CLINICAL LAB SCIENEC

CHAPTER 16: IMMUNOLOGY AND SEROLOGY 403
Quantitative and Semiquantitative Procedures
Quantitative procedures are tests that attempt to quantify the numbers,
amounts, or levels of certain constituents of the blood. Some serological procedures
are sophisticated enough to qualify as quantitative, but the majority
of common serological tests are semiquantitative, in that they measure only
the dilutional strength of an antibody level, rather than
actual numbers of antibodies. Antibody titers (dilutions
of antibodies) are indirect and semiquantitative tests.
Because antibodies formed against disease vary widely
from individual to individual, a result may be significant
or nonsignificant according to the level of antibodies
present. Some individuals will not produce an antibody
level sufficient to support the diagnosis of a certain disease.
Other serological tests are available for determining
if white cells themselves are competent for performing
their destructive work on foreign invaders. This set of
procedures is rarely requested, and is extremely specialized
work that is usually beyond the scope of a routine
hospital laboratory. Most of these tests are performed in
large commercial reference laboratories that perform a
sufficient quantity of tests to make it economical to set
up the procedure.
Serial dilutions (dilutions of serum) reacted with
specific antigens available in commercially prepared laboratory
test kits comprise the bulk of serological tests.
A titer, which includes dilutions of serum to determine
the approximate level of antibodies present, yields valuable
data as to the stage of the disease. The acute phase
for most organisms infecting the body occurs within
hours to days of exposure, depending on previous exposure
to a certain organism, which if present may cause
a more rapid rise in antibody levels. The convalescent
phase comes later when the level of antibodies subsides
and the infected person has overcome the disease and
is usually free of symptoms of the disease. This information
enables the physician to determine whether the
patient’s immune system has responded adequately or if
a chronic, or long-term, disease process is in place.
Critical Reminders
Prozone and Postzone Reactions
Two phenomena–prozone and postzone reactions–
can produce false-negative results in common
serological reactions called agglutination and
precipitation, and the laboratory professional must
be alert to these possibilities. There is an optimum
proportion of antibody-antigen levels in a sample
that yields valid results, but prozone and postzone
reactions yield erroneous results. For some
procedures, both undiluted and diluted samples are
tested for these conditions before a quantitative
antibody titer is performed. A prozone reaction
occurs when there is an excess of antibodies
compared with the amount of antigens present. In
undiluted and low-dilution samples, this condition
creates a zone with relatively high antibody
concentrations where no reaction occurs, yielding
a false-negative result. In addition, if low levels of
antibody are present, the agglutination may be too
weak to observe. A postzone reaction occurs when
excess antigen is present, so there is a lack of
agglutination or precipitation and a false-negative
result is also obtained. In the serial dilution test
for antibodies called an antibody titer, a prozone
reaction will result in a negative value for the
low dilution portions of the test and will become
positive at higher dilutions.
Methodologies Employed in Immunology
and Serology
It is important for the clinical laboratory student to understand that the basic serological
test is an indirect test to determine the presence of antibodies against antigens,
not to identify the antigen itself. Most tests in serology are of the indirect
type, where a known antigen, purchased most often from a commercial provider,
is used to test for antibodies against the organism. If antibodies exist against a
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