DOI: 10.1373/clinchem.2015.252320


5-week-old white boy presented to the emergency department after referral from his primary care physician with concerns for failure to thrive. The patient's birth weight was 2892 g at 37 weeks of gestation (10th percentile). He was 2665 g at 5 weeks of age (<second percentile). He had been exclusively breastfed for the first 3 weeks of his life.