SLC2A1 and hyperinsulinemic hypoglycemia, familial, 4: Cerebrospinal fluid abnormalities may point towards several inherited metabolic disorders (Table 5): low serine levels (serine biosynthesis defects), hypoglycorrhachia (glucose transporter GLUT1 deficiency), elevated lactate (mitochondrial disorders), elevated pipecolic acid, reduced 5-methylenetetrahydrofolate (cerebral folate deficiency), and abnormal pterin profile (tetrahydrobiopterin metabolism defects, pyridoxine-dependent epilepsy and peroxisomal diseases).