For each organic acid stock solutions of a concentration of10 mg/mL were prepared in absolute methanol. PDA was used as internal standard; its stock solution (1 mg/mL) was also prepared inabsolute methanol. From these stock solutions three standard solu-tions were prepared, to correspond to lower, medium and highertarget concentration for each acid. The stock solutions were used tospike urine control samples to generate calibration curves. The con-centration levels for the calibration curve were covering the normaland up to three times the normal concentration range in urine. Nor-mal ranges were specified for each acid based on data from theHuman Metabolome Database (www.hmdb.ca) (see Table 1). Clin-ical samples were diluted with water prior to spiking in order tocorrect for creatinine concentration differences (at 0.25 mg creati-nine in 3 mL urine see below). All standard solutions were stored inglass vials at −24◦C.