Incidence of a Type AB Infant Born to a Type O Mother

Abstract
Routine cord blood testing may include ABO,
Rh, Direct Antigen Testing (DAT) and Indirect
Coombs testing (IDC), among other testing, but
varies widely from blood bank to blood bank. In
this case, the routine testing revealed that an
AB infant was born to an O mother. It is not
routine to find an AB infant born to an O
mother, as the mother carries an O,O type and
would have to pass an O to the infant, leaving
the possibilities of O,O; A,O; or B,O type for the
infant, depending on the father’s blood type.
Because of the rarity of an AB infant being
born to an O mother, several troubleshooting
steps were taken prior to the release of test
results. It is important to investigate whenever
there is a discrepancy discovered during
laboratory testing. An AB and O couple
producing an AB child cannot be explained in
terms of the usual inheritance patterns. One
would not expect the cord blood result to be
AB when the mother is type O, however, in very
rare instances, such as the cis-AB blood type,
it is possible. Extensive troubleshooting
revealed the infant to have this very rare cisAB blood type.