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Hemolytic disease of the fetus and newborn

17 min

Skills you'll build3 skills
  • Explain how maternal IgG and newborn bilirubin clearance produce HDFN findings
  • Recognize suppression of erythropoiesis in anti-K-associated fetal anemia
  • Call a maternal titer change only when the same method retests the earlier sample

Try first

Try first

A mother has anti-D, and her D-positive baby's cord bilirubin is 3.4 mg/dL. At 18 hours of age the bilirubin is 11.8 mg/dL and still rising. Why does bilirubin rise after delivery?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Mechanism of HDFN

Only IgG crosses the placenta. The neonatal Fc receptor transports it, and transfer rises through the second trimester to a peak near term.1 IgG against an antigen the fetus inherited from the father coats fetal red cells. Macrophages in the fetal spleen clear the coated cells, and anemia follows.2

Before birth, unconjugated bilirubin from the hemolysis crosses the placenta, and the mother's liver conjugates it. After delivery that route ends. The newborn liver conjugates poorly, so bilirubin rises in the first hours and days.2,3 Maternal IgG has a half-life of about 25 days, so hemolysis continues after birth.1

Anti-K and suppressed red-cell production

Kell antigens are present on erythroid precursors in the marrow. Anti-K suppresses red-cell production as well as destroying circulating cells.4 Less hemolysis produces less bilirubin, and the anti-K titer correlates less predictably with fetal anemia. Both can underestimate the severity of fetal anemia. The laboratory reports anti-K to the obstetric service promptly.5,6 A 2025 guideline recommends a critical titer of 4 or more for anti-K and 16 or more for most other antibodies. Each laboratory validates the critical titer for its own method.6

Monitoring antibody titers

The titer is the reciprocal of the highest dilution that still reacts. It reflects the amount of antibody available to cross the placenta. Titers from different methods or laboratories are not interchangeable.5 To assess a change:

  1. Titrate the new sample by the laboratory's validated method.
  2. Titrate the stored earlier sample alongside it, in the same run.
  3. Count the doubling dilutions between them. A fourfold change is two doubling dilutions, for example 16 to 64.
  4. Compare the change with the laboratory's significant-change criterion.

Cold-reacting IgM antibodies such as anti-Lea, anti-Leb, anti-I and anti-P1 usually need no HDFN follow-up.2

Follow the bench demonstration

Sources for this step
References
  1. Palmeira P, Quinello C, Silveira-Lessa AL, Zago CA, Carneiro-Sampaio M. IgG placental transfer in healthy and pathological pregnancies. Clin Dev Immunol. 2012;2012:985646. doi:10.1155/2012/985646
  2. Bloch EM, Campbell-Lee S, McKenna DH Jr, Montemayor-Garcia C, Schwartz J, Shaz B, Storry J, eds. Technical Manual. 22nd ed. AABB; 2026.
  3. Kemper AR, Newman TB, Slaughter JL, et al. Clinical practice guideline revision: management of hyperbilirubinemia in the newborn infant 35 or more weeks of gestation. Pediatrics. 2022;150(3):e2022058859. doi:10.1542/peds.2022-058859
  4. Vaughan JI, Manning M, Warwick RM, Letsky EA, Murray NA, Roberts IAG. Inhibition of erythroid progenitor cells by anti-Kell antibodies in fetal alloimmune anemia. N Engl J Med. 1998;338(12):798-803. doi:10.1056/NEJM199803193381204
  5. Regan F, Veale K, Robinson F, et al. Guideline for the investigation and management of red cell antibodies in pregnancy: a British Society for Haematology guideline. Transfus Med. 2025;35(1):3-23. doi:10.1111/tme.13098
  6. Moise KJ Jr, Markham KB, Spinella PC, et al. A clinical practice guideline for the management of pregnancy alloimmunized to red blood cell antigens. JAMA Netw Open. 2025;8(11):e2544649. doi:10.1001/jamanetworkopen.2025.44649

Watch one

Read each decision and its explanation.

Priya Geller, 31, MRN 4418203, is referred at 24 weeks of pregnancy with anti-c. The referring hospital reported a titer of 16 at 18 weeks by its own tube method, and sent that sample with the referral. Your laboratory titrates today's sample and gets 64. Its validated critical titer is 16, and its procedure counts a fourfold change as significant.

