Antibody Screen Ordering for Pregnant Women
Order an "Indirect Coombs Test" or "Antibody Screen" on the lab requisition form for routine prenatal antibody screening in pregnant women. This test detects circulating maternal antibodies against red blood cell antigens that could cause hemolytic disease of the fetus and newborn 1, 2, 3.
Understanding the Correct Test
- The indirect Coombs test (antibody screen) is the appropriate test for pregnant women to identify alloimmunization by detecting circulating antibodies in maternal serum 2, 3.
- This differs from the direct Coombs test, which detects antibody-coated red blood cells and is used for suspected hemolysis in the mother or newborn, not for routine prenatal screening 1, 2.
- The indirect Coombs test should be performed as part of routine prenatal care along with blood type determination and Rh(D) antigen status 3.
When to Order
- First trimester: Perform initial antibody screen on all pregnant women regardless of Rh status 1, 3, 4.
- Third trimester: Repeat antibody screen at 28 weeks gestation, particularly for Rh-negative women 1.
- After delivery: Test cord blood with direct Coombs if mother is Rh-negative or has no prenatal typing 1.
What Happens After Ordering
If the antibody screen is positive:
- The laboratory will automatically perform antibody identification to determine the specific antibody (e.g., anti-Kell, anti-E, anti-c, anti-M) 2, 5.
- Serial antibody titers should be measured to track antibody levels over time, with increasing titers suggesting higher risk of fetal anemia 1, 2.
- When titers reach critical levels, initiate middle cerebral artery Doppler monitoring 2.
- Detailed ultrasound with fetal echocardiography may be needed to evaluate for fetal anemia 2, 3.
Common Pitfalls to Avoid
- Don't confuse direct and indirect Coombs tests: The indirect test screens maternal serum for antibodies; the direct test detects antibodies already coating red blood cells 2, 3.
- Don't skip screening in Rh-positive women: Although the yield is low (0.2% positive rate), antibodies other than anti-D can cause hemolytic disease 6, 4.
- Don't misinterpret positive screens after Rh immune globulin: Anti-D antibodies can be detected for 4-11 weeks after antepartum Rh prophylaxis using sensitive methods, but this doesn't necessarily indicate true sensitization 7.
- Maternal direct Coombs is rarely indicated: Only order if the mother has clinical evidence of active hemolysis, suspected autoimmune hemolytic anemia, or transfusion reaction 2.
Special Considerations
- For Rh-negative mothers: A positive antibody screen requires identification of whether anti-D antibodies are present, which determines need for Rh immune globulin and intensity of fetal monitoring 1, 2.
- Low-titer antibodies (such as anti-M) may not require intensive surveillance if levels remain stable, though guidelines traditionally call for close monitoring 5.
- Extended red cell antigen profiling facilitates antibody identification and compatible donor unit selection if transfusion becomes necessary 2.