Background: Pregnant women are routinely screened for red blood cell (RBC) antibodies early in pregnancy. If RBC-alloantibodies are detected, repeated laboratory testing is advised to timely identify pregnancies at high risk for severe hemolytic disease of the fetus and newborn (HDFN). We assessed for RBC alloantibodies, other than anti-D or anti-K, cut-offs for the titer and the antibody dependent cellular cytotoxicity (ADCC) test to select high-risk cases. To advise on test repeat intervals, and to avoid unnecessary testing, we evaluated the chance for exceeding the cut-offs for Rh antibodies other than anti-D, Jk, Fy, and S/s antibodies.
Study design and methods: Diagnostic value of antibody titer and ADCC test was determined with data from a prospective index-cohort study, conducted in 2002-2004. Laboratory test outcomes were from a recent observational cohort (2015-2016).
Results: A titer cut-off of ≥16 showed a sensitivity of 100% (95% CI:73-100%) and a positive predictive value (PPV) of 17% (95% CI:14%-20%). The percentage of pregnancies reaching a titer above the cut-off of ≥16 varied from 0% for anti-Jka /Jkb (n = 38) to 36% for anti-c (n = 97). The ADCC test showed no cut-off with a 100% sensitivity. However, in cases with a titer ≥16 and an ADCC test ≥30% a PPV of 38% was obtained to detect severe HDFN.
Conclusion: A titer cut-off of ≥16 is adequate to detect all cases at risk for severe HDFN; the ADCC test may add a more accurate risk estimation. Repeated testing is recommended in pregnancies with anti-c. In pregnancies with other Rh antibodies a repeated test in the third trimester is recommended.
© 2019 AABB.