The following points highlight the top four indices of red blood cells. The indices are: 1. Mean Cell Volume 2. Increased MCV 3. Decreased MCV 4. Mean Cell Hemoglobin.
1. Mean Cell Volume (MCV):
The mean cell volume indicates the volume of the “average” red cell in a sample. It is expressed in femtoliters (fl; 10-15 liters). Traditionally, MCV was a calculated parameter, derived by using the following formula:
MCV = (PCV ÷ RBC) × 10
Present-day automated hematology analyzers provide a more accurate, direct measure of MCV, based on the actual volume of the cell as it passes through a laser (newer laser- based hematologic analyzers) or an electronic beam.
The amount of laser light scattered in a forward direction or the amplitude of pulses created in the electronic field as the cells through the detector is equivalent to the cell volume, which is averaged based on the number of cells analyzed by the instrument.
The instruments also “channelize” the scatter or impulses, segregating them into channels representing relative ranges of cell size. This data is then assembled into a cell size histogram, such as shown at right. The histogram provides additional useful information about the characteristics of the red cell population.
Red cell populations with the MCV above the reference interval are termed macrocytic. Conversely, red cell populations with the MCV below the reference interval are ermedmicrocytic.
2. Increased MCV (Macrocytosis):
i. Red Cell Clumping or Agglutination:
With impedance based analyzers, agglutinated RBC are detected as single large red cells, resulting in very high MCVs (> 90 fl) and low MCHCs. This occurs rarely with the newer laser-based analyzers, since the agglutinated clumps are excluded from the analysis (which may decrease the RBC count but does not affect the MCV).
ii. Storage-Related Changes:
Red cells swell with storage, increasing the MCV and decreasing MCHC. This will occur quite rapidly, within 24 hours of collection particularly if the blood sample is not kept cool until analysis.
With the ADVIA hematology analyzer, macrocytosis can be observed in animals with severe hyperosmolality, e.g. hypernatremia. This is attributed to dehydration of red blood cells which occurs in vivo due to the hyperosmotic environment.
Once these dehydrated cells are placed in an iso-osmolar diluent for counting within the analyzer, they are now actually hypertonic compared to the diluent and swell in vitro in the diluent, thus increasing the MCV and decreasing the MCHC.
Breed-associated macrocytosis has been reported in Greyhounds (around 81 fl) and Miniature and Toy Poodles (up to 90 fl), without any evidence of anemia.
v. Regenerative Anemia:
Since young red blood cells are usually larger, the MCV can be increased above reference intervals (and the MCHC decreased). These findings are less apparent with laser-based hematology analyzers.
Coats that have recovered from a recent anemia can be macrocytic, since punctate reticulocytes (which are frequently larger than normal red blood cells) can persist for up to 3 weeks in the circulation. This is called a post-regenerative macrocytosis.
Red cell swelling due to Osmotic Effects/Membrane Abnormalities:
ii. Hereditary Stomatocytosis:
This inherited defect has been reported in Malamutes, Miniature Schnauzers, Pomeranians. Drentje-patrishjond and other breeds. There are breed-specific membrane defects in lipid content or the sodium/potassium pump, resulting in macrocytic and hypochromic red blood cells. Affected animals may not even be anemic.
Defects in Nuclear Maturation/DNA Synthesis:
i. Primary Myelodysplasia:
This is a clonal disorder that in cats is usually caused by FeLV.
ii. Folate or Vitamin B12 Deficiency:
Both are required for DNA synthesis (thymidine and nucleoproteins). Impaired DNA synthesis delays cell division resulting in macrocytosis. These deficiencies can occur with intestinal disorders and small intestinal bacterial overgrowth (although macrocytosis is not a feature of these diseases), drugs which inhibit folate/vitamin B12 absorption or metabolism (e.g. trimethoprim sulphur, hydroxyurea) or because of other mineral deficiencies or excess, such as cobalt deficiency (primary or secondary to molybdenum excess) in ruminants. Cobalt is essential in the molecular structure of vitamin B12.
iii. Inherited Abnormalities in Erythropoiesis:
Congenital dyserythropoietic anemia (CDA) is an inherited defect in humans that results in macrocytosis. This has been reported in Poll Hereford cattle.
iv. Unknown Mechanism or Miscellaneous:
Hyperthyroidism has been associated with macrocytosis in cats in some studies. This was attributed to thyroid hormone induced red cell production, with decreased maturation time and premature release of larger red blood cells.
