An 8-day-old former 38 weeks' gestation male was admitted to the neonatal intensive care unit for indirect hyperbilirubinemia and anemia. He was born to a 32-year-old mother (gravida 3, para 3) of Vietnamese descent. Baby and mother had the same blood type (B, Rh positive). There were no maternal medical problems or medications except prenatal vitamins and no complications with the pregnancy. The baby was born at 38 weeks via spontaneous vaginal delivery without complication; APGAR scores were 8 and 9 at 1 and 5 minutes, respectively. The infant was large for gestational age. He was discharged at 48 hours of life.

He went to his primary care physician for follow-up, and his weight was 4% below birth weight. He was breastfeeding well with formula supplementation with the expected number of wet diapers and stools. He had no signs or symptoms of infection. Due to jaundice on exam, labs were obtained and revealed a bilirubin level of 29.4 mg/dL (indirect 27.0 mg/dL), so he was referred for admission. Direct antiglobulin test (DAT) was negative. Complete blood count was notable for hemoglobin (Hb) 11.8 g/L with mean cell volume 103.7 fL. Reticulocyte count was 1.47%. Upon admission, he was started on intensive phototherapy, and hematology was consulted.

Based on the differential diagnosis, which of the following would be the next best step in your diagnostic workup?

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Alexandra Prosser, MD, and Lauren Amos, MD
Division of Pediatric Hematology/Oncology & Blood and Marrow Transplant
Children's Mercy Hospital, Kansas City, MO

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