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Acidosis. The differential diagnosis of metabolic acidosis is broad (see Chapter 100 Renal and
Electrolyte Emergencies ) but IEMs must be considered in patients with unexplained or greater-thanexpected metabolic acidosis. Clinical manifestations of acidosis are nonspecific and include vomiting and
tachypnea. Primary metabolic acidosis is diagnosed by low pH, low PCO 2 , and low bicarbonate. An
elevated anion gap (>16 mmol/L), which is characteristic of acute metabolic crisis with many IEMs,
helps distinguish among causes of metabolic acidosis. An elevated anion gap with a normal chloride
usually reflects excess acid production, most often of lactate, ketone bodies, and/or other organic acids.
Organic acidemias are characterized by metabolic acidosis, usually severe, with marked ketonuria, with
or without hyperammonemia or hypoglycemia. In neonates believed to have pyloric stenosis, the
diagnosis of IEM, particularly an organic acidemia, should be considered if the patient has metabolic
acidosis rather than metabolic alkalosis. Fatty acid oxidation disorders may also present with metabolic
acidosis, but usually with hypoglycemia and absent ketones or hypoketosis. The IEMs in which a primary
lactic acidosis is the cause of the metabolic acidosis include disorders of gluconeogenesis and
mitochondrial disorders of oxidation. In patients with metabolic acidosis, concentration of serum
ammonia and glucose, and presence or absence of urine ketones and reducing substances will also help
direct further metabolic workup. Plasma amino acids, acylcarnitines and urine organic acids, and
acylglycines should be measured. Measurement of serum lactate, pyruvate, ketones, and organic acids
may also be helpful.