The first useful marker is the ammonia level. Urea cycle defects have extremely elevated ammonia levels, sometimes in excess of 2000 ug/dL. Organic acidemias and benign transient hyperammonemia of the newborn (THAN) have ammonia elevations that can overlap, but are not usually as high as those found in urea cycle defects.

 

The next useful laboratory marker is the presence or absence of hypoglycemia. Infants with elevated ammonia levels in the presence of hypoglycemia have a reasonable likelihood of having an organic acidemia. Infants with hyperammonemia without hypoglycemia tend to have urea cycle defects. Hypoglycemia without hyperammonemia can signal a carbohydrate metabolism defect (e.g., galactosemia, defect in gluconeogenesis, or a glycogen storage disease) or a fatty acid oxidation deficiency in the older infant. 

source=http://www.hawaii.edu/medicine/pediatrics/pedtext/s04c04.html