We describehere anovelmethodology for rapiddiagnosis of metabolic changes,which is basedonprobabilistic equations that relate GC-MS-derived mass distributions in proteino-genic aminoacids toin vivoenzyme activities. Thismetabolic flux ratio analysis by GC-MS provides a comprehensive perspective on central metabolism by quantifying 14 ratios of fluxes through converging pathways and reactions from [1-13 C] and [U-13 C]glucose experiments.