Close window to return to IVIS
Proceeding of the NAVC
North American Veterinary Conference
Jan. 8-12, 2005, Orlando, Florida
Reprinted in the IVIS website with the permission of the NAVC
http://www.ivis.org/
Published in IVIS with the permission of the NAVC
Small Animal - Hematology
USING THE CBC TO YOUR ADVANTAGE IN
ANEMIC PATIENTS
Ann E. Hohenhaus, DVM Diplomate, ACVIM
The Animal Medical Center
New York, NY
Anemia is a decrease in oxygen carrying capacity of the
blood. It is characterized by a decreased hemoglobin,
hematocrit and red blood cell count. Clinical signs of anemia
are not related to number of RBCs, but the rapidity with which
the decrease occurred. Anemia is not a diagnosis, but a
manifestation of an underlying disease process.
The
diagnosis of anemia requires a full medical evaluation and
oftentimes, additional special testing. The complete and
differential blood counts are underutilized components of the
patient’s medical database. The following discussion will
outline the typical parameters of the CBC and how they can
be used to aid in identifying the underlying cause of anemia.
EQUIPMENT TO PERFORM BLOOD CELL COUNTING
Manual methods have been used for hundreds of years.
They are still an important component of evaluating the
hemogram and should be performed on every case, even if
automated counting is performed. Most commercial and
university reference laboratories are using impedance or flow
cytometry techniques.
These machines are quick and
accurate, but expensive.
Some machines have been
designed for use in veterinary practices. The QBC machine
uses quantitative buffy coat analysis and is very accurate
within the normal range. Outside the normal range the
results vary significantly, making the machine less useful in
sick dogs and cats. The Mascot multispecies hematology
machine is an impedance analyzer specifically designed for
veterinary practices. It seems to be more accurate in pets
with abnormal hemograms. It requires only a few µl of blood
for testing and is simple to operate.
CLASSIFICATION OF ANEMIA
Anemia can be classified several ways. Based on the
presence or absence of a bone marrow response, anemia
can be classified as regenerative or non-regenerative. This
is helpful since decreased anemia from bone marrow failure
is never a regenerative anemia. This classification may also
help owner decision-making, because non-regenerative
anemias are more likely to be transfusion dependant
complicated by a long course of recovery when compared to
regenerative anemia. Anemia has also been described
based on the degree of anemia: mild, moderate and severe.
Severe anemia (Hct <13%) is common in bone marrow
failure and mild anemia (Hct 25-30%) is typical of anemia of
inflammatory disease. Morphologic classification of anemia
uses cell size (MCV) and hemoglobin content (MCH or
MCHC) to describe the anemia as macro-, normo-, microcytic
and hypo- or normochromic. Hyperchromic anemias do not
exist because RBCs cannot synthesize an excess of
hemoglobin. If the MCH and MCHC are elevated over
normal, it is due to laboratory error such as hemolysis or the
presence of Heinz bodies. I find this classification scheme is
most useful in a few select cases such as iron deficiency
anemia that is microcytic, hypochromic; FeLV related anemia
that is macrocytic and normochromic; or anemia of chronic
disease that is normocytic and normochromic.
The
classification I find clinical useful in developing a diagnostic
383
Close window to return to IVIS
www.ivis.org
plan is the pathophysiologic method. This divides anemia
into hemolytic, blood loss and decreased production (bone
marrow failure) and can be done with a CBC, biochemistry
profile, urinalysis and reticulocyte count. Once the anemia
has been classified, a differential diagnosis list can be
developed and a diagnostic plan executed.
RED BLOOD CELL EVALUATION
The ability of the blood to carry oxygen to the tissues is
assessed in the laboratory by measurement of the RBC
count, hemoglobin or hematocrit. In anemic patients, all 3
values move in parallel, decreasing by a similar amount;
however, the degree of decrease is not reflected in the
clinical signs. For example, a dog hit by a car with acute
hemorrhage may be extremely symptomatic for anemia with
a Hct of 20%. A dog with slowly progressive bone marrow
failure will not even appear abnormal to the owners until the
Hct falls much lower than 20%.
Evaluating Red Blood Cell Mass
Because of its universal availability, limited equipment
requirements and ease of interpretation, veterinarians have
typically used Hct when discussing anemia.
With the
approval of a hemoglobin based oxygen carrier for dogs, Hgb
will need to be measured more often to fully assess oxygen
carrying capacity. Hgb is approximately 3 times the Hct.
Red blood cell count is accurate only if the equipment has
been optimized for the species being tested. Feline RBCs
are small and can be counted as platelets, falsely lowering
the RBC count.
Red Blood Cell Indices
Mean Corpuscular Volume (MCV) is the average side of
the red blood cell. It may measured directly or be calculated
by (PCV X 10)/RBC = MCV fl. Larger cells are typically
immature and can indicate regeneration. Smaller cells
develop in response to abnormal iron metabolism.
Microcytosis and macrocytosis may also be breed specific.
