HDFN: prevention (Rh immune globulin)

6 ASCP MLS practice questions on HDFN: prevention (Rh immune globulin), part of Blood Banking. The first 6 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   A Kleihauer-Betke test counts 30 fetal cells among 2,000 cells. The mother's blood volume is taken as 5,000 mL. How many 300 µg vials of Rh immune globulin are needed?

   1. A3 vials
   2. B4 vials
   3. C2 vials
   4. D1 vial

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   Answer: B · 4 vials

   Fetal cells = 30 ÷ 2,000 = 1.5%. Fetal whole blood = 1.5% × 5,000 mL = 75 mL, and 75 ÷ 30 mL per vial = 2.5. A value of .5 or more is rounded up to 3, and one extra vial is added for the test's imprecision, giving 4. Too little Rh immune globulin risks anti-D formation.

   • A: 3 vials leaves out the extra vial added for imprecision.
   • C: 2 vials rounds 2.5 down and leaves out the extra vial.
   • D: 1 vial covers only 30 mL of fetal blood.

2. Question 2 · Easy

   A D-negative pregnant woman without anti-D is usually given Rh immune globulin:

   1. AOnly once, at 12 weeks of pregnancy
   2. BAt delivery only, if the baby is D-negative
   3. CAt 28 weeks and after a D-positive birth
   4. DAt 36 weeks and at the 6-week checkup

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   Answer: C · At 28 weeks and after a D-positive birth

   Antenatal Rh immune globulin at about 28 weeks covers small bleeds in the third trimester. A further dose is given within 72 hours of delivering a D-positive baby, adjusted to the size of any bleed. Doses are also given after events such as miscarriage, amniocentesis or abdominal trauma. It prevents her from making anti-D.

   • A: A single dose at 12 weeks would not protect later in pregnancy.
   • B: A D-negative baby cannot immunize her, so no postpartum dose is needed then.
   • D: Doses at 36 weeks and 6 weeks after birth are not the standard schedule.

3. Question 3 · Medium

   After delivery, the fetal bleed screen (rosette test) is positive. What is the next step?

   1. ANothing more; she is already immunized
   2. BGive one standard vial and stop there
   3. CQuantify the bleed (Kleihauer-Betke or flow)
   4. DRepeat the rosette test after 72 hours to confirm

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   Answer: C · Quantify the bleed (Kleihauer-Betke or flow)

   The rosette test screens for a bleed too large to be covered by one standard dose. A positive result must be quantified with the Kleihauer-Betke acid elution test or flow cytometry for fetal hemoglobin. The size of the bleed sets the number of vials. A negative rosette test means a single standard dose is enough.

   • A: A positive rosette shows fetal cells, not maternal anti-D.
   • B: One vial may be too little for a large bleed.
   • D: The dose must be given within 72 hours, so waiting is not useful.

4. Question 4 · Medium

   A D-negative woman received Rh immune globulin at 28 weeks. At delivery her screen shows weak anti-D (titer 2), and the baby is D-positive. Should she receive postpartum Rh immune globulin?

   1. AYes; the weak anti-D is most likely passive
   2. BNo; she is already immunized
   3. CNo; the baby's DAT must be negative first
   4. DOnly if the titer rises above 16

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   Answer: A · Yes; the weak anti-D is most likely passive

   Antenatal Rh immune globulin often leaves a low titer of passive anti-D, usually 4 or less, for weeks. This does not mean she has made her own anti-D. She should still receive the postpartum dose within 72 hours. A history of recent Rh immune globulin is essential when interpreting the antibody screen.

   • B: A low titer after antenatal Rh immune globulin is usually passive, not immune.
   • C: A positive baby DAT from passive antibody does not prevent the dose.
   • D: Waiting for a rising titer would miss the 72-hour window.

5. Question 5 · Hard

   A 29-year-old D-negative woman without anti-D delivers a D-positive baby. Her postpartum rosette fetal bleed screen is positive. On the Kleihauer-Betke slide, a trainee reports 1.0% fetal cells, but the reviewing technologist notes that most of the mother's own cells stain an even pale pink instead of appearing as ghosts. Her records show hereditary persistence of fetal hemoglobin, with hemoglobin F 22%. Her blood volume is taken as 5,000 mL, and each 300 µg vial covers 30 mL of fetal whole blood. What is the best next step?

   1. AWithhold RhIG, as the stained cells are maternal
   2. BGive 3 vials based on the 1.0% Kleihauer-Betke result
   3. CGive a single 300 µg vial without further testing
   4. DMeasure fetal D-positive cells by flow cytometry

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   Answer: D · Measure fetal D-positive cells by flow cytometry

   In hereditary persistence of fetal hemoglobin, the mother's own red cells contain HbF, so they resist acid elution and stain on the Kleihauer-Betke slide. Fetal cells can no longer be counted reliably, and the 1.0% result (50 mL of fetal blood, so 3 vials) cannot be trusted. Because the mother is D-negative and the baby D-positive, flow cytometry with anti-D counts only the fetal cells; anti-HbF flow can often separate brightly stained fetal cells from dimly stained maternal cells. The positive rosette test shows D-positive fetal cells are present, so RhIG must not be withheld. A single vial is not enough until the bleed is measured, because a positive rosette test means the bleed may be larger than one vial covers.

   • A: The positive rosette test shows that D-positive fetal cells are present.
   • B: The count is unreliable because the mother's own cells contain HbF.
   • C: A positive rosette test means the bleed may exceed what one vial covers.

6. Question 6 · Hard

   A Kleihauer-Betke test on a D-negative mother shows 1.2% fetal cells. Her blood volume is taken as 5,000 mL. How many 300 µg vials of Rh immune globulin are indicated?

   1. A2
   2. B4
   3. C3
   4. D1

   Answer and explanation in LearnFree account