The measurement of total homocysteine (tHcy) is a sensitive marker of folate and cobalamin (vitamin B12) deficiency and an independent risk factor for cardiovascular disease. Plasma tHcy concentrations are also related to birth defects, pregnancy complications, psychiatric disorders, and cognitive impairment in the elderly.

Concentrations of Methylmalonic acid are not only elevated in vitamin B12 deficiency, but are also known to increase during renal dysfunction. Moreover, methylmalonyl-CoA is an intermediate in the metabolism of propionic acid (PA). Studies with PA in isolated hepatocytes suggest that toxicity of PA is due to accumulation of propionyl-CoA and methylmalonyl-CoA. Intracellular accumulation of propionyl-CoA and methylmalonyl-CoA was found to be associated with impairment of several hepatic metabolic pathways, including gluconeogenesis, ureagenesis, pyruvate oxidation, and fatty acid oxidation.

Methylcitric acid is generally accepted as a pathognomonic marker for propionate metabolic defects and has been shown to improve screening for Propionic acidemia, methylmalonic acidemia and Cobalamin C defects.

Amino acids are the basic structural units that comprise proteins and are found throughout the body. Many inborn errors of amino acid metabolism, such as phenylketonuria and tyrosinemia, have been identified. Amino acid disorders can manifest at any age, but most become evident in infancy or early childhood. These disorders result in the accumulation or the deficiency of 1 or more amino acids in biological fluids, which leads to the clinical signs and symptoms of the particular amino acid disorder. In addition, plasma amino acid analysis may have clinical importance in the evaluation of several acquired conditions including endocrine disorders, liver diseases, muscle diseases, neoplastic diseases, neurological disorders, nutritional disturbances, renal failure, and burns. Test includes quantitation of the following amino acids: taurine, threonine, serine, asparagine, glutamic acid, glutamine, proline, alanine, citrulline, a-amino-n-butyric acid, valine, cystine, methionine, isoleucine, leucine, tyrosine, phenylalanine, b-alanine, ornithine, lysine, histidine, argininosuccinic acid, allo-isoleucine, arginine, phosphoserine, phosphoethanolamine, hydroxyproline, glycine, aspartic acid, ethanolamine, sarcosine, 1-methylhistidine, 3-methylhistidine, carnosine, anserine, homocitruline, a-aminoadipic acid, g-amino-n-butyric acid, b-aminoisobutyric acid, hydroxylysine, cystathionine, and tryptophan.