs or cats.

This value indicates the mean hemoglobin content of all reticulocytes (mass in picograms), just as mean corpuscular hemoglobin (MCH) indicates the mean mass of hemoglobin of all erythrocytes. The CHr is calculated as the product of MCVr (mean corpuscular volume of reticulocytes) and CHCMr (hemoglobin concentration mean of reticulocytes). Within a few days of release into blood, circulating reticulocytes mature into erythrocytes that lack detectable RNA.

Therefore, CHr and other reticulocyte indices reflect the current state of erythropoiesis (rather than that of all circulating erythrocytes), making it more reliable when erythropoiesis is altered. Decreased CHr occurs with acute or ongoing decreases in erythrocyte hemoglobin production, which is most commonly caused by iron deficiency or decreased iron availability due to inflammation or portosystemic shunts.

The greatest reported decreases in CHr occur with iron deficiency. Detection of increasing CHr on serial assessments may reflect clinical improvement (e.g., effective iron supplementation, resolving inflammation). Decreased CHr also occurs in dogs with breed-associated microcytosis (e.g., Akitas, Chow Chows, Shiba Inus).

Bibliography for further reading

Fry MM, Kirk CA. Reticulocyte indices in a canine model of nutritional iron deficiency. Vet Clin Pathol. 2006 Jun;35(2):172-81.

Steinberg JD, Olver CS. Hematologic and biochemical abnormalities indicating iron deficiency are associated with decreased reticulocyte hemoglobin content (CHr) and reticulocyte volume (rMCV) in dogs. Vet Clin Pathol. 2005;34(1):23-7.

Schaefer DM, Stokol T. The utility of reticulocyte indices in distinguishing iron deficiency anemia from anemia of inflammatory disease, portosystemic shunting, and breed-associated microcytosis in dogs. Vet Clin Pathol. 2015 Mar;44(1):109-19.

The plateletcrit (Pct) or thrombocrit is the percentage of blood volume filled by platelets, just as the hematocrit indicates the percentage of blood volume filled by erythrocytes. The Pct is calculated from the product of the platelet concentration and mean platelet volume (MPV). Compared to platelet concentration, Pct is a better indicator of circulating platelet mass and the body’s capacity for primary hemostasis because the total volume, and therefore mass, of all platelets correlates with platelet functional capacity.

The Pct is of particular value in dogs with genetic macrothrombocytopenia (e.g., Cavalier King Charles Spaniels and Norfolk terriers), as affected dogs have a decreased platelet concentration that might imply impaired hemostasis. However, when the Pct is within the reference interval (due to increased MPV), this supports appropriate capacity for platelet plug formation with no need for further investigation of the thrombocytopenia. Similarly, in thrombocytopenic patients with increased MPV from stimulated thrombopoiesis, Pct may be a better indication of primary hemostatic function than platelet concentration.

However, because Pct is calculated from two values that are unreliable when platelet clumping is present, Pct also is unreliable when there is platelet clumping. Platelet clumping may be minimized by “clean” venipuncture, prompt exposure to EDTA, adequate gentle sample mixing, and prompt analysis. Prompt testing also minimizes effects of EDTA and temperature on MPV and Pct.

Bibliography for further reading

Tvedten H, Lilliehöök I, Hillström A, et al. Plateletcrit is superior to platelet count for assessing platelet status in Cavalier King Charles Spaniels. Vet Clin Pathol. 2008 Sep;37(3):266-71.

Gelain ME, Tutino GF, Pogliani E, et al. Macrothrombocytopenia in a group of related Norfolk terriers. Vet Rec. 2010 Sep 25;167(13):493-4.

Tvedten HW, Lilliehöök IE, Oberg J, et al. Validation of Advia plateletcrit for assessing platelet mass in dogs, including Cavalier King Charles spaniels. Vet Clin Pathol. 2012 Sep;41(3):336-43.

Kelley J, Sharkey LC, Christopherson PW, et al. Platelet count and plateletcrit in Cavalier King Charles Spaniels and Greyhounds using the Advia 120 and 2120. Vet Clin Pathol. 2014 Mar;43(1):43-9.

Handagama P, Feldman B, Kono C, Farver T. Mean platelet volume artifacts: the effect of anticoagulants and temperature on canine platelets. Vet Clin Pathol. 1986;15(4):13-7.