Glucose transport into the brain is mediated by a facilitative glucose-transporter protein, GLUT-1. A GLUT-1 defect results in the Glucose-Transporter-Protein Syndrome (GTPS), characterized by infantile epilepsy, developmental delay, and acquired microcephaly. The diagnosis is currently based on clinical features, low to normal lactate levels and low glucose levels (hypoglycorrhachia) in the cerebrospinal fluid, and the demonstration of impaired GLUT-1 function in erythrocytes as described here. Blood samples were collected in sodium-heparin or citrate-phosphate-dextrose solution and uptake of 14C-labeled 3-O-Methyl-D-glucose (3OMG into erythrocytes (0.5 mmol/L 3OMG; 1 microCi/mL) was measured at 4C and pH 7.4. Three-OMG influx was terminated at 5-second intervals, washed cells were lysed, and uptake was quantitated by liquid scintillation counting. Patients' uptake (n = 22) was 44 +/- 8% of controls (100 +/- 22%, n = 70). Statistical analyses showed an uptake cut-off point at 60% uptake, a sensitivity of 86% (95%-confidence interval 78 to 94%), and a specificity of 97% (95%-confidence interval 93 to 100%). Gender, age, and ketosis did not influence 3OMG uptake. This assay provides a reproducible and accurate laboratory test for diagnosing the GTPS.