What is a fetal-maternal hemorrhage?
A FMH is a bleed across the placenta from fetus to mother before or during delivery. Other than during normal delivery, this can result due to a trauma, amniocentesis, fetal blood sampling, miscarriage, or abortion.
Why do we need to know if this has occurred?
We need to establish if a FMH has occurred in RhD negative mothers who have RhD positive babies in order to help prevent the mothers from becoming sensitized to the D antigen which is present on the red blood cells of RhD positive babies.
What is given to RhD negative mothers to prevent this sensitization?
Rh immune globulin or RhoGAM is given to RhD negative mothers to prevent D antigen sensitization.
When is Rh immune globulin or RhoGAM given?
One dose of RhoGAM is given at 28 weeks of gestation and again within 72 hours of delivery of a RhD positive baby.
What is RhoGAM?
Rh(D) immune globulin of mostly IgG anti-D from human plasma made by Ortho-Clinical Diagnostics and has been around since 1968. Prior to 1968, HDFN was a major cause of infant mortality in the U.S., but the incidence of HDFN currently, for the most part, has been eliminated. The dosing frequency of RhoGAM brand is 12 weeks, so patients getting RhoGAM at the 28 weeks should be covered through the normal 40-week gestation period. If RhoGAM is given before 28 weeks of gestation, it is advisable to repeat the dose every 12 weeks to ensure adequate levels until delivery.
When is the K-B test done?
This test is performed on a specimen from a Rh negative mother who delivered a Rh positive baby and has a positive FETALSCREEN or rosette test. The positive rosette test indicates occurrence of a large FMH.
What is the principle of the K-B test?
Fetal hemoglobin is resistant to acid treatment. A slide is made from the mother’s specimen which is then dried, fixed, and immersed in a citrate/phosphate buffer with a pH of 3.2. This acid buffer will dissolve the adult hemoglobin (HbA) but not the fetal hemoglobin (HbF). The smear is then stained with 0.1% Erythrosine, where the fetal cells will now appear as a dark reddish/pink while adult cells will appear white to light pink with a slightly darker center when viewed microscopically.
What is the formula for calculating a fetal bleed in milliliters (mL)?
The fetal and maternal cells are counted separately for a total of at least 2000 cells on the prepared smear. These numbers are then plugged into the following formula:
Fetal cells/total cells counted × maternal blood volume (5000 mL arbitrarily assigned mother’s blood volume) = mL of fetal whole blood
What is the formula for calculating the % of fetal cells?
Total fetal cells counted/total red blood cells (RBC) counted × 100 = %
How many milliliters of Rh D positive fetal whole blood will one regular dose of Rh immune globulin protect against during a transplacental hemorrhage in a Rh negative woman?
30 mL
How is the Rh immune globulin dosage calculated?
Volume of fetal whole blood/30 (volume in mL protected by 1 dose of Rh immune globulin) = dose
Note: When the number to the right of the decimal point is <5 round down and add on one dose of Rh immune globulin. When the number to the right of decimal point is 5 or >, round up to the next number and add one dose of Rh immune globulin.
What specimens are acceptable to use with this procedure?
Maternal blood collected in EDTA, amniotic, and other body fluids. However, if testing amniotic and other body fluids, the red cells must be isolated by washing with saline and adding albumin to the final suspension.
What quality control is necessary to run with patient specimens?
The Clinical Laboratory Improvement Amendments (CLIA) recommends a high and a low-positive control and a negative control.
A fetal-maternal hemorrhage is a potentially dangerous condition that may lead to complications for a mother. Identification of this condition is typically done using the Modified Kleihauer-Betke (K-B) acid/elution test. In this module, you will perform the K-B test and then calculate the % of fetal cells present and the number of vials of Rh immune globulin or RhoGam that will need to be given to a Rh negative mother who experienced a fetomaternal hemorrhage (FMH).
