Examination of the blood in the laboratory
Physicians rely upon laboratory analysis to obtain
measurements of many constituents of the blood, information
useful or necessary for the detection and recognition
of disease. Hemoglobin contains a highly coloured pigment
that interferes with the passage of a beam oflight.
To measure hemoglobin concentration, blood is accurately
diluted and the red cells broken down to yield a clear
red solution. A photoelectric instrument is used to
measure the absorbance of transmitted light, from which
hemoglobin concentration can be calculated. Changes
in the hemoglobin concentration of the blood are not
necessarily directly paralleledby changes in the red
cell count and the hematocrit value, because the size
and hemoglobin concentration of red cells may change
in disease. Therefore, measurements of the red cell
count and the hematocrit value may provide useful information
as well. These three tests canbe carried out rapidly
and in large numbers by automated machines. Electronic
particle counters for determining red cell, white cell,
and platelet counts are widely used. Only a drop of
blood is needed for the analyses, which are completed
within a minute; results are printed in a laboratory
report that is sent to the physician. Although expensive,
the equipment increases the output of the laboratory
and saves the technician valuable time.
Adequate examination of the blood cells requires that
a thin film of blood be spread on a glass slide, stained
with a special blood stain (Wright's stain), and examined
under the microscope. Individual red cells, white cells,
and platelets are examined, and the relative proportions
of the several classes of white cells are tabulated.
The results may have important diagnostic implications.
In iron deficiency anemia, for example, the red cells
look paler than normal because they lack the normal
amount of hemoglobin; in malaria the diagnosis is established
by observing the malarial parasites within the red cells.
In pneumonia and many infections the proportion of neutrophilic
leukocytes is usually increased, while in others, such
as whooping cough and measles, there is an increase
in the proportion of lymphocytes.
Chemical analyses measure many of the constituents
of plasma. Often serum rather than plasma is used, however,
since serum can be obtained from clotted blood without
the addition of an anticoagulant. Changes in the concentrations
of chemical constituents of the blood can indicate the
presence of a disease process. For example, quantitative
determination of the amount of sugar in the blood is
essential for the diagnosis of diabetes, a disease in
which the blood sugar tends to be elevated. Nitrogenous
waste products, in particular urea, tend to accumulate
in patients with diseased kidneys that are unable to
excrete these substances at a normal rate. An increase
in the concentration of bilirubin in the serum often
reflects a disorder of the liver and bile ducts or an
increased rate of destruction of hemoglobin. Measurements
of these and many other serum constituents are so valuable
inmedical diagnosis that often multiple tests are performed.
Tests can be performed manually using an individual
procedure for each analysis; however, the autoanalyzer,
a completely automated machine, increases the number
of chemical analyses that can be performed in laboratories.
A dozen analyses may be made simultaneously by a single
machine employing a small amount of serum. The serum
is automatically drawn from a test tube and is propelled
through plastic tubing of small diameter. As the serum
specimen advances, it is divided; appropriate reagents
are added; chemical reactions occur with formation of
a product that can be measured with a photoelectric
instrument; and the result appears as a written tracing
from which serum concentration of various substances
can be read directly. The data acquired by the machine
may be fed automatically into a computer and the numerical
results printed on a form that is submitted to a physician.
Many of the available analyses are not performed routinely
but are invaluable in special circumstances. In cases
of suspected lead poisoning, for example, detection
of an elevated level of lead in the blood may be diagnostic.
Some analytical procedures have specific diagnostic
usefulness. These include assays for certain hormones,
including measurement of the thyroid hormone in the
serum of patients suspected of having thyroid disease.
Other important laboratory procedures are concerned
with immunologic reactions of the blood. Careful determinations
of the blood groups of the patient and of the blood
donor, and cross matching of the cells of one with the
serum of the other to ensure compatibility, are essential
for the safe transfusion of blood. The Rh type of a
pregnant woman is regularly determined and is necessary
for the early detection of fetal–maternal incompatibility
and for proper prevention or treatment of hemolytic
disease of the newborn. The diagnosis of certain infectious
diseases depends upon the demonstration of antibodies
in the patient's serum.
Many other kinds of blood examination yield useful
results. Enzymes normally present in themuscle of the
heart may be released into the blood when the heart
is damaged by a coronaryocclusion (obstruction of the
coronary artery) with consequent tissue necrosis. Measurement
of these enzymes in the serum is regularly performed
to assist in diagnosis of this type of heart disease.
Damage to the liver releases other enzymes, measurement
of which aids in evaluation of the nature and severity
of liver disease. Inherited abnormalities of proteins
are increasingly recognized and identified by use of
sophisticated methods. Accurate diagnosis of hemophilia
and other bleeding disorders is made possible by investigations
of the coagulation mechanism. Measurements of the concentration
of folic acid and vitamin B12 in the blood provide the
basis for diagnosis of deficiencies of those vitamins.
The number of potentially useful blood tests is so vast
that they must be selected judiciously in the evaluation
of the individual patient.
C. Lockard Conley
Robert S. Schwartz
