HDFN: detection

6 ASCP MLS practice questions on HDFN: detection, 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 · Medium

   Once a maternal anti-D titer reaches the critical level, how is the fetus usually monitored for anemia?

   1. ARepeated maternal antibody titers alone
   2. BAmniotic fluid bilirubin from 10 weeks
   3. CMiddle cerebral artery Doppler
   4. DWeekly cordocentesis

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   Answer: C · Middle cerebral artery Doppler

   An anemic fetus has thinner blood that flows faster, so a raised middle cerebral artery peak systolic velocity on Doppler ultrasound signals anemia. This noninvasive test has largely replaced amniocentesis for bilirubin. The critical anti-D titer is usually 16 or 32, depending on the laboratory. Cordocentesis is kept for confirming anemia and giving transfusion.

   • A: Titers above the critical level do not track fetal anemia well.
   • B: Amniotic fluid bilirubin was used later in pregnancy and is now largely replaced.
   • D: Cordocentesis is invasive and carries risk, so it is not used for routine monitoring.

2. Question 2 · Medium

   How should a new antibody titer in pregnancy be compared with earlier results?

   1. ATest the new sample at room temperature only
   2. BTest the previous frozen sample in parallel
   3. CUse a different method each time
   4. DCompare it with the last reported titer

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   Answer: B · Test the previous frozen sample in parallel

   Titers vary with technique and reagent cells, so the previous sample is kept frozen and tested side by side with the new one. A rise of fourfold or more (two tubes) is considered significant. The same method and cells should be used each time. Gel or tube methods give different titers and are not interchangeable.

   • A: Clinically significant IgG antibodies are titrated at the antiglobulin phase.
   • C: Changing methods makes titers impossible to compare.
   • D: Comparing with a reported number ignores day-to-day variation.

3. Question 3 · Easy

   A pregnant woman's antibody screen is positive. What should follow?

   1. AGive Rh immune globulin straight away
   2. BTest the father's plasma for antibodies
   3. CIdentify it, then titrate if significant
   4. DRepeat the screen only after delivery

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   Answer: C · Identify it, then titrate if significant

   The antibody must be identified, because only some antibodies can harm the fetus. For clinically significant antibodies, such as anti-D, anti-c or anti-K, a titer is done and repeated during pregnancy. Typing the father for the antigen, or the fetus by cell-free DNA, shows whether the fetus is at risk. Passive anti-D from antenatal Rh immune globulin must be recognized.

   • A: Rh immune globulin does not help a woman already immunized.
   • B: The father's red cell antigens matter, not his plasma antibodies.
   • D: Waiting until delivery misses fetal anemia that could be treated.

4. Question 4 · Hard

   A group O, D-negative mother has anti-D. Her group A, D-positive baby's DAT is 3+ with anti-IgG. The eluate reacts with D-positive cells and with A1 cells. What does this show?

   1. AOnly ABO HDFN is present
   2. BThe eluate was contaminated with residual plasma
   3. CThe baby has made its own autoantibody
   4. DBoth maternal anti-D and anti-A coat the cells

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   Answer: D · Both maternal anti-D and anti-A coat the cells

   A group O mother has IgG anti-A,B as well as anti-D, and both can cross the placenta to coat a group A, D-positive baby's cells. The eluate shows which antibodies are actually on the cells. Here both contribute to the hemolysis, although anti-D is likely the main cause. Red cells for the baby must be group O and D-negative.

   • A: The eluate also contains anti-D, so this is not ABO HDFN alone.
   • B: A properly washed eluate reflects antibody bound to the cells.
   • C: Newborns rarely make red cell antibodies; these are maternal.

5. Question 5 · Hard

   A D-negative woman's anti-D titer rises from 4 to 32 between 20 and 26 weeks. Her partner is heterozygous for RHD. What is the best next step?

   1. ARepeat the antibody titer at delivery
   2. BFetal RHD typing by cell-free DNA, then Doppler
   3. CGive Rh immune globulin to the mother
   4. DPlan only an exchange transfusion at birth, not before

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   Answer: B · Fetal RHD typing by cell-free DNA, then Doppler

   A heterozygous father gives a 50% chance that the fetus is D-negative and not at risk. Fetal RHD typing on maternal plasma cell-free DNA answers this without invasive testing. If the fetus is D-positive, the titer of 32 calls for middle cerebral artery Doppler monitoring. Severe anemia may need intrauterine transfusion long before birth.

   • A: Waiting until delivery risks missing severe fetal anemia.
   • C: She is already immunized, so Rh immune globulin will not help.
   • D: A fetus may need treatment before birth, not only at delivery.

6. Question 6 · Hard

   A 31-year-old woman, gravida 2, para 1, was transfused during abdominal surgery 4 years ago. At 16 weeks of her current pregnancy, her antibody screen is positive and anti-K is identified, with a titer of 8. Her first child, born before the surgery, was unaffected. Her partner types K+k+, and fetal genotyping on maternal plasma predicts a K-positive fetus. The center's critical titers are 16 for anti-D and 8 for anti-K. The obstetrician asks the laboratory how the fetus should be followed. What is the best plan?

   1. AStart serial MCA Doppler studies now
   2. BAmniocentesis for ΔOD450 bilirubin monitoring
   3. CBegin plasma exchange to lower the titer
   4. DRepeat the titer monthly; act when it reaches 16

   Answer and explanation in LearnFree account