Diagnosis of congenital lactic acidosis (CLA) meeting the following criteria: Three basal venous lactates at least 2.5 mM, arterial lactates at least 2.0 mM, or CSF lactates at least 2.5 mM OR any combination of these, obtained over at least 1 month and within 6 months OR Increase in blood lactate at least 1.0 mM over basal following a carbohydrate meal challenge