Non-invasive Tests to Predict Fetal Anemia in Kell-alloimmunized Pregnancies

Abstract

Marked fetal anemia may develop if there are antibodies to the Kell group of antigens, and may lead to hydrops and fetal death. Adverse outcomes can be prevented if timely intervention is possible. The accepted test for fetal anemia, fetal blood sampling (FBS), itself carries a risk of fetal loss and may promote fetal–maternal hemorrhage, increasing the severity of alloimmunization. The most reliable noninvasive tests for predicting fetal anemia in RhD disease are Doppler studies of blood flow velocity in the fetal middle cerebral artery (MCA) and intrahepatic umbilical vein (IHUV). This study evaluated Doppler parameters and ultrasound in a prospective series of 27 fetuses at risk of anemia because of Kell alloimmunization. The perimeter of the spleen, length of the liver, peak systolic velocity in the MCA, and maximum velocity in the IHUV were estimated. Fetal hemoglobin was measured at the initial intrauterine blood sampling or at delivery. Severe fetal anemia was considered present if there was a hemoglobin deficit of 5 or more standard deviations below the mean for gestational age. It proved possible to measure the fetal liver and spleen in 22 of the 27 fetuses. Whereas 21 women had only Kell antibodies, 6 had anti-Kell with additional non-D alloantibodies. In 7 pregnancies, Kell alloimmunization probably resulted from transfusing unmatched blood rather than developing in the current or a previous pregnancy. FBS confirmed severe anemia in all but one of 18 fetuses having the procedure, making intrauterine transfusion necessary. Hydrops was evident in 10 of these fetuses. Flow velocities in the MCA and IHUV were the best predictors of fetal anemia; both had a sensitivity of 89%. Sensitivity was only 15% for splenic perimeter and 14% for liver length. After excluding fetuses with signs of hydrops, sensitivity was 89% for the MCA, 78% for IHUV, 29% for spleen parameter, and zero for liver length. Specificity was 89% for the MCA, 56% for the HUV, 89% for the spleen, and 75% for the liver. These figures did not differ significantly from those for the entire group. The best available noninvasive tests for predicting fetal anemia in Kell-alloimmunized pregnancies are Doppler measurements of peak systolic velocity in the MCA and maximum velocity in the IHUV.