Table 10. Comparison of urine concentrations of 23 quantified amino acids by different assays: DFI/LC-MS/MS (BIOCRATES kit), NMR and GC-MS.

Amino acids*	DFI/LC-MS/MS	NMR	GC-MS
Glycine	101.0±53.5	106±66	75.7±51.7
L-Alanine	21.7±10.6	21.8±10.3	NA
L-Arginine	7.4±3.0	8.6±3.6	NA
L-Asparagine	8.8±3.5	10.1±4.0	9.5±5.5
L-Aspartic acid	10.9±7.7	10.9±5.8	NA
L-Citrulline	0.8±0.4	NA	NA
L-Cysteine	NA	65.8±24.0	NA
L-Cystine	NA	12.5±5.4	NA
L-Glutamic acid	6.4±4.9	8.5±3.6	NA
L-Glutamine	39.9±14.3	37.2±14.8	NA
L-Histidine	45.6±15.6	43.9±19.5	NA
L-Isoleucine	1.3±0.4	1.3±0.5	NA
L-Leucine	3.0±1.0	2.8±0.9	NA
L-Lysine	17.9±12.2	17.2±11.5	NA
L-Methionine	0.8±0.3	1.4±0.6	NA
L-Ornithine	4.4±5.0	NA	5.3±1.9
L-Phenylalanine	5.9±2.0	6.4±2.3	7.8±2.0
L-Proline	1.1±0.6	NA	NA
L-Serine	24.5±9.2	25.3±9.7	19.0±7.0
L-Threonine	14.6±6.1	13.3±5.3	NA
L-Tryptophan	5.6±2.1	6.3±2.2	NA
L-Tyrosine	8.7±4.2	9.5±4.5	15.1±5.4
L-Valine	4.3±1.9	3.4±1.4	5.5±2.0

^*Concentration (Mean ± SD) is expressed by µM/mM creatinine. NA: not available.