The initial laboratory assessment of infants and children with suspected inborn errors of metabolism (IEM) is reviewed by the Department of Medical Genetics, Mayo Clinic, Rochester, MN. Classes of IEM include organic acidemias, aminoacidopathies, urea cycle defects, glycogen storage diseases, lysosomal storage diseases, B-oxidation defects, and peroxisomal disorders. Signs and symptoms of IEM include failure to thrive, loss of milestones, vomiting, seizures, coma, hepatosplenomegaly, dysmorphic features, sparse or abnormal textured hair, cataract and other eye findings, and urine or body odor. Initial tests suggested include blood gases, glucose, urinary ketones, ammonia, electrolytes, uric acid, liver function, lactate and pyruvate, carnitine, free fatty acids, B-hydoxybutyrate, and acetoacetate. 
COMMENT. Examples of IEM requiring additional preliminary tests include Menkes’ kinky-hair disease (serum copper and ceruloplasmin), and molybdenum cofactor deficiency (urine sulfite dipstick).