Various stable isotope labeling studies have been used to estimate the TCA cycle flux by measuring one or more labelled intermediate metabolites within the cycle. Unfortunately, these labelled intermediates are often present through only partial segments of the cycle due to exchange, anaplerosis (entrance into the cycle), cataplerosis (export out of the cycle) or incomplete cycling. Though these previous isotope labeling studies of the TCA cycle flux were qualitatively informative, many were quantitatively inaccurate due to unexpected dilutions of the TCA cycle intermediates arising from unlabeled precursors.