Macrocytosis – Increased MCV
Greyhounds, normal finding, average MCV = 81 fl
Poodle macrocytosis, average MCV = 94 fl
FeLV induced anemia
B12/folate deficiency (congenital disease)
Microcytosis – Decreased MCV
Akita, Shiba Inu, Jindo dog
Iron deficiency anemia
Portosystemic shunts
Mean corpuscular hemoglobin (MCH) is the Hgb/RBC X 10
= MCH pg. Mean corpuscular hemoglobin concentration is
the Hgb/PCV X 100 = MCHC g/dl. Either of these is useful in
determining the adequacy of cellular hemoglobin.
Red Blood Cell Morphology
Since the advent of automated cell counting, the
microscopic appearance of RBCs is less carefully evaluated
than previously. Microscopic morphology can still provide
clues useful in the diagnosis of anemia. Acute anemia is
followed 2-4 days later by polychromasia, which is an
indicator for the presence of reticulocytes and a regenerative
response. Basophillic stippling is classically thought of in
cases of lead poisoning, but highly regenerative anemia may
also be associated with basophilic stippling. Heinz bodies
www.ivis.org
Published in IVIS with the permission of the NAVC
The North American Veterinary Conference – 2005 Proceedings
are protrusions on the surface of RBCs resulting from
oxidative damage to hemoglobin. Heinz bodies can occur
from exposure to propylene glycol, acetaminophen
(paracetamol), onions, vitamin K or diabetes. They are much
more common in cats because feline hemoglobin has 8
sulfhydral groups compared to 2 in other species and the cat
has a limited ability to reverse oxidative damage. Heinz
bodies can also cause a false elevation in MCH.
Nucleated Red Blood Cells
The presence of nRBCs is termed normoblastemia. They
do not indicate regeneration in dogs and cats as they do in
ruminants. The bone marrow endothelial cells prevent
release of nRBCs into circulation. In cases of anemia, the
endothelial cells become hypoxic and the nRBCs are
erroneously released. NRBCs can also be seen in bone
marrow disease and splenic dysfunction. The presence of
nRBCs without anemia suggests lead poisoning. Increased
nRBCs may falsely elevate the WBC.
WHITE BLOOD CELL COUNT
Leukocytosis is common in anemic patients. It may occur
as part of a stress response, secondary to bone marrow
stimulation or the presence of necrotic tissue in the body.
When anemia occurs secondary to infection, the WBC is
typically elevated. Leukopenia may also occur in association
with anemia. It may be an indicator of bone marrow failure or
neoplastic cell infiltrate in the bone marrow.
Differential Count
The differential WBC can be expressed in either absolute
numbers or as a percentage of the total number. Both are
useful, but care must be used in interpretation. Assessment
of the percentage allows the veterinarian to see which cell
type predominates but without calculating the absolute
numbers, you cannot determine if the particular cell type
predominates because of a lack of one cell type or a
overabundance of another cell type.
For example, assume a differential count of 75%
lymphocytes and 25% neutrophils. If the total WBC is 2000,
there are 1500 lymphocytes and 500 neutrophils. The patient
should be suspect for a parvovirus infection or chemotherapy
toxicity. If the total WBC is 28,000, and there are 21,000
lymphocytes and 7,000 neutrophils, then diseases like
ehrlichiosis, lymphocytic leukemia and hypoadrenocorticsm
should be considered.
Close window to return to IVIS
www.ivis.org
Some diseases have a “classic” pattern on the differential
count. Dogs and cats with hypoadrenocorticsm will have a
lymphocytosis, and an eosinophillia. Dogs with splenic
hemangiosarcoma can have a marked leukocytosis with a
mature neutrophilia as well as a regenerative anemia.
Although most veterinarians believe patients with leukemia
have a tremendously elevated WBC, in actuality, patients
may have a normal, low or high WBC. The clue is an
abnormal ratio of cells and the presence of atypical cells in
the peripheral circulation.
OTHER PARAMETERS OFTEN INCLUDED IN A CBC
Total Solids
The difference between total solids and total protein is total
solids are measured on plasma in the microhematocrit tube
and total solids on serum. Total solids include measurement
of clotting factors that are lost when serum is formed.
Typically, total solids are 1.0 g/dl higher than total protein.
Either parameter can be low when anemia is due to blood
loss of greater than 1-2 days. Assessment of total solids also
allows the plasma to be viewed and assessed for features
such as bilirubinemia or hemoglobinemia.
Platelet Count
An actual count or platelet estimate is often included in the
CBC since automated counting has become widespread.
Changes in platelet counts are non-specific, but
thrombocytopenia is often associated with anemia due to
hemangiosarcoma, immune disorders and neoplastic
infiltrates in the bone marrow. Marked elevation platelet
counts are seen with iron deficiency anemia.
The
mechanism is unknown.
Reticulocytes
Immature RBCs are termed reticulocytes. They are larger,
and when stained are a bluish-pink compared to mature
RBCs. They are often called polychromatophillic RBCs. An
increase in reticulocytes is considered a regenerative
anemia. There are 2 types of reticulocytes aggregate, which
are most commonly counted and punctate which occur in
cats and are not often counted in laboratories. Although
many formulas can be used to assess regeneration, the
presence of >50,000-60,000 reticulocytes/µl indicates
regeneration. To calculate the number of reticulocytes,
multiply the reticulocyte percentage by the number of red
blood cells. Feline reticulocytes are difficult to count and are
best counted by flow cytometery.
www.ivis.org
384