Case 97

1. Presented by Sintawat Wangsiricharoen, MD and reviewed by Lorraine N. Blagg, MA, MLS(ASCP)SBB

An adult patient presented for a routine prenatal visit during the first trimester of pregnancy. A sample was received for type and screen.

Blood bank results:
Antibody identification:
Paternal genotype: K/k

Amniocentisis was not performed. The Anti-K titration results are as follows.
Middle cerebral artery peak systolic velocity (MCA-PSV) was performed during the second trimester and was 1.58 MoM (normal range <1.5 MoM). Cordocentesis was performed to assess fetal hematocrit.
Fetal sample:
Genotype: heterozygous KEL*K/KEL*k; predicted phenotype K/k

What is the most likely explanation of the discrepancy between phenotype and genotype?