The possibility of an inborn error of metabolism is very unlikely if there is no ketonuria (excludes propionic and methylmalonic acidemias) and the following laboratory values are in the normal range: (1) blood glucose (excludes carbohydrate and fatty acid abnormalities); (2) ammonia level (excludes a urea cycle defect); (3) lactic acid (excludes pyruvate, citric acid, and mitochondrial respiratory chain abnormalities); and (4) sulfite in blood and urine (excludes sulfite oxidase deficiency and molybdenum cofactor deficiency). A neonate with these findings should be tentatively considered to not have an inborn error of metabolism until more specific tests such as biotinidase level, acyl and total carnitine, pyruvate, serum for organic and amino acids, urine organic acids, and cerebrospinal fluid lactic acid and glycine results are available.