| Mean Cell Volume (MCV)
|
The MCV is the average volume of a single
RBC. It is determined by direct measurement with an electronic cell counter
such as the VetScan HMT. Increases are usually due to reticulocytes and
indicate a responsive anemia.
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| Mean Platelet Volume
(MPV) |
The MPV is a machine calculation of platelet
size. In thrombocytopenic dogs, increased mean platelet volume gives indirect
evidence of increased megakaryocyte response. High mean volume (>12 fl)
indicates increased response but decreased volume (<12 fl) is not accurate in
predicting lack of bone marrow megakaryocyte production.
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| Monocytes |
Monocytes are the immature blood stage of
tissue macrophages. Increased numbers occur in response to inflammation. Their
main function is phagocytosis of foreign material, cellular debris and
pathogens that are not effectively controlled by neutrophils. They engulf
intracellular organisms and those causing a granulomatous inflammatory
response. They are effective scavengers, removing tissue debris, cellular
remnants and foreign material. Monocytes are also active in regulating the
immune response, processing antigen, and activating killer cells and
macrophages. Monocytes are the most commonly misidentified leukocyte in blood
smears, often being placed into the lymphocyte category.
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| Monocytosis
|
Increased numbers of circulating monocytes (monocytosis)
occur in chronic suppurative, pyogranulomatous, necrotic, malignant,
hemolytic, hemorrhagic or immune-mediated diseases. Monocytosis also occurs in
dogs as a corticosteroid-induced response from stress, adrenal hyperfunction
or exogenous corticosteroids. Some animals with chronic disease have
persistent monocytosis. Decreased numbers of circulating monocytes (monocytopenia)
is rare and has no diagnostic significance.
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| Nucleated Red Blood
Cells, (nRBC) |
Nucleated RBCs (nRBCs) are larger and more
immature than reticulocytes and mature RBCs. These immature, nucleated stages
of the erythrocyte generally occur within the bone marrow, and are rarely
observed in the peripheral blood of normal dogs and cats. They appear as
metarubricytes in small numbers in response to acute blood loss or anemia.
Nucleated RBCs without concurrent anemia or reticulocytosis are a sign of
disease. They are found in splenic disease, extramedullary hemopoiesis, lead
poisoning, hyperadrenocorticism, leukemia and bone marrow disease. Circulating
nucleated RBCs can be metarubricytes or younger cells, such as rubricytes.
RBCs of birds and reptiles are all nucleated and are counted as WBCs by
automatic counters.
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| Neutrophils |
Neutrophils phagocytize and kill
microorganisms. They also initiate and modify the acute inflammatory process,
cause tissue damage and are cytotoxic. Production and storage in the bone
marrow, margination of cells in the capillary beds, and the demands of
peripheral tissues affect the numbers of circulating neutrophils.
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| Neutropenia |
Neutropenia indicates decreased numbers of
circulating neutrophils. It may be due to insufficient production or increased
destruction of neutrophils. Conditions that cause neutropenia include
endotoxemia, viral infections, overwhelming bacterial infections, and
administration of drugs that cause bone marrow suppression.
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| Neutrophilia |
Neutrophilia indicates increased numbers of
circulating neutrophils. It can be physiologically induced by exercise and
corticosteroids, or pathologically induced by infections and tissue
destruction. The primary differential diagnoses for neutrophilia are
inflammation (both septic and non-septic), stress, exercise or excitement.
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| Packed Cell Volume (PCV) |
The packed cell volume (PCV) or hematocrit (Hct)
is a measure of RBC numbers, expressed as a percentage of the total volume of
blood. The PCV, by common usage, has become synonymous with the Hct.
Traditionally the PCV is obtained by centrifuging an anticoagulated blood
sample (spun crit); with automated counters this value is calculated from the
measured Mean Corpuscular Volume (MCV) and RBC count. This is the reason that
laboratory values may differ slightly from in-clinic values. The column of
packed RBCs (PCV) is measured in millimeters and expressed as a percentage of
the total blood volume. Anemia exists when the PCV falls below the reference
range for the species. Hemoconcentration may exist when the PCV exceeds the
reference range. There is normally a 3:1 ratio of PCV to hemoglobin value.
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| Platelet clumping |
Platelet clumping is an aggregation of
thrombocytes that produces inaccurate counts with electronic counters. This is
caused by activation of the platelets from poor collection but sometimes
occurs spontaneously in cats. Storage in EDTA (lavender top tubes) may
increase this tendency. Rapid processing with electronic counters decreases
this tendency. The histogram may show an abnormal distribution of large cells
indicating platelet clumping. Because of this clumping in samples the
reference labs usually give only an estimation of platelet numbers as seen on
blood smears. An adequate count of 8-10 platelets/100X objective field would
suggest platelet numbers greater than 150,000. Thrombocytopenic slides show <7
per 100X objective indicating counts less than 100,000.
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| Platelet Count |
Counts below 100,000/ml are significant.