Has the titer risen significantly?

  1. Confirm the specificity. Anti-c reacts at the antiglobulin phase and is a recognized cause of HDFN, so it is titrated.

    Titration and follow-up are for IgG antibodies that can cause HDFN.

  2. Check where each titer came from. The 16 is another laboratory's result by another method, so it cannot be compared with today's 64.

    Titers from different methods or laboratories are not interchangeable.

  3. Thaw the stored 18-week sample and titrate it alongside today's. The 18-week sample reacts through 1:32, a titer of 32. Today's sample again gives 64.

    A parallel titration tests both samples by the same method in the same run.

  4. Count the steps. From 32 to 64 is one doubling dilution, a twofold change.

    Each doubling dilution is a twofold step.

  5. Report today's titer of 64 with the method and the parallel result. Both titers are at or above the critical titer of 16.

    The laboratory reports the titer and the method, and the obstetric service decides on fetal surveillance.

The titer has not risen significantly. Tested in parallel, it went from 32 to 64, a twofold change below the fourfold criterion. The apparent fourfold rise from 16 came from comparing two methods.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A patient at 26 weeks of pregnancy has a reactive antibody screen and a nonreactive autocontrol. Cross out what the panel rules out, then name the antibody.

Antibody panel: + the cell carries the antigen, 0 it lacks it
CellDCEceKkFyaFybJkaJkbMNSsP1LeaLebIS37AHG
1++00+0++0+0+0+0+0+000
2+0++00+0+0+++0+++00w+3+
30+0++++++++0+++00+000
4000++0+0+0+++0+++0000
5+00++0++++00++000+000
600+++0++0+++++++0+002+
7+0++0++++0++0+++000w+3+
8++0++0+0+++++0++0+000
9+++++0++0+++0++00+002+
10000+++++++++++++0+000
11+0+++0+0+0+++++++0002+
Autocontrol000
Hint
  1. Start with the nonreactive cells. For C, c, E, e, Kidd, Duffy and MNS, cross out only on a nonreactive cell that is homozygous for the antigen.
  2. Compare the reactive cells' strengths. Look for an antigen whose double-dose cells react more strongly than its single-dose cells.
Show the answer and the reasoning

Cell 1 is nonreactive and rules out D, C, e, k, Fya, Jka, M, S, P1, Leb.

Cell 3 is nonreactive and rules out e, K, k, N, Leb.

Cell 4 is nonreactive and rules out c, e, k, Fyb, Jkb, s, P1, Lea.

Cell 5 is nonreactive and rules out D, c, e, k, Jka, N, S, Leb.

Cell 8 is nonreactive and rules out D, e, k, Fyb, s, P1, Leb.

Cell 10 is nonreactive and rules out c, e, K, k, P1, Leb.

Anti-E

Every reactive cell is E-positive and every nonreactive cell is E-negative. The E+e− cells 2 and 7 react 3+, and the E+e+ cells react 2+, the dosage anti-E often shows. Anti-E is IgG and can cause HDFN, so it is titrated and followed. Type her red cells for E to confirm she is E-negative.

Review Antibody investigation

Problem 2 of 3 · Hints available

Why can anti-K produce severe fetal anemia with less bilirubin elevation than expected from hemolysis alone?

Incorrect. Anti-K can suppress erythroid precursors, reducing the marrow response.

Incorrect. The disease depends on maternal IgG reaching fetal circulation. Anti-K does not block that transfer.

Correct. Kell expression on erythroid precursors allows reduced production to contribute to anemia. Bilirubin trends can understate its severity.

Hint
  1. Ask where Kell antigens are carried besides mature red cells.
  2. Consider what fewer new red cells means for the amount of hemoglobin broken down to bilirubin.

Review Disease-associated antibody specificities

Problem 3 of 3 · On your own

A patient with anti-E transfers her care at 30 weeks. The referring hospital reported a titer of 8 by gel at 22 weeks and kept no sample. Your laboratory's tube method gives a titer of 32 today. How does the laboratory report the titer?

