CLINICAL LAB SCIENEC

422
ESSENTIALS OF CLINICAL LABORATORY SCIENCE
In the Lab
Reporting of Results
When cell suspensions are “too heavy” and excess RBCs are present in the suspension,
reactions may be masked by excess antigen based on the number of antibodies
in the anti-sera used in a procedure. If a cell suspension is “too light,” a reaction may
not be strong enough to macroscopically (with the naked eye) observe clumping of
RBCs and a microscope may be required.
MANUAL IMMUNOHEMATOLOGY PROCEDURE #2
Direct Blood Grouping by Test Tube Method
Principles
The blood groups are A, B, and O and are referred to as the ABO grouping. Blood typing
really has a different meaning, although many call the ABO grouping the blood
“type.” Human blood contains either A or B antigens, sometimes in combination, or
neither A or B antigens, which is then classified as “O” (actually a zero), indicating
neither A nor B antigens are present on the surface of the RBCs.
A person who has the A antigen will have antibodies in his plasma or serum
against the B antigen. One who has the B antigen will have antibodies against the
A antigen. This is true with a few exceptions, as there are blood groups other than
A and B. The person with group O blood will have antibodies against both the A and B
antigens. Antibodies are used to indicate reverse grouping, and antigens are used as
the forward, or direct, grouping.
Performing both a forward grouping and a reverse grouping provides a check for
accuracy. There are two major methods for performing blood grouping by the direct
method: the slide method and the tube method. Both may be used. Most immunohematology
professionals consider the tube method to be more reliable and more sensitive
than the slide test. Slide testing is more commonly used for biology students and
in some clinical laboratory classrooms, but most blood banks and clinical laboratories
rely on tube testing for the ABO group and the Rh type.
Washing the RBCs prior to treating with anti-sera to determine the blood group
of the RBCs provides an additional safeguard, as almost all the protein-rich plasma is
removed by washing.
A specialized centrifuge called a serofuge is used to speed up the reaction of
agglutination or clumping of the RBCs in the tube. It usually requires an approximately
30-second spin to effect a valid result.
Antigens may be found on the surfaces of the RBCs of some patients and results
may be extremely weak, or antibodies may be absent or diminished in strength due to
the inability of the body’s immune system to efficiently produce normal levels of antibodies.
The results in these patients may require careful testing and even referral to a
specialty reference laboratory where results of testing are in doubt.
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