The patients with sepsis are in the state of hypermetabolism, increased resting energy expenditure, protein and fat catabolism disorder, negative nitrogen balance, insulin resistance, hyperglycemia, and amino acid metabolism disorders. However, it is remain unclear the changes of amino acids expression profiling in sepsis patients. In this study, the investigators has planned to enroll 100 subjects, including 20 cases with systemic inflammatory response syndrome (SIRS), 20 cases with sepsis, 20 cases with severe sepsis and 20 with septic shock. In addition, this study also include 20 normal cases as control. The serum sample of patients with sepsis is draw on days 1, 3, 5, 7, 10, and 14 after first ICU admission. High-performance liquid chromatography and tandem mass spectrometry was used to detect the quantification of amino acids. The amino acids expression profiling contain Arginine, Ornithine, Histidine, Cystine, Isoleucine, Cystathionine, Leucine, Homocystine, Lysine, α-Amino-n-Butyric Acid, Methionine, Alanine, Phenylalanine, Anserine, Threonine, β-Alanine, Tryptophan, β-Amino-Isobutyric Acid, Valine, Carnosine, γ-Amino-n-Butyric Acid, Ethanolamine, Glycine, δ-Hydroxylysine, Serine, Hydroxy-L-Proline, Taurine, 1-Methyl-L-Histidine, Tyrosine, 3-Methyl-L-Mistidine, α-Amino-Adipic, Phospho-Ethanolamine, Asparagine, Phospho-L-Serine, Aspartic acid, Proline, Citrulline, Sarcosine, Glutamic acid, Argininosuccinic Acid, Glutamine, Homocitrulline. The investigators speculate that measurement of amino acids expression profiling could be taken as an indicator for assessment in critically ill patients.