Enzyme in red blood cells for which flavin adenine dinucleotide (FAD; derived from vitamin B2) is the essential cofactor. It converts oxidised to reduced glutathione, with reduced nicotinamide dinucleotide phosphate (NADPH) as co-substrate. Activation of this enzyme in vitro from red cell extracts by added FAD provides a means of assessing vitamin B2 nutritional status, sometimes known as the erythrocyte glutathione reductase activation coefficient (EGRAC) test.