Organic Acids (urine)

General Information

The term "organic acidemia" or "organic aciduria" (OA) applies to a group of disorders characterized by the excretion of non-amino organic acids in urine. Most organic acidemias result from dysfunction of a specific step in amino acid catabolism, usually the result of deficient enzyme activity. The majority of the classic organic acid disorders are caused by abnormal amino acid catabolism of branched-chain amino acids or lysine. They include

• Maple syrup urine disease (MSUD)
• Propionic acidemia
• Methylmalonic acidemia (MMA)
• Methylmalonic aciduria and homocystinuria
• Isovaleric acidemia
• Biotin-unresponsive 3-methylcrotonyl-CoA carboxylase deficiency
• 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase deficiency
• Ketothiolase deficiency
• Glutaricacidemia type I (GA I)

A neonate affected with an OA is usually well at birth and for the first few days of life. The usual clinical presentation is that of toxic encephalopathy and includes vomiting, poor feeding, neurologic symptoms such as seizures and abnormal tone, and lethargy progressing to coma. Outcome is enhanced by diagnosis and treatment in the first ten days of life. In the older child or adolescent, variant forms of the OAs can present as loss of intellectual function, ataxia or other focal neurologic signs, Reye syndrome, recurrent ketoacidosis, or psychiatric symptoms.

Clinical laboratory findings that suggest an organic acidemia include acidosis, ketosis, hyperammonemia, abnormal liver function tests, hypoglycemia, and neutropenia. First-line diagnosis in the organic acidemias is urine organic acid analysis. The urinary organic acid profile is nearly always abnormal in the face of acute illness with decompensation; however, in some disorders diagnostic analytes may be present only in small or barely detectable amounts when the affected individual is not acutely ill. Confirmatory testing involves assay of the activity of the deficient enzyme in lymphocytes or cultured fibroblasts and/or molecular genetic testing.
Taken from The Organic Acidemias: An Overview by Margretta R Seashore, MD.