Glyceryl tri- [4-phenylbutyrate] (HPN-100) for use in: A) a method of treating a nitrogen retention disorder selected from the group consisting of urea cycle disorders (UCD), hepatic encephalopathy (EH) , and end stage renal disease (ESRD) in a subject comprising: (a) the administration of a first dose of glyceryl tri- [4-phenylbutyrate] (HPN-100), (b) the measurement of phenylacetic acid from plasma (PAA) and phenylacetylglutamine (PAGN) levels, (c) the calculation of a PAA plasma ratio: PAGN, (d) the determination of whether the dosage of glyceryl tri- [4-phenylbutyrate] (HPN-100) It needs to be adjusted based on whether the PAA: PAGN ratio falls within a range of 1 to 2.5, where a PAA: PAGN ratio below the target range indicates that the dose needs to be increased and a PAA: PAGN ratio per above the target interval indicates that the dose needs to be decreased, and (e) the administration of a second dose of gli ceryl tri- [4-phenylbutyrate] (HPN-100) based on the determination in (d)； or B) a method for treating a nitrogen retention disorder selected from the group consisting of urea cycle disorders (UCD), hepatic encephalopathy (EH), and end stage renal disease (ESRD) in a subject to that a first dose of glyceryl tri- [4-phenylbutyrate] (HPN-100) has previously been administered, comprising: (a) the measurement of PAA and PAGN plasma levels, (b) the calculation of a PAA plasma ratio: PAGN, (c) the determination of whether the first dosage of glyceryl tri- [4-phenylbutyrate] (HPN-100) has to be adjusted based on whether the PAA: PAGN ratio falls within a range of 1 to 2, 5, where a PAA: PAGN ratio below the target range indicates that the dose needs to be increased and a PAA: PAGN ratio above the target range indicates that the dose needs to be decreased and (d) the administration of a second dose of glyceryl tri- [4-phenylbutyrate] (HPN-100) based on the determination in (c).Glicerilo tri-[4-fenilbutirato] (HPN-100) para uso en: A) un método para tratar un trast
