Monophenyl Succinate is an impurity in the synthesis of Succinylacetone (S692060), a specific marker for the inherited metabolic disease, hepatorenal tyrosinemia. It was developed a stable-isotope for the determination of SA in dried blood spots (DBS) and liquid urine using a 13C4-SA as internal standard. The method was applied retrospectively and prospectively for the diagnosis of hepatorenal tyrosinemia and for follow-up of patients under treatment.