||Blood pyruvate (MET10) is useful for screening for possible disorders of mitochondrial metabolism, when used in conjunction with blood lactate. CLINICAL Pyruvic acid, an intermediate metabolite, plays an important role in linking carbohydrate and amino acid metabolism to the tricarboxylic acid cycle, the fatty acid beta-oxidation pathway, and the mitochondrial respiratory chain complex. Though isolated elevated pyruvate is not diagnostic of any inborn error of metabolism, analysis with lactate may suggest an inborn error of metabolism as some present with lactic acidosis and/or a high Lactate:Pyruvate ratio. The Lactate:Pyruvate ratio is elevated in several, but not all, mitochondrial respiratory chain disorders. Mitochondrial disorders vary widely in presentation and age of onset. Many mitochondrial disorders have neurologic and myopathic features and may involve multiple organ systems. Determination of lactate, pyruvate, and Lactate:Pyruvate ratio in cerebrospinal fluid is helpful in directing attention toward a possible mitochondrial disorder in cases with predominantly neurologic dysfunction and normal blood lactate levels, though further confirmatory testing will be required to establish a diagnosis. A low Lactate:Pyruvate ratio is observed in inherited disorders of pyruvate metabolism including pyruvate dehydrogenase complex (PDHC) deficiency. Clinical presentation of PDHC deficiency can range from fatal congenital lactic acidosis to relatively mild ataxia or neuropathy. The most common features in infants and children with PDHC deficiency are delayed development and hypotonia, and seizures and ataxia are also frequent features. Other manifestations can include congenital brain malformations, degenerative changes including Leigh disease, and facial dysmorphism. An elevated Lactate:Pyruvate ratio may indicate inherited disorders of the respiratory chain complex, tricarboxylic acid cycle disorders and pyruvate carboxylase deficiency. Respiratory chain defects usually result in Lactate:Pyruvate ratios >20. A low Lactate:Pyruvate ratio (disproportionately elevated pyruvic acid) may indicate an inherited disorder of pyruvate metabolism. Defects of the pyruvate dehydrogenase complex result in Lactate:Pyruvate ratios <10. The Lactate:Pyruvate ratio is characteristically normal in other patients. An artifactually high ratio can be found if the patient is acutely ill. Cerebrospinal fluid (CSF) Lactate:Pyruvate ratio may assist in evaluation of patients with neurologic dysfunction and normal blood Lactate:Pyruvate ratios. Blood and CSF specimens should be collected at the same time.