What is the purpose of an antibody screen in a 28-year-old pregnant woman?

Antibody Screen in Pregnancy

An antibody screen in a 28-year-old pregnant woman is a blood test performed to detect irregular red blood cell antibodies that could cause hemolytic disease of the fetus and newborn (HDFN), which can lead to fetal anemia, hydrops fetalis, and even fetal death. 1

Purpose and Clinical Significance

The antibody screen serves two critical functions:

• Identifies maternal alloantibodies that can cross the placenta and destroy fetal red blood cells, potentially causing severe complications including hypoxia, heart failure, and death 2
• Enables early referral of at-risk mothers to obstetricians experienced with high-risk pregnancy care, allowing for appropriate monitoring and intervention 1

Standard Testing Protocol

All pregnant women should undergo ABO blood type, RhD typing, and antibody screening during the first trimester. 1

For RhD-Negative Women (Additional Requirements)

• Repeat antibody screening in the third trimester (preferably before 36 weeks' gestation) is recommended 3
• Rh immune globulin (RhIG) administration at approximately 28 weeks' gestation to prevent alloimmunization 4
• Fetal-maternal hemorrhage (FMH) testing at delivery and following any antepartum events that could cause bleeding (amniocentesis, abdominal trauma, threatened abortion) to determine appropriate RhIG dosing 4, 1

For RhD-Positive Women

• Routine antibody screening is performed but the clinical yield is extremely low 5
• In one study of 522 RhD-positive pregnant women screened, only 0.2% had a positive antibody screen 5
• Despite low incidence, screening remains standard practice to detect rare but clinically significant antibodies 1

High-Risk Populations Requiring Enhanced Surveillance

Repeat third-trimester screening is specifically indicated for women with: 3

• Residence in states with high HIV prevalence in women aged 15-45 years
• Delivery in hospitals with HIV prevalence ≥1 per 1000 pregnant women
• Sexually transmitted infection diagnosed during pregnancy
• Injection drug use or partners who inject drugs
• Multiple or new sexual partners during pregnancy
• Signs or symptoms of acute HIV infection

Clinically Significant Antibodies

When antibodies are detected, the specific type determines management intensity:

• Anti-D antibodies are most common and can be rapidly identified using RhD-negative screening cells 6
• Anti-Kell, anti-c, anti-E, anti-C can cause severe HDFN requiring intensive fetal monitoring 2, 7
• Anti-M antibodies are naturally occurring and usually require less intensive monitoring unless titers are high, as they account for fewer than 15 reported cases of HDFN 2
• Rare antibodies (such as anti-Kpb) may require specialized blood products for transfusion if needed 7

Critical Timing Considerations

Early identification is essential because: 1

• Allows quantification of antibody titers to assess risk level
• Enables timely referral to maternal-fetal medicine specialists
• Permits planning for potential intrauterine transfusions or early delivery
• Ensures availability of compatible blood products if neonatal transfusion is needed

Common Pitfalls to Avoid

• Never assume previous pregnancy history is accurate - one case report described a child's death attributed to "cot death" that was likely undiagnosed erythroblastosis fetalis from anti-Kell antibodies 7
• Do not skip screening in subsequent pregnancies even if previous pregnancies were uncomplicated, as antibodies can develop between pregnancies 7
• Ensure proper follow-up when antibodies are detected - failure to take precautions in a twin pregnancy with known anti-Kpb antibodies put both mother and children at risk 7
• Document all RhIG administration as passive anti-D from immunoprophylaxis will cause positive antibody screens and must be distinguished from true alloimmunization 6

Laboratory Testing Algorithm

When antibody screen is positive: 6

• For RhD-negative patients with reactive screening: test with RhD-negative reagent red cells first
• If no reaction with RhD-negative cells: identify as anti-D only
• If reaction with RhD-negative cells: perform full antibody panel to identify non-D antibodies
• Quantify antibody titers to establish baseline and monitor for rising titers throughout pregnancy