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, hepatic encephalopathy, and end-stage renal disease (ESRD) in a subject comprising: (a) administering a first dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100), (b) measuring plasma phenylacetic acid and phenylacetylglutamine (PAGN) levels, (c) calculating a plasma PAA:PAGN ratio, (d) determining whether the glyceryl tri-[4-phenylbutyrate] (HPN-100) dosage needs to be adjusted based on whether the PAA:PAGN ratio falls within a target range of 1 to 2.5, where a PAA:PAGN ratio below the target range indicates that the dosage needs to be increased and a PAA:PAGN ratio above the target range indicates that the dosage needs to be decreased, and (e) administering a second dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100) based on the determination in (d); or B) a method of treating a nitrogen retention disorder selected from the group consisting of urea cycle disorders, hepatic encephalopathy, and end-stage renal disease (ESRD) in a subject who has previously been administered a first dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100) comprising: (a) measuring plasma PAA and PAGN levels, (b) calculating a plasma PAA:PAGN ratio, (c) determining whether the first glyceryl tri-[4-phenylbutyrate] (HPN-100) dosage needs to be adjusted based on whether the PAA:PAGN ratio falls within a target range of 1 to 2.5, where a PAA:PAGN ratio below the target range indicates that the dosage needs to be increased and a PAA:PAGN ratio above the target range indicates that the dosage needs to be decreased, and (d) administering a second dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100) based on the determination in (c). A) a method of treating a nitrogen retention disorder selected from the group consisting of urea cycle disorders, hepatic encephalopathy, and end-stage renal disease (ESRD) in a subject comprising: (a) administering a first dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100), (b) measuring plasma phenylacetic acid and phenylacetylglutamine (PAGN) levels, (c) calculating a plasma PAA:PAGN ratio, (d) determining whether the glyceryl tri-[4-phenylbutyrate] (HPN-100) dosage needs to be adjusted based on whether the PAA:PAGN ratio falls within a target range of 1 to 2.5, where a PAA:PAGN ratio below the target range indicates that the dosage needs to be increased and a PAA:PAGN ratio above the target range indicates that the dosage needs to be decreased, and (e) administering a second dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100) based on the determination in (d); or (a) administering a first dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100), (b) measuring plasma phenylacetic acid and phenylacetylglutamine (PAGN) levels, (c) calculating a plasma PAA:PAGN ratio, (d) determining whether the glyceryl tri-[4-phenylbutyrate] (HPN-100) dosage needs to be adjusted based on whether the PAA:PAGN ratio falls within a target range of 1 to 2.5, where a PAA:PAGN ratio below the target range indicates that the dosage needs to be increased and a PAA:PAGN ratio above the target range indicates that the dosage needs to be decreased, and (e) administering a second dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100) based on the determination in (d); or B) a method of treating a nitrogen retention disorder selected from the group consisting of urea cycle disorders, hepatic encephalopathy, and end-stage renal disease (ESRD) in a subject who has previously been administered a first dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100) comprising: (a) measuring plasma PAA and PAGN levels, (b) calculating a plasma PAA:PAGN ratio, (c) determining whether the first glyceryl tri-[4-phenylbutyrate] (HPN-100) dosage needs to be adjusted based on whether the PAA:PAGN ratio falls within a target range of 1 to 2.5, where a PAA:PAGN ratio below the target range indicates that the dosage needs to be increased and a PAA:PAGN ratio above the target range indicates that the dosage needs to be decreased, and (d) administering a second dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100) based on the determination in (c). (a) measuring plasma PAA and PAGN levels, (b) calculating a plasma PAA:PAGN ratio, (c) determining whether the first glyceryl tri-[4-phenylbutyrate] (HPN-100) dosage needs to be adjusted based on whether the PAA:PAGN ratio falls within a target range of 1 to 2.5, where a PAA:PAGN ratio below the target range indicates that the dosage needs to be increased and a PAA:PAGN ratio above the target range indicates that the dosage needs to be decreased, and (d) administering a second dosage of glyceryl tri-[4-phenylbutyrate] (HPN-100) based on the determination in (c).