A. After exposure to acid, only fetal hemoglobin remains, such that after staining, the fetal erythrocytes appear red and adult erythrocytes appear as "ghosts" (Fig. 15-3). he fetal cells are then counted and expressed as a percentage of adult cells. he KB test is labor intensive. Importantly, there are two scenarios in which it may not be accurate: (1) maternal hemoglobinopathies such as �-thalassemia in which the fetal hemoglobin level is elevated and (2) pregnancies at or near term, when the fetus has already started to produce hemoglobin A.
he volume of fetomaternal hemorrhage is calculated from the KB test result using the following formula:
One method is to estimate the maternal blood volume (MBV) as 5000 mL for a normal-size, normotensive women at term. Thus, for 1.7 -percent positive KB-stained cells in a woman of average size with a hematocrit of 35 percent and whose fetus has a hematocrit of 50 percent: 5000 X 0.35 X 0.017 0.5 he fetal-placental blood volume at term approximates 125 mLlkg. For a 3000-g fetus, that would equate to 375 mL. Thus, this fetus lost approximately 15 percent (60 -375 mL) of the fetal-placental volume. Because the hematocrit is 50 percent in a term fetus, this 60 mL of whole blood represents 30 mL of red cells lost into the maternal circulation. This loss should be well tolerated hemodynamically but would require two 300-lLg doses of anti-D immunoglobulin to prevent alloimmunization. A more precise method to estimate the maternal blood volume includes a calculation based on the maternal height, weight, and anticipated physiological maternal blood volume accrual (Table 41-1, p. 756).
Fetomaternal hemorrhage can also be quantified using low cytometry, which uses monoclonal antibodies to hemoglobin F or to the D antigen, followed by quantiication of luorescence (Chambers, 2012; Welsh, 2016). Flow cytometry is an automated test that can analyze a greater number of cells than the KB test. Further, it is unafected by maternal levels of fetal hemoglobin or by fetal levels of hemoglobin A. Flow cytometry has been reported to be more sensitive and accurate than the KB test, however, it uses specialized technology not routinely available in many hospitals (Chambers, 2012; Corcoran, 2014; Fernandes, 2007).
This condition is also referred to as neonatal alloimmune throm cytopenia (FNAIT). Alloimmune thrombocytopenia (AIT) is the most common cause of severe thrombocytopenia among term newborns, with a frequency of 1 to 2 per 1000 births (Kamphuis, 2010; Pacheco, 2013; Risson, 2012). FNAIT is caused by maternal alloimmunization to paternally inherited fetal platelet antigens. he resulting maternal antiplatelet anti bodies cross the placenta in a manner similar to red cell allo immunization (p. 301). Unlike immune thrombocytopenia, the maternal platelet count is normal with FNAIT. And, unlike anti-D alloimmunization, severe sequelae may afect the initial at-risk pregnancy.