3. Decreased MCV (Microcytosis):
i. Excess EDTA:
EDTA is hypertonic and will cause cellular dehydration (decreasing the MCV and increasing the MCHC).
An artifact of the AD VIA used at Cornell University is caused by a hypoosmolar environment in vivo, e.g. hyponatremia, to which erythrocytes adjust by increasing cytoplasmic water content. When put in a diluent prior to counting in the machine in vitro, osmosis results in water loss from RBCs within the analyzer, causing cell shrinkage (low MCV and high MCHC).
i. Young Animals:
Puppies and kittens < 8-16 weeks old and calves and foals up to 1-2 months old can be microcytic and anemic. Calves and foals can remain microcytic (but not anemic) for up to 1 year of age. This is associated with low iron stores (and is called a “physiologic iron deficiency”). Neonatal alpacas can be anemic but are not microcytic.
ii. Breed Associations:
Microcytosis without anemia is found in Akitas, and possibly other Oriental breeds, such as the Shiba or Sharpeis, and Siberian Huskies
iii. Iron Deficiency:
Iron is an essential component of many enzymes in cells and is also part of the heme group in hemoglobin (which consists of a porphyrin ring containing iron). Much of the body’s iron stores are within red blood cells where iron is critical for hemoglobin synthesis. Iron deficiency could be due to inadequate intake or absorption of iron, excessive loss with external hemorrhage, or interference with iron metabolism. For more information on iron, refer to the chemistry section of this webpage.
iv. Lack or Loss of Iron:
In adult animals, iron deficiency usually results from chronic external blood loss, often gastrointestinal in origin. Young animals are predisposed to iron deficiency due to low iron intake (in milk), low body iron stores, and rapid growth rate.
Iron deficiency is thought to result in microcytosis by the following hypothesized mechanism: Erythrocyte division in the bone marrow is governed by the hemoglobin concentration.
Cell division stops when a critical hemoglobin concentration has been reached. Therefore, if hemoglobin production is defective (as occurs in iron deficiency), erythrocytes continue to divide until that hemoglobin concentration is reached. With each division they become successively smaller.
v. Interference with Iron Use/Uptake:
Drugs, such as chloramphenicol and lead can interfere with iron uptake into hemoglobin. Nutritional deficiencies (or excesses) are an important cause of iron deficiency in animals, particularly herbivores. A deficiency of pyridoxine or copper can result in iron deficiency anemia.
Pyridoxine is required for heme synthesis in red blood cells, whereas copper is an essential component of ceruloplasmin and haephestin which are required for the transfer of iron to/from macrophages and intestinal epithelial cells, respectively. An excess of some minerals, in particular zinc, can also inhibit copper absorption resulting in a secondary copper and then iron deficiency.
vi. Liver Disease:
Acquired or congenital hepatic shunts may produce microcytosis with or without anemia. The MHCH is usually normal but can be low. The microcytosis in shunting is thought to be due to altered iron metabolism from liver dysfunction and usually resolves with successful shunt closure.
vii. Unknown Mechanisms or Miscellaneous:
An inherited dyserythropoietic disorder in English Springer Spaniels is associated with microcytic erythrocytes. Severe fragmentation anemias can result in microcytosis. Note, that although spherocytes in immune-mediated hemolytic anemia (IMHA) appear smaller on blood films (because they have reduced diameter), they usually have normal volumes.
MCH = Hg in gms × 10
RBC number in millions
4. Mean Cell Hemoglobin:
MHC is the mean cell hemoglobin. This represents the absolute amount of hemoglobin in the average red cell in a sample. Its units are picograms (pg) per cell.
