Background: Severe cases of HDN occur after the immunization of the mother with K (KEL1) antigen. To date, the only means of evaluating the concentration of anti-K in maternal serum is by titration with an indirect antiglobulin test (IAT). A more accurate estimation of the serum anti-K concentration is needed.
Study design and methods: An ELISA technique was developed for the determination of the absolute concentration of anti-K IgG and IgG subclasses in the sera of alloimmunized patients. In this technique, after absorption of anti-K on K-positive RBCs and subsequent elution at acid pH, the concentration of anti-K in the eluate was measured with a sensitive and reproducible ELISA. This method was validated with monoclonal and polyclonal anti-K. It was then used to assay the sera of eight pregnant women with anti-K immunization, associated with early fetal anemia (Hct, 7-17%) detected between the 20th and the 31st week of pregnancy. In addition, in most of these cases, the anemia was associated with fetal hydrops.
Results: The anti-K IgG concentration measured by ELISA in the sera of the eight women varied from 1.0 to 4.1 microg per mL (mean, 2.2 microg/mL). Therefore, severe and early forms of fetal anemia can be observed with a relatively low concentration of anti-K (as compared to the concentration of anti-D in similar cases of fetal anemia due to anti-D). The mean proportion of each IgG subclass of anti-K in these sera was IgG1, 95.9 percent; IgG2, 2.4 percent; IgG3, 1.3 percent; and IgG4, 0.4 percent.
Conclusion: A simple method for quantitative estimation of anti-K in human serum has been developed. Low concentrations of anti-K can cause fetal anemia relatively early in pregnancy. This method should lead to a better identification of pregnant women whose fetuses are at risk for severe fetal anemia due to anti-K.