2-25. PREVENTION OF Rho(D) IMMUNIZATION
a. Very few Rh-negative women form anti-Rho(D) after an Rh-positive pregnancy
nowadays because the women are given antepartum (at 28 weeks gestation) and
postpartum (within 72 hours of delivery) injections of Rh immune globulin (RhIG), which
provides specific, effective immunosuppression. Rh immune globulin is also effective in
suppressing immunization after inadvertent or deliberate transfusion of Rh-positive
blood. The recommended immunosuppressive dose is 300 ug of immune globulin for
each 15 ml of Rh-positive cells transfused. Manufacturers state that a single vial of
RhIG provides adequate immunosuppression for up to 15 ml of Rh-positive cells. Note
that this dose refers to the red blood cells, not whole blood. To calculate the number of
vials of RhIG needed, the volume of transfused red blood cells should be divided by 15.
Example: A Rh-negative person is inadvertently given a unit of Rh-positive whole
blood. Transfusion is discontinued when the error is discovered, after 175 ml has been
QUESTION: How many Rh-positive cells have been given?
EXPLANATION: It is not necessary to calculate red cell volume precisely. A generous
rule-of-thumb is to consider that the whole blood is 50 percent red cells
and that the red cell concentrates are 100 percent red blood cells.
maximum volume of red blood cells received by the above patient is
(175 ml/2) = 88 ml.
QUESTION: How much RhIG to give?
EXPLANATION: 88 ml red blood cells divided by 15 ml/vial = 6 vials.
Giving excess RhIG will not harm the Rh-negative patient, whereas giving too
little and allowing immunization to occur is certainly harmful. In calculating
the dose of RhIG, it is better to err by overestimating the volume of Rh-
positive cells given, than by underestimating.
b. Rh immune globulin shortens the life span of transfused Rh-positive cells, but
does not usually cause clinical findings more severe than transient temperature rise,
muscle pain, and splenomegaly. Since Rh-positive cells continue to circulate for at
least 7 days after anti-Rho(D) has been given, giving immune globulin does not abolish
the immediate therapeutic benefit of the transfusion. The recipient's direct antiglobulin
test will be positive for as long as the Rh-positive cells continue to circulate. The
bilirubin may rise within hours to days of injection, with increments as high as 2 to 6
mg/dl. Passively administered antibody remains detectable up to five months after