A 20-year-old woman, G3, P2, has a screening ultrasound at 18 weeks' gestation that shows hydrops fetalis but no malformations. The woman's two previous pregnancies ended at term in live births. The current pregnancy results in a live birth at 36 weeks. Physical examination shows marked hydrops of the neonate and placenta. Laboratory studies show a cord blood hemoglobin level of 9.2 g/dL and total bilirubin concentration of 20.2 mg/dL. Which of the following laboratory findings is most likely to be present in this case?