The two titers come from different laboratories and methods. The difference may be due to the change in method.

Compared titers from different methods as a true change

Titers from different methods or laboratories are not interchangeable, so a rise from 16 in one laboratory to 64 in another may reflect the difference in method. A fourfold change is significant only when the stored earlier sample is tested in parallel with the new one by the same method.

No fixed conversion links two methods. A change can be assessed only when both samples are tested by one method.

A gel method in another laboratory is still a different method and run. Only a parallel test of both samples shows a true change.

Without the earlier sample, no parallel test is possible. Today's sample is stored and titrated in parallel with the next one.

Review Antibody identification in pregnancy

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Lena Miller, 29, MRN 3307416, is 20 weeks into her second pregnancy. She has never been transfused.
  • Her antibody screen reacts at the antiglobulin phase, and her red cells type K-negative.
  • The laboratory titrates her antibody by its validated tube method and freezes each sample.
Antibody panel: + the cell carries the antigen, 0 it lacks it
CellDCEceKkFyaFybJkaJkbMNSsP1LeaLebIS37AHG
1++00+0++0+0+0+0+0+000
2+0++0++0+0+0+0+++00w+3+
30+0++0+0+0+0+0+0+0000
4000++0++++++++++0+000
5+0++00++++++++++0+000
600++++++0++++0+00+002+
7++0++0+++0++++++00000
8000++++0++0+0+++0+0w+3+
9++00+0+0+++0++0++0000
10++0+++++++0++++00+002+
11+00++0++00+++0++0+000
Autocontrol000
Decision 1 of 3

Her panel is below, and her titer is 8. What does the laboratory report?

Cell 3 carries Lea and cell 1 carries Leb, and both are nonreactive. The reactive cells 2, 6, 8 and 10 share K, and the reactions are at the antiglobulin phase.

Only the K-positive cells react, and she types K-negative. Anti-K suppresses fetal red-cell production, so a low titer can understate fetal anemia, and a 2025 guideline sets its critical titer at 4.

The specificity is correct. The threshold of 16 applies to most antibodies. Anti-K is reported promptly because its titer correlates less predictably with fetal anemia.

Judged anti-K severity by titer and bilirubin

Kell antigens are expressed on erythroid precursors, so anti-K suppresses red-cell production as well as destroying circulating cells. With less hemolysis there is less bilirubin, and the titer predicts the degree of anemia less reliably, so both can understate the severity of fetal anemia.

Review Disease-associated antibody specificities

Decision 2 of 3

At 26 weeks her new sample titrates to 32. The frozen 20-week sample, tested in the same run, gives 8. How does the laboratory report the change?

From 8 to 16 is one doubling dilution, and from 16 to 32 is a second. Two doubling dilutions make a fourfold change.

A rise in a titer already above the critical titer is still reported, because it shows the obstetric service that the antibody is increasing.

Both samples were tested by the same method in one run, so the change is valid. From 8 to 32 meets a fourfold significant-change criterion.

Review Antibody identification in pregnancy

Decision 3 of 3

Her baby is born at 37 weeks, K-positive with an IgG-positive DAT. The cord bilirubin is 2.9 mg/dL, and at 20 hours it is 10.6 mg/dL. Why was the cord value low?

Maternal IgG coated fetal cells for weeks before birth. Hemolysis was already occurring, and the placenta cleared its bilirubin.

Assumed bilirubin handling is unchanged after delivery

Before birth, unconjugated bilirubin from fetal hemolysis crosses the placenta and the mother's liver clears it. After delivery that route ends and the newborn liver conjugates poorly, so bilirubin rises after birth as maternal IgG keeps destroying red cells.

Cord blood is the baby's own blood. Its bilirubin reflects what remained after placental clearance.

The mother's liver conjugated the fetal bilirubin. After birth the newborn liver must clear it, and bilirubin rises as long as maternal anti-K persists.

Review Maternal antibody transfer to the fetal circulation

Because the antibody was anti-K, a titer of 8 was reported promptly to the obstetric service. The frozen 20-week sample allowed a parallel titration, so the later fourfold rise could be reported with confidence.

For reference

Sources checked