Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

VII. Tests for Evaluating Iron Metabolism
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hemoglobin value (in g/dl) by the hematocrit (as a
percentage) and multiplying by 100. The MCHC is
expressed as g/dl of erythrocytes. It may be normal in iron
deficiency anemia when MCV values are only slightly
decreased, but the MCHC is typically low when the MCV
is moderately to markedly decreased.
The mean cell hemoglobin (MCH) represents the average
amount of hemoglobin in a single erythrocyte. It is
calculated by dividing the Hb value (in g/dl) by the RBC
count (in millions per μ l) and multiplying by 10. The
MCH generally provides little added information beyond
that obtained from the MCV and MCHC because the MCH
depends on both the size and hemoglobin concentration of
erythrocytes. It usually correlates directly with the MCV,
except in animals with macrocytic hypochromic erythrocytes.
Exceptionally low MCH values strongly suggest true
iron deficiency is present.
Although quite useful when abnormal, MCV and
MCHC values are relatively insensitive in identifying the
presence of erythrocytes with abnormal volumes or hemoglobin
concentrations. Many microcytic or macrocytic
erythrocytes are required to move the MCV below or above
the reference interval, respectively, and many hypochromic
erythrocytes are needed to move the MCHC below the reference
interval. In addition to counting cells, electronic cell
counters can determine and plot the volume of individual
erythrocytes, and examination of these erythrocyte volume
histograms can reveal the presence of increased numbers
of microcytes or macrocytes even when the MCV is within
the reference interval ( Weiser and Kociba, 1983 ). Some
electronic cell counters such as the Advia 120 (Siemens
Medical Solutions Diagnostics, Tarrytown, NY) can also
determine the hemoglobin concentration of individual
erythrocytes from the deflection of light that occurs when
a laser beam strikes individual cells. Inspection of hemoglobin
concentration histograms can reveal the presence
of increased numbers of hypochromic erythrocytes, even
when MCHC has not decreased below the reference interval.
Individual erythrocytes can be further characterized
by creating a cytogram in which the erythrocyte volumes
of individual cells are plotted against their respective
hemoglobin concentrations. In addition, the percentages of
microcytes, macrocytes, hypochromic erythrocytes, hyperchromic
erythrocytes, erythrocytes with low hemoglobin
content, and erythrocytes with high hemoglobin content can
be determined, and these same parameters can also be determined
for reticulocytes in a blood sample. Because reticulocytes
are recently formed immature erythrocytes, reticulocyte
parameters may best reflect the current state of iron sufficiency
in an animal. Alterations in several reticulocyte indices—including
reticulocyte MCV (MCV retic ), reticulocyte
hemoglobin content (CH retic ), percentage of hypochromic
reticulocytes (% Hypo retic ), percentage of reticulocytes with
low hemoglobin content (% Low CH retic ), percentage of
reticulocytes with high hemoglobin content (% High CH retic ),
and percentage of macrocytic reticulocytes (% Macro retic )—
appear to be of value in the diagnosis of iron deficiency in
dogs ( Fry and Kirk, 2006 ; Steinberg and Olver, 2005 ).
B. Serum Iron
Serum iron can be measured to assess the transport compartment
of iron ( Table 9-1 ). Serum iron concentration is
increased in animals with hemolytic anemia and dyserythropoiesis,
in which iron transfer from macrophages to
plasma is increased ( Harvey and Smith, 1994 ; Smith, 1992 ;
Steffen et al ., 1992 ; Stewart et al ., 1953 ; Watanabe et al .,
1998 ; Weiss and Lulich, 1999 ); in hypoplastic or aplastic
anemia, in which iron transfer from plasma is decreased
( Lange et al ., 1976 ; Smith, 1992 ; Stokol and Blue, 1999 ;
Stokol et al ., 2000 ); in iron overload ( Arnbjerg, 1981 ;
House et al ., 1994 ; Lavoie and Teuscher, 1993 ; Paglia
et al ., 2001 ; Sprague et al ., 2003 ); in pigs with experimental
pyridoxine deficiency ( Deiss et al ., 1966 ); in dogs and
horses following the administration of glucocorticoid steroids
( Fig. 9-7 ) ( Adamama-Moraitou et al ., 2005 ; Harvey
et al ., 1987b ; Smith et al ., 1986b ); and in dogs with
chronic hepatopathy ( Soubasis et al ., 2006 ). Serum iron
concentrations are high at birth in foals, with most of the
transferrin saturated with iron ( Fig. 9-8 ), but serum iron
TABLE 9-1 Serum Iron Analyte Reference Intervals in Domestic Animals a
Species Iron ( μ mol/l) TIBC ( μ mol/l) Ferritin ( μ g/l) Reference(s)
Dog 5.9–26.3 50.5–69.1 80–800 Weeks et al . (1988)
Cat 5.9–24.2 30.3–58.2 32–123 Andrews et al . (1994)
Horse 9.0–35.4 41.3–81.4 43–261 Smith et al . (1986a)
Cattle 7.0–27.7 33.3–48.3 33–55 Kolb (1963), Smith (1997)
Pig 9.8–33.5 43.1–70.3 20–125 Kolb (1963), Smith et al . (1984a)
a
Iron and total iron-binding capacity (TIBC) values can be converted from SI units to conventional units ( μ g /dl) by multiplying by 5.59.