c. Methamoglobin. This compound is formed when the ferrous state of the
heme is oxidized to the ferric state. This compound is incapable of oxygen transport.
d. Sulfhemoglobin. This compound results from the combination of inorganic
sulfides and hemoglobin. This compound is incapable of oxygen transport. This is an
e. Cyanmethemoglobin. This compound results when methemoglobin
combines with the cyanide radical. This compound is used in hemoglobinometry.
VARIATIONS OF HEMOGLOBIN
The variations of hemoglobin occur due to structural differences in the globin
protein. These differences are genetically controlled. The normal hemoglobin
components are hemoglobin A (HbA), hemoglobin A2 (HbA2), and fetal hemoglobin
(HbF). HbA constitutes most of the hemoglobin of a normal adult while HbA2
constitutes a much smaller amount. HbF is present during the first 4 to 6 months of life
and not normally present in adults. Hemoglobin S and hemoglobin C are the most
commonly occurring abnormal hemoglobins. Others (D, E, H, etc.) are found in rare
occurrences associated with several types of anemia. The various types of hemoglobin
are separated by electrophoresis.
The hemoglobin concentration is directly proportional to the oxygen-combining
capacity of blood. Therefore, the measurement of the hemoglobin concentration in the
blood is important as a screening test for diseases associated with anemia and for
following the response of these diseases to treatment. There are four basic ways to
measure the hemoglobin concentration: (1) measurement of the oxygen-combining
capacity of blood (gasometric), (2) measurement of the iron content (chemical method),
(3) colorimetric measurement of specific gravity (gravimetric method), and (4) method is
the most widely used. The cyanmethemoglobin method is the method of choice and is
recommended by the Technical Subcommittee on Hemoglobinometry of the
International Committee for Standardization in Hematology.
6-10. CHYANMETHEMOGLOBIN METHOD
a. Principle. Blood is diluted with a dilute solution of potassium ferricyanide and
potassium cyanide at a slightly alkaline pH. The ferricyanide converts the hemoglobin
to methemoglobin. The cyanide then reacts with the methemoglobin to form the stable
cyanmethemoglobin. The color intensity is measured in a spectrophotometer at a
wavelength of 540 mm. The optical density is proportional to the concentration of