10 hours after birth a child developed jaundice, hypotonia, hyporeflexia, and moderate hepatosplenomegaly. Feces and urine are of normal color. Umbilical cord blood bilirubin is 51 mcmol/L due to unconjugated bilirubin levels. In venous blood: erythrocytes – 3.5 × 1012/L, Hb- 140 g/L. reticulocytes – 1.5%, bilirubin – 111 mcmol/L, conjugated – 11 mcmol/L, ALT-40 U/L, AST- 30 U/L. Mother’s blood group is A(II) Rh(-), child’s blood group is A(II) Rh(+). What laboratory test can confirm the diagnosis?