How concerning is a 15.3 bilirubin level jaundice in a new born?

Hyperbilirubinemia. Bilirubin levels are interpreted in context with other factors such as age, previous therapy, bilirubin level trends, response to therapy and other conditions such as organ disease, immunological issues (ABO incompatability), hemolysis, etc. Please provide more specific information for a more complete answer.
High bilirubin. Depends when it was drawn, whether the baby is full term, whether the baby is being breast or bottle fed, whether there were any problems with the pregnancy or delivery.