No Apnea Or Cyanosis in Pediatrics — Case | Neonatology | SMLE Q#15367

A full-term 2-day-old male newborn is evaluated in the newborn nursery for intermittent jitteriness and tremulous movements of the arms and legs. The episodes are stimulus-sensitive and stop when the limbs are gently held. He was born via uncomplicated vaginal delivery to a healthy 27-year-old G1P1 mother. Pregnancy and labor were uneventful, and there is no history of maternal diabetes or substance use. The infant is feeding poorly today and appears mildly irritable but otherwise alert, with no apnea or cyanosis. Vital signs: T: 36.8 °C, BP: 68/40 mmHg, HR: 150/min, RR: 40/min, SpO2: 98% (room air). Physical examination shows normal tone and no dysmorphic features. Which laboratory value should be checked first in this infant?