Analysis of CSF is daily routine in patients with acute neurologic disorders like CNS infections. In those patients, the finding of a low CSF glucose may influence further diagnostic workup and therapeutic choices. The interpretation of a low CSF glucose in patients with a chronic neurologic disorder, however, is a less common practice. We present a practical overview on the differential diagnosis of a low CSF glucose and stress the importance of recognizing a low CSF glucose as the diagnostic marker for GLUT1 deficiency syndrome, a treatable neurometabolic disorder.