WHAT IS HDFN DIAGNOSIS?
Antibodies screening test
Any woman, even before pregnancy, should carry out the determination of the blood type and should know if it’s Rh negative or Rh positive.
If it’s Rh negative, it should be subjected to anti-Rh D antibody test (Coombs indirect test). Their presence indicates that the woman has been exposed to Rh positive blood and is at risk of immunization. If the woman is Rh negative and has no antibody D, even the child’s father needs to be studied for the Rh group.
If the father is also Rh negative, the child will probably not be Rh positive and therefore there is no risk of incompatibility.
If, on the other hand, the dad is Rh positive, the child will have about 75% chance of being Rh positive and consequently is at risk of Rh incompatibility and possible Hemolityc Disease of the Fetus and Newborn. If the antibody detection is positive, after the identification of the antibody type, the title should be repeated several times during pregnancy. The higher the antibody levels are the higher the risk of an hemolytic anaemia for the fetus.
Detect fetus RH
It has been seen that during pregnancy, the fetal DNA released by the placenta’s trophoblast is present in the maternal plasma and, thanks to the development of new molecular biology techniques, it is now possible to determine, by a non-invasive method in maternal blood the genotype Fetal RH, avoiding more risky methods, such as amniocentesis (Lo, 1994). (Amniocentesis consists in taking a sample of amniotic fluid, with a needle, through the abdomen and the wall of the uterus)
The diagnostic accuracy of fetal genotype determination reaches elevated levels after the eleventh week of gestation with a sensitivity greater than 99% and a specificity of about 95% (Chitty et al., 2014). The incidence of false negatives is low after the 11th week of gestational age, but remains stable over the next few weeks because the concentration of fetal DNA in maternal blood increases only marginally between 10 and 20 weeks of gestational age (Chitty et al., 2014).
Prenatal diagnosis of HDFN
To evaluate the severity of HDFN Rh and the amount of haemolysis in the past was usual recurring to the spectrophotometric analysis of the amniotic fluid. This examination, introduced in the 1960s, is based on the determination of bilirubin absorption peak. This method is invasive and requires, for a reliable pregnancy monitoring, the performance of repeated amniocentesis, which may however result in an increased risk of further immunological stimuli with an increase in the antibody title.
More recently, with the aim of discovering always more precise and possibly non-invasive methods to diagnose the health of the fetus, an ultrasound study was conducted, based on the determination of the peak velocity of the systolic blood flow to identify a state of fetal anaemia. This measurement is performed by studying the average cerebral artery of the fetus, which is easy to visualize (Mari, 2005)
This method over time has completely suppressed the spectrophotometric examination of the amniotic fluid. It’s currently considered reliable and valid until the gestational age of 35 weeks. It must be associated, of course, with a general ultrasonographic assessment to identify the presence of severe HDFN, such as hydrops foetalis, ascites, pericardial effusion, and edema.
Newborn clinical tests
At the delivery of a newborn with suspected or certain maternal-fetal incompatibility for the Rh group, the following exams should be performed:
- Blood-test type to confirm incompatibility.
- Direct Coombs Test, that consents to individuate the presence of all antibodies, directly attached to the surface of red blood cells.
- A complete hemocromocytometric examination, with emoglobin levels and platelet determination. A complication of HDFN is neutropenia and thrombocytopenia
- The count of reticulocytes. The count is high when the infant produces more red blood cells to fight the hemolysis-induced anemia.
- Indirect and direct bilirubin levels in the umbilical cord blood.
Chitty LS, Finning K, Wade A, et al. Diagnostic accuracy of routine antenatal determination of fetal RHD status across gestation: population based cohort study. BMJ 2014;349:5243-49.
Lo Y. Non-invasive prenatal diagnosis using fetal cells in maternal blood. J Clin Pathol 1994;47:1060-5.
Mari G. Middle cerebral artery peak systolic velocity is it the standard of care for the diagnosis of fetal anemia? J Ultrasound Med 2005;24:697-702.