a. Hypoglycemia. Ketotic hypoglycemia is seen in endocrine disorders, some organic acidemias, primary lactic acidoses, and some glycogen storage diseases. Hypoketotic or ake-totic hypoglycemia is seen in hyperinsulinism. Hypoglycemia is seen in metabolic disorders, including type I ("classic") glycogen storage disease (Von Gierke), and is the hallmark of FAO defects. When considering hypoglycemia due to energy production disorders, the length of the fast may be helpful: glycogen is a fuel that is necessary shortly after meals (~3-4 hours); fatty acid metabolism is the next obligatory fuel (~4-8 hours); and gluconeogenesis is utilized thereafter. Prolonged fast or intercurrent vomiting and diarrheal illness is typical of hypoglycemia with FAO defects; whereas a short fast (3-4 hours) may result in hypoglycemia in patients with glycogen storage disease. Fasting tolerance increases with age.

b. Secondary hyperglycemia. Can also accompany organic acidemias. Ketosis may be seen in these disorders as well, making the presentation difficult to distinguish from neonatal diabetic ketoacidosis.

2. Urine ketones. Neonates make and use ketones highly efficiently, so they are a rare finding before 2-3 months of age.

Ketosis in a neonate suggests an organic acidemia. Outside of the neonatal period, inappropriate ketones in the face of a normal or elevated blood glucose level suggests organic acidemia. Conversely, absence of ketones in a hypoglycemic child suggests glycogen storage disease and FAO defects. See earlier discussion.

3. Electrolytes. Low bicarbonate suggests acidosis. ABGs should be obtained to confirm this, because hyperpnea caused by hyperammonemia can result in hypocarbia and compensatory renal wasting of bicarbonate.

4. ABGs. Metabolic acidosis is typically seen in acutely ill neonates, often due to lactic acidosis with respiratory or circulatory compromise. Organic acidemias or lactic acidosis from metabolic disease should be considered. Respiratory alkalosis is unusual in an acutely ill child and is typical of the primary urea cycle defects (see III, C, 3, earlier).

5. Anion gap. Calculated as follows: Na - (Cl + HCO3); normal anion gap is 12-15. In confirmed acidosis, an elevated anion gap is seen the presence of an unmeasured ion, such as an organic acid, lactate, excessive ketones, or toxic ingestion.

6. Blood ammonia. Typically significantly elevated in primary urea cycle defects. May be secondarily elevated in organic acidemias. Mild to modest elevations can be seen in FAO defects or primary lactic acidosis.

7. CBC. Elevated WBC count can suggest infection. Bone marrow suppression can occur in some organic acidemias and severe infections.

8. Liver function tests. May be elevated in many metabolic disorders (see IV, A, 3, earlier).

9. BUN. In urea cycle disturbances (primary or secondary), patients are unable to make urea; therefore, BUN is low even in the presence of dehydration.

10. Lactic acid. Can be elevated in tissue hypoxia from sepsis, seizure, and trauma. Often excessive in mitochondrial disease, primary lactic acidoses, and glycogen storage diseases.

11. Pyruvate. Lactate and pyruvate are in equilibrium, depending on the redox potential of the cell. In lactic acidosis, pyruvate elevations and lactate-to-pyruvate ratios may help to localize the enzymatic defect. These levels should be obtained simultaneously.

12. Uric acid. May be elevated in energy-deficient states such as the primary lactic acidoses, FAO defects, and glycogen storage diseases. Often excessive in glycogen storage diseases due to both overproduction and underexcretion.

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