The page below is a sample from the LabCE course Hemolytic Disease of the Fetus and Newborn. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Introduction

Fetomaternal hemorrhage (FMH) greater than 30 mL of whole blood occurs in only about 0.3% of cases but must be detected to prevent the mother from producing anti-D. Once the mother has become immunized to D, RhIg is of no use.

A typical test protocol is to first screen for a large FMH and then quantitate the bleed if the screen is positive. Some laboratories proceed directly to a test that can quantitate the size of the FMH.

Once the size of the FMH is determined, a formula is used to determine how much RhIg is needed. Recall that

  • A standard vial of RhIg contains 1500 IU (300 µg) of IgG anti-D;
  • 300 µg of RhIg can suppress immunization to approximately 30 mL of D-positive whole blood.