Which laboratory test should be prioritized in addition to sending a newborn screen for a 2-week-old presenting with jaundice?

In the context of a 2-week-old infant presenting with jaundice, prioritizing the serum ammonia test is significant due to the potential for liver dysfunction or metabolic conditions that can lead to an elevated ammonia level. This is crucial in this age group because congenital conditions such as urea cycle disorders can manifest with jaundice and may lead to life-threatening hyperammonemia.

While total and direct bilirubin measurements are also important in evaluating the cause of jaundice, they are not as immediate in terms of ruling out a potentially serious condition as serum ammonia is. Elevated bilirubin levels in this setting can indicate conditions such as physiological jaundice, breastfeeding jaundice, or hemolytic disease, but they do not provide insight into metabolic derangements that can be indicated by elevated ammonia levels.

In contrast, serum glucose and serum sodium may be helpful in other contexts but are less directly related to the immediate priorities concerning the risks associated with jaundice in a newborn. The focus on serum ammonia in this scenario underscores the need for careful assessment of metabolic function and the risk of acute decompensation in an infant presenting with jaundice.