Neonatal alloimmune thrombocytopenia due to anti-HPA-2b (anti-Ko^a)

Most severe cases of neonatal alloimmune thrombocytopenia (NAIT) are due to anti-HPA-la (anti-Pl^A1) antibodies. We report a case of NAIT due to anti-HPA-2b that resulted in in utero intracranial hemorrhage. A 33-year-old G2P1A0 Caucasian woman had a routine ultrasound at 34 weeks. The fetus appeared to have a left hemispheric hematoma. ΓVIG, lg/kg, was started immediately and administered weekly until delivery. One day after receiving the first dose of ΓVIG, fetal platelet count was 18 × 10⁹/L, and Hb was 116 g/L. Eleven mL of matched platelets compatible by monoclonal antibody immobilization of platelet antigens (MAIPA) assay were transfused in utero, raising the platelet count to 62 × 10⁹/L. Repeat transfusions were done later that week and 1 week later, with pretransfusion counts of 19 × 10⁹/L and 16 × 10⁹/L, respectively. Delivery by C section was done at 35.5 weeks, after the third platelet transfusion. Platelet count at birth was 77 × 10⁹/L. Drainage of the hematoma was performed after transfusion. Testing with a solid phase ELISA revealed reactivity against GPlb/IX. MAIPA testing after platelet treatment with the protease inhibitor leupeptin demonstrated the presence of anti-HPA-2b. On PCR-SSP the mother was HPA-2a homozygous, the father was HPA-2a/2b. Antibodies against the HPA-2b antigen located on the GPlb/IX complex have been reported in rare cases of NAIT Testing is complicated by proteolytic degradation of the antigen-bearing fragment. Compatible platelets are easily found since approximately 85 percent of donors are HPA-2a/2a. Immunohematotogy 2003;19:43-46.