(P-TS-131) Two Patients with Anti-Scianna 3 Alloantibodies in Pregnancy

Background/Case Studies: The Scianna blood group (Sc), described in 1962 and designated the 13th blood group system by the International Society for Blood Transfusion (ISBT), contains seven antigens. Sc1 is present in >99% of all populations; Sc2 occurs in ~1% of individuals of European ancestry. Sc3 is expressed on all Sc1 and 2 positive cells, making its absence rare. Anti-Sc3 antibodies have been reported in approximately six patients globally, typically with Sc:-1, -2 phenotypes. One case was identified during pregnancy. Anti-Sc3 is associated with hemolytic transfusion reactions and mild hemolytic disease of the fetus and newborn (HDFN). Most cases occurred in individuals of Pacific Islander descent.

Study

Design/Methods: A retrospective chart review from March 2024-May 2025 conducted at an academic medical center in Utah identified two pregnant patients (that notably had limited prenatal care) with anti-Sc3 antibodies. Clinical courses and outcomes are reported here.

Results/Findings: Patient 1: 28-year-old G4P1201 blood type A positive Pacific Islander woman presented at 34w5d with abdominal pain and no prenatal care. The immunohematology reference laboratory (IRL) identified anti-Sc3 (titer 2) and anti-Jkb (unable to titer). History included fetal demise at 22w5d, preeclampsia with severe features, and gestational diabetes. She underwent emergent Caesarean section at 39w2d with 1L estimated blood loss, for which she was not transfused.
The male infant was A positive, DAT positive, with a 2+ panreactive eluate identifying anti-Sc3. At delivery, the infant weighed 3645 kg, hematocrit was 54.9%, and total bilirubin was 4.0 mg/dL; bilirubin peaked at 15.3 mg/dL at day of life four. Phenotype confirmed presence of Sc3 and Jkb.
Patient 2: 42-year-old G17P11-2-3-1, blood type A positive Pacific Islander woman presented at 28w1d for fetal growth restriction. Her history included 11 vaginal births, three spontaneous abortions, and two preterm deliveries. Anti-Sc3 was detected at an external IRL (not titered). At 38w6d, she delivered a live male infant weighing 2438 kg, O positive, DAT positive, hematocrit 56.0%, and total bilirubin of 11.1 mg/dL. Both were lost to follow up.

Conclusions: These cases represent additional occurrences of anti-Sc3 in pregnancy in Pacific Islander women. One neonate, positive for Sc3 and Jkb, exhibited mild HDFN (elevated bilirubin but normal hematocrit). The second infant had mild hyperbilirubinemia, further evaluation was not possible. Findings add to the limited body of literature on clinical significance of anti-Sc3. Given the antibodies rarity and limited follow-up in these cases, its clinical significance remains unclear.