The MCH is calculated from the [Hb] and the RBC using the following equation:
MCH (pg) = (Hb × 10) ÷ RBC
This value is not that useful, in that the combination of MCV and MCHC give more specific information. For example, a low MCH could be due to smaller than normal cells with normal Hb concentration, or normal sized cells with lower than normal Hb concentration. It is better to know the values for cell volume and Hb concentration directly.
MCHC = Hg × 100
Mean Cell Hemoglobin Concentration (MCHC):
MCHC is the mean cell hemoglobin concentration, expressed in g/dL.
It can be calculated from the [Hb] and the PCV using the following formula:
MCHC = (Hb ÷ PCV) x 100
The normal value for MCHC is about 33%. Red cell populations with values below the reference interval can be termed “hypochromic”. This can occur in a strongly regenerative anemia, where an increased population of reticulocytes with low Hb content “pull” the average value down (an increased MCV would be expected under this scenario). Low MCHC can also occur in iron deficiency anemia, where microcytic, hypochromic red cells are produced as a result of the lack of iron to support hemoglobin synthesis.
Values for MCHC significantly above the reference interval are not physiologically possible due to limitations on the solubility of Hb. Sample-related problems of analysis, however, can result in spurious high values.
Lipemia or other causes of turbidity in the lysate can cause falsely high [Hb] values, which raises the apparent MCHC. In the latter cases we would provide a CHCM which is optically measured directly from the red blood cells by our hematology analyzer.
Red cells from animals of the Camillidae family (camels, llamas, alpacas), however, truly do have higher MCHC (around 40-45 fL) compared to those of common domestic animals. This is possible due to higher solubility of the Hb molecule in these species.
Red Cell Distribution Width (RDW):
The RDW is an index of the variation in cell volume within the red-cell population. It is a parameter provided by automated hematology analyzers and is the electronic equivalent of anisocytosis or variation in red blood cell size that is judged by smear examination. Mathematically, it is the coefficient of variation, i.e.,
RDW = (Standard deviation ÷ mean cell volume) x 100
A high RDW indicates that the red blood cells are more variable in volume than normal. This may be due to the presence of smaller or larger red blood cells or a combination of either scenario.
For example, increased numbers of immature red blood cells during a regenerative response to an anemia will increase the RDW, because immature anucleate red blood cells are larger than normal. Conversely, the presence of increased numbers of smaller cells (e.g. in iron deficiency anemia) will also increase the RDW (see image below).
The cause for a high RDW may be revealed by examination of a blood smear to identify the presence of small or large red blood cells. Note that low numbers of smaller or larger red blood cells may increase the RDW BEFORE you see an increase or decrease in the mean cell volume (MCV) result on the hemogram.
An RDW within reference intervals provides little information on variation in red blood cell size. An RDW below the reference interval is not a clinically relevant finding.
Red blood cell volume histograms from an impedance-based hematology analyzer (left images) and representative pictures (right images) of Wright’s-stained peripheral blood smears from a dog with iron deficiency anemia before(top images) and after transfusion (bottom images).
Before transfusion, there are large numbers of microcytic hypochromic red blood cells with a small degree of polychromasia and fragmentation (acanthocytes, schistocytes and keratocytes) evident in the dog’s peripheral blood.
The MCV (37 fL) and MCHC (28 g/dL) are markedly decreased and the RDW, which is graphically illustrated as the width of the histogram curve (delineated by the red double arrow), is also increased (24%) compared to a healthy dog.
After transfusion, there are two populations of red blood cells in the dog’s peripheral blood: the dog’s own microcytic hypochromic fragmenting cells and normocytic normochromic transfused red blood cells. This results in two peaks on the red blood cell volume histogram, with the left peak representing the dog’s own microcytic red blood cells and the right peak representing the transfused cells.
The transfusion has naturally increased the MCV and MCHC to 49 fL and 32 g/dL, respectively. The RDW has almost doubled as seen by the much larger width of the combined curves on the histogram. In general, MCV is the most useful index and divide the anemias into microcytic, normocytic and macrocytic types. The MCH and MCHC add very little to the information provided by MCV.