Platelets can be counted directly, or numbers can be estimated from the blood
smear (>5 per oil-immersion field). Decreased platelet numbers
(thrombocytopenia) occur with disseminated intravascular coagulation, bone
marrow depression, autoimmune hemolytic anemia, systemic lupus erythematosus
and severe hemorrhage. Thrombocytosis (increased platelet numbers) is caused
by excess bleeding (from trauma, blood sucking parasites or neoplasia), iron
deficinecy anemia and myeloproliferative syndromes.
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| Platelet
Distribution Width (PDW) |
Platelet distribution width (PDW) is a
number that measures platelet anisocytosis (variation in size). A mixture of
large and small platelets may give a normal Mean Platelet Volume (MPV) but a
high PDW. This would be an indication of active platelet release.
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| Platelets (Thrombocytes) |
Platelets or thrombocytes are small flat
disks, which are produced by megakaryocytes. They adhere to exposed
subendothelial collagen within seconds of injury to form a hemostatic plug.
Low platelet counts predispose an animal to hemorrhage.
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|
Poikilocytes |
Poikilocytes are abnormally shaped RBCs.
Poikilocyte is a general term that encompasses all categories of abnormal RBC
shapes, including more specific terms, such as echinocyte, acanthocyte,
schizocyte and crenation. RBC distortion may occur with improperly prepared
blood films and should not be confused with poikilocytosis. Poikilocytosis is
a nonspecific change seen in chronic blood loss, iron-deficiency anemia,
diseases characterized by RBC fragmentation, and chronic lead poisoning. A
stained blood smear will show the abnormally shaped RBCs.
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| Polycythemia
|
Polycythemia is an increase in the red cell
mass of the blood. This is seen as an increase in PCV, hemoglobin
concentration and RBC count. Absolute polycythemia results from increased bone
marrow production of RBCs and may be primary, as with polycythemia vera or
myeloproliferative disease, or secondary to hypoxia and renal disease.
Absolute polycythemia must be distinguished from relative polycythemia that
occurs with dehydration (high plasma protein), hypovolemia (low plasma
protein), shock or splenic contraction (normal plasma protein).
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| Potassium
|
Potassium and sodium are the two cations
mainly responsible for the osmotic pressures of intracellular and
extracellular fluid (ICF and ECF, respectively). Sodium, found primarily in
ECF, is actively moved from ICF to ECF by sodium-potassium pumps in the cells.
The ICF is the principal location of potassium, where it can not be clinically
measured. The movement of potassium across the cell membrane is crucial to
cardiac and neuromuscular excitability. Membrane potential is altered by
changing the ratio of ICF potassium to ECF potassium. Homeostasis of potassium
is controlled by the ICF : ECF potassium ratio and by body intake and output.
The ICF : ECF ratio is affected by acid-base balance, glucose and insulin
administration, exercise, and catecholamine release.
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| Red Blood Cells (RBC) |
Red blood cells (RBCs) transport oxygen from
the lungs to body tissues. Their production is stimulated by erythropoietin,
secretion of which is controlled by the blood oxygen tension. Erythropoietin
stimulates maturation of RBC precursors in bone marrow into mature RBCs. Blood
loss, parasitism, renal failure, RBC damage, chronic inflammatory disease,
hematopoietic malignancies and insufficient dietary iron, copper or vitamin
B12 cause a deficiency of RBCs (anemia). Shock, fluid loss or increased RBC
production can cause increased RBC numbers (polycythemia). Dehydration or
protein fluid extravasation causes a relative decrease in the fluid portion of
the blood and a relative increase in the cellular portion. Carbon monoxide,
lung disease, heart disease and high altitude cause excessive RBC production
by stimulating erythropoietin secretion. Erythrocytic malignancies and
polycythemia vera cause excessive RBC production without normal stimulation.
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| Red Cell
Distribution Width (RDW) |
The red cell distribution width is an
electronic measure of anisocytosis (variation of cell size). It increases
where the degree of anisocytosis is increased. In regenerative anemia, it
increases when large cells are produced even before the MCV exceeds the
reference range. It also increases when small cells are produced as with iron
deficiency anemia.
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| Reticulocytes |
Reticulocytes are immature RBCs without a
nucleus. They retain a fine network of endoplasmic reticulum that stains with
reticulocyte stains. These immature cells are slightly larger than mature RBCs
and normally circulate in small numbers. Elevated numbers of circulating
reticulocytes (reticulocytosis) occur in chronic hemorrhagic or hemolytic
anemia with increased erythropoiesis. A lack of circulating reticulocytes in
chronic anemia indicates bone marrow depression. Reticulocytosis without
evidence of anemia may indicate reduced oxygenation of blood. This leads to
increased erythropoietin levels, which in turn stimulate erythropoiesis and
release of reticulocytes from the bone marrow. Reticulocytes are not counted
by the VetScan HMT but are suggested by a high MCV. When present they indicate
the animal is responding to blood loss by red cell regeneration.
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| Right Shift |
The term right shift indicates increased
numbers of circulating hypermature neutrophils in neutrophilic blood samples.
These are cells showing hypersegmentation. This is usually seen in
noninfectious inflammatory processes such as inflammation secondary to a
malignancy.
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