||CSF Pyruvate (MET11) is useful for investigating possible disorders of mitochondrial metabolism, when used in conjunction with cerebrospinal fluid lactate collected at the same time to determine the Lactate:Pyruvate ratio. The CSF Lactate:Pyruvate ratio is considered a helpful (not diagnostic) tool in the evaluation of patients with possible disorders of mitochondrial metabolism, especially in patients with neurologic dysfunction and normal blood Lactate:Pyruvate ratios. Pyruvic acid levels alone have little clinical utility. CLINICAL 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 L:P 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. 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 in acutely